INSTRUCTION Ga ae ee MANUAL MODELS 2120B and 2125A 30 MHz DUAL-TRACE OSCILLOSCOPESTEST INSTRUMENT SAFETY Normal use of test equipment exposes you to a certain amount of danger from electrical shock because testing must often be performed where exposed high voltage is present. An electrical shock causing 10 milliamps of current to pass through the heart will stop most human heartbeats. Voltage as low as 35 volts dc or ac rms should be considered dangerous and hazardous since it can produce a lethal current under certain conditions. Higher voltage poses an even greater threat because such voltage can more easily produce a lethal current. Your normal work habits should include all accepted practices that will prevent contact with exposed high voltage, and that will steer current away from your heart in case of accidental contact with a high voltage. You will significantly reduce the risk factor if you know and observe the following safety precautions: Dont expose high voltage needlessly in the equipment under test. Remove housings and covers only when necessary. Turn off equipment while making test connections in high-voltage circuits. Discharge high-voltage capacitors after removing power. If possible, familiarize yourself with the equipment being tested and the location of its high voltage points. However, remember that high voltage may appear at unexpected points in defective equipment. Use an insulated floor material or a large, insulated floor mat to stand on, and an insulated work surface on which to place equipment; make certain such surfaces are not damp or wet. Use the time-proven one hand in the pocket technique while handling an instrument probe. Be particularly careful to avoid contacting a nearby metal object that could provide a good ground return path. When using a probe, touch only the insulated portion. Never touch the exposed tip portion. When testing ac powered equipment, remember that ac line voltage is usually present on some power input circuits such as the on-off switch, fuses, power transformer, etc. any time the equipment is connected to an ac outlet, even if the equipment is turned off. Some equipment with a two-wire ac power cord, including some with polarized power plugs, is the hot chassis type. This includes most recent television receivers and audio equipment. A plastic or wooden cabinet insulates the chassis to protect the customer. When the cabinet is removed for servicing, a serious shock hazard exists if the chassis is touched. Not only does this present a dangerous shock hazard, but damage to test instruments or the equipment under test may result from connecting the ground lead of most test instruments (including this oscilloscope) to a hot chassis. To make measurements in hot chassis equipment, always connect an isolation transformer between the ac outlet and the equipment under test. The B+K Precision Model TR-110 or 1604A Isolation Transformer, or Model 1653A or 1655A AC Power Supply is suitable for most applications. To be on the safe side, treat all two wire ac powered equipment as hot chassis unless you are sure it has an isolated chassis or an earth ground chassis. Never work alone. Someone should be nearby to render aid if necessary. Training in CPR (cardio-pulmonary resuscitation) first aid is highly recommended.Instruction Manual for BK pee OnciOar Models 2120B and 2125A 30 MHz Dual-Trace Oscilloscopes 2000 B+K Precision Corp. This symbol on oscilloscope means refer to instruction manual for further precautionary information. This symbol appears in the manual where the corresponding information is given. BK | = ge os ge ria WivtWVi 1031 Segovia Circle, Placentia, CA 92870TABLE OF CONTENTS Page TEST INSTRUMENT SAFETY ...... inside front cover FEATURES... 0... ccc cee eee ee eee eens 3 SPECIFICATIONS .... 26.6.0 eee eee eee eee ees 5 CONTROLS AND INDICATORS ..............005- 7 General Function Controls...............2++066- 7 Vertical Controls... 0.0.00... cee eee eee ee eee eee 7 Horizontal Controls ............0.ceccecceeeees 9 Triggering Controls ......... 2.2.02 cece eee ees 9 Rear Panel Controls ............- 2-00 -e ee eeeee 10 OPERATING INSTRUCTIONS. ........-.-2-0002+ ll Safety Precautions ..........0. 0.00 cece e cence 1 Equipment Protection Precautions..............- 11 Operating Tips .................-.- Leese eee 12 Initial Starting Procedure... ...... 00.0. eee e eee 12 Single Trace Display .............. 000 e eee eeee 12 Dual Trace Display.......... 0.0. e eee cee eee 12 TrIZQZering.. 2... cee eee ce ee eee eens 13 Magnified Sweep Operation ................... 15 Page OPERATING INSTRUCTIONS (Continued) X-Y Operation 2.0.00... ce eee cee ee ee eee 15 Video Signal Observation ..............0ee0005 15 Application Guidebook ..............-2.00005- 15 Delayed Sweep Operation. ......... 0200 5eee eee 15 Component Test Operation ..........-.00000 00s 16 MAINTENANCE. .... 2.0.0 ee eee eee eee eens 20 Fuse Replacement .......... 0.00 ec ee eee eeeees 20 Line Voltage Selection........... 0.0 e eee ecnee 20 Periodic Adjustments. ........0. 2 0c ce eee eee 20 Calibration Check ......... 2. cc eee cee eens 21 Instrument Repair Service..........0.0eeeeeee 21 APPENDIX .......0 0. cece eee cece eee e eee eneene 22 Important Considerations for Rise Time and Fall Time Measurements ............2000005- 22 Service Information ........... Lecce tence eeeneee 23 Limited Warranty .......0.0 ccc ee cece eee eee 24 Guidebook To Oscilloscopes Availability ..............-..006- inside back coverFEATURES LOW COST, HIGH PERFORMANCE B+K Precisions entry level oscilloscopes, Models 2120B and 2125A, are economically priced to equal the competi- tion, but offer higher performance than the competition. For example, most competitors entry level oscilloscopes have a 20 MHz bandwidth, while B+K Precisions Models 2120B and 2125A offer a 30 MHz bandwidth. These oscilloscopes are built by and backed by B+K Precision, a company that has been selling reliable, durable, value priced test instru- ments for over 50 years. CRT FEATURES Rectangular CRT Rectangular CRT with large 8 x 10 centimeter viewing area. On Model 2125A, graticule is equipped with variable scale illumination. Convenience Trace rotation electrically adjustable from front panel. 0%, 10%, 90%, and 100% markers for rise time meas- urements. DUAL TRACE FEATURES Dual Trace Models 2120B and 2125A each have two vertical input channels for displaying two waveforms simultane- ously. Selectable single trace (either CH | or CH 2) or dual trace. Alternate or chop sweep selectable at all sweep rates. Sum and Difference Capability Permits algebraic addition or subtraction of channel 1 and channel 2 waveforms, displayed as a single trace. Useful for differential voltage and distortion measure- ments. HIGH FREQUENCY FEATURES Wide Bandwidth Conservatively-rated -3 dB bandwidth is de to 30 MHz. Fast Rise Time Rise time is less than 12 ns. Fast Sweep Maximum sweep speed of 10 ns/div (with X10 MAG) assures high frequencies and short-duration pulses are displayed with high resolution. VERTICAL FEATURES High Sensitivity 5 mV/div sensitivity for full bandwidth. High-sen- sitivity 1 mV/div and 2 mV/div using PULL XS gain control. Calibrated Voltage Measurements ~ Accurate voltage measurements (3%) on 10 cali- brated ranges from 5 mV/div to 5 V/ div. Vertical gain fully adjustable between calibrated ranges. SWEEP FEATURES Calibrated Time Measurements Accurate (+3%) time measurements. The main sweep has 23 calibrated ranges from 2 S/div to 0.1 wS/div. The delayed sweep on the Model 2125A has 23 cali- brated ranges from 2 S/div to 0.1 uS/div. Sweep time is fully adjustable between calibrated ranges. X10 Sweep Magnification Allows closer examination of waveforms, increases maximum sweep rate to 10 nS/div. DUAL TIME BASE FEATURES (Model 2125A) Dual Sweep Generators Main sweep gives normal waveform display, delayed sweep may be operated at faster sweep speed to expand a portion of the waveform. * Four Sweep Modes Choice of main sweep only, delayed sweep only, main sweep and delayed sweep sharing the trace (percentage of main/delayed sweep adjustable), or X-Y. Adjustable Start Of Delayed Sweep DELAY TIME POSition control allows adjustment of delayed sweep starting point. TRIGGERING FEATURES Two Trigger Modes Selectable normal (triggered) or automatic sweep modes. Triggered Sweep Sweep remains at rest unless adequate trigger signal is applied. Fully adjustable trigger level and (+) or (-) Slope. . FEATURES AUTO Sweep Selectable AUTO sweep provides sweep without trig- ger input, automatically reverts to triggered sweep operation when adequate trigger is applied. Five Trigger Sources Five trigger source selections, including CH 1, CH 2, alternate, EXT, and LINE. Video Sync Frame (TV V) or Line (TV H) triggering selectable for observing composite video waveforms. TV-H position can also be used as low frequency reject and TV-V position can be used as high frequency reject. Variable Holdoff Trigger inhibit period after end of sweep adjustable. Permits stable observation of complex pulse trains. OTHER FEATURES X-Y Operation Channel 1 can be applied as horizontal deflection (X-axis) while channel 2 provides vertical deflection (Y-axis). Built-in Probe Adjust Square Wave A2V p-p, 1 kHz square wave generator permits probe compensation adjustment. Component Test Function (Model 2125A) Built-in XY type component tester applies fixed level ac signal to components for display of signature on CRT. Channel 2 (Y) Output (Model 2125A) A buffered 50 output of the channel 2 signal is available at the rear panel for driving a frequency counter or other instruments. The output is 50 mV/div (nominal) into 50Q. Z-Axis Input (Model 2125A) Rear panel Z-Axis input allows intensity modulation. Supplied With Two ProbesSPECIFICATIONS CRT: Type: 6-inch rectangular with integral graticule, P31 phosphor. Display Area: 8 x 10 div (1 div = 1 cm). Accelerating Voltage: 2 kV. Phosphor: P31. Trace Rotation: Electrical, front panel adjustable. Scale Illumination: Continuously variable (Model 2125A). Beam Finder (Model 2125A). VERTICAL AMPLIFIERS (CH 1 and CH 2) Sensitivity: 5 mV/div to 5 V/div, | mv/div to 1 V/div at X5 MAG. Attenuator: 10 calibrated steps in 1-2-5 sequence. Vernier control provides fully adjustable sensitivity between steps; range I/{ to at least 1/3. Accuracy: +3%, 5 mV to 5 V/div; 5%, at XS MAG. Input Resistance: 1 MQ +2%. Input Capacitance: 25 pF +10 pF. Frequency Response: 5 mV/div to 5 V/div: DC to 30 MHz (3 dB). X5 MAG: DC to 10 MHz (-3 dB). Rise Time: 12 nS; 35 nS at X5 MAG. Overshoot: Less than 5%. Operating Modes: CH 1: CH 1, single trace. CH 2: CH 2, single trace. DUAL: CH 1 and CH 2, dual trace. Alternate or Chop selectable at any sweep rate. ADD: Algebraic sum of CH 1 + CH 2. Chop Frequency: Approximately 500 kHz. Polarity Reversal: CH 2 invert. Maximum Input Voltage: 400 V (dc + ac peak), HORIZONTAL AMPLIFIER (Input through channel | input) X-Y mode: CH 1 = X axis. CH 2 = Y axis. Sensitivity: Same as vertical channel 2. Input Impedance: Same as vertical channel 2. Frequency Response: DC to | MHz (3 dB). X-Y Phase Difference: 3 or less at 50 kHz. Maximum Input Voltage: Same as vertical channel |. SWEEP SYSTEM Operating Modes: Model 2125A: Main, Mix (both main and delayed sweep displayed), Delay (only delayed sweep displayed), XY. Model 2120B: Main only. Main Time Base: 0.1 pS/div to 2.0 S/div in 1-2-5 sequence, 23 steps. Vernier control provides fully adjustable sweep time between steps. Delayed Time Base (Model 2125A only): 0.1 S/div to 2.0 S/div in 1-2-5 sequence, 23 steps. Accuracy: +3%, except +6% on 0.2 S/div and +20% on 0.1 pS/div. Sweep Magnification: X10 +10%. Holdoff: Continuously adjustable for main time base from NORM to 5 times normal. Delay Time Position: Control sets percentage of display that is devoted to main and delayed sweep. Delay Jitter: 1/10,000 of full scale sweep time. TRIGGERING Trigger Modes: AUTO (free run), NORM, TV-V, TV-H. Trigger Source: CH 1, CH 2, Alternate, EXT, LINE. Slope: (+) or (~).SPECIFICATIONS Trigger Coupling: AUTO: Sweep free-runs in absence of suitable trigger signal. NORM: Sweep triggered only by adequate trigger signal. TV-V: Video vertical sync pulses are selected. Also usable for high frequency reject. TV-H: Video horizontal sync pulses are , selected. Also usable for low frequency reject. Trigger Sensitivity: Auto: 1.5 div (internal) 20.5 Vp-p (external) 100 Hz 40 MHz (2125A) 100 Hz 30 MHz (2120B) Norm: 1.5 div (internal) 20.5 Vp-p (external) 100 Hz - 40 MHz (2125A) DC 30 MHz (2120B) TV-V: 1.0 div (internal) 20.5 Vp-p (external) DC 1 kHz (2125A) 20 Hz | kHz (2120B) TV-H: 1.0 div (internal) 20.5 Vp-p (external) 1 kHz 100 kHz Maximum External Trigger Voltage: 300 V (dc + ac peak). COMPONENT TESTER (Mode! 2125A) Components Tested: Resistors, capacitors, inductors, and semiconductors. Test Voltage: 6 V rms maximum (open). Test Current: 11 mA maximum (shorted). Test Frequency: Line frequency (60 Hz in USA). OTHER SPECIFICATIONS Cal/Probe Compensation Voltage: 2 V p-p 3% square wave, | kHz nominal. CH 2 (Y) Output (Model 2125A): Output Voltage: 50 mV/div (nominal into 50 ohm load). Output Impedance: Approximately 50 ohms. Frequency Response: 20 Hz to 30 MHz, 3 dB. Intensity Modulation (Model 2125A) Input Signal: TTL level, intensity increasing with more positive levels, decreased intensity with more negative levels. Input Impedance: Approximately 50 k@2. Usable Frequency Range: DC to 5 MHz. Maximum Input Voltage: 30 V (dc + ac peak). Power Requirements: 100-130 VAC or 200-260 VAC, 50/60 Hz, 38 watts. Dimensions (H < W xX D): 5.2 X 12.8 X 15.7 (132 X 324 X 398 mm). Weight: 16.8 lbs (7.6 kg). Environment: : Within Specified Accuracy: +10 to +35 C, 10-80% relative humidity. Full Operation: 0 to +50 C, 10-80% relative humidity. Storage: 30 to +70 C, 10-90% relative humidity. ACCESORIES SUPPLIED: Two Switchable X1/X10 Probes. Instruction Manual. AC Line Cord.CONTROLS AND INDICATORS Fig, 1. GENERAL FUNCTION CONTROLS 1. 2; ON Indicator. Lights when oscilloscope is on. 2125A Only. POWER/Scale [LLUMination Con- trol: Clockwise rotation from OFF position turns os- cilloscope on. Further clockwise rotation increases amount of graticule illumination. 2120B Only. POWER Pushbutton. Tums oscillo- scopeon and off. INTENSITY Control. Adjusts brightness of trace. TRACE ROTATION Control. Adjusts to maintain trace at a horizontal position. FOCUS Control. Adjusts trace focus. 2125A Only. COMPonent TEST Pushbutton. With pushbutton set to in position, Component Test mode is enabled. Normal scope operation is enabled with pushbutton in out position. 2125A Only. COMPonent TEST Jack. Banana- type HI-side input jack for connection to component: in Component Test operating mode. GND = Terminal. Oscilloscope chassis ground jack, and earth ground via three-wire ac power cord. On Model 2125A, also serves as LO-side Component Test jack. 10. 1. Model 2125A Controls and Indicators. CAL Terminal. Terminal provides 2V p-p, 1 kHz (nominal) square wave signal. This signal is useful for checking probe compensation adjustment, as well as providing a rough check of vertical calibration. 2125A Only. BEAM FIND Pushbutton. Momentary- contact pushbutton speeds setup of trace positioning by bringing the beam into graticule area; operates independently of other display controls. VERTICAL CONTROLS: 12. VERTical MODE Switch. Selects vertical display mode. Four-position lever switch with the following positions: CHI: Displays the channel | signal by itself. CH2X-Y: CH2: displays the channel 2 signal by itself. X-: used in conjunction with the X-Y control and Trigger SOURCE switch to enable X-Y display mode. DUAL: Displays the channel | and channel 2 signals simul- taneously. Dual-trace mode may be either alternate or chopped sweep: see the description under HOLDOFF/PULL CHOP control.CONTROLS AND INDICATORS / i / \ \ \ \ \ i og \ 10 2 \ 21 22 gy j i 14 41 13 15 16 42 49 20 Fig. 2. Model 2120B Controls and Indicators. ADD: the X-Y mode of operation is selected, this control The inputs from channel 1 and channel 2 are provides step adjustment of X-axis sensitivity. summed and displayed as a single signal. If the : trol: Channel 2 POSition/PULL INVert control is CH VARiable/PULL X5 MAG Control: pulled out, the input from channel 2 is subtracted VARiable: from channel | and the difference is displayed as a Rotation provides vernier adjustment of channel | single signal. vertical sensitivity. In the fully-clockwise (CAL) 13. CH1 AC-GND-DC Switch. Three-position lever position, the vertical attenuator is calibrated. Coun- terclockwise rotation decreases gain sensitivity. In X-Y operation, this control becomes the vernier X-axis sensitivity control. PULL X5 MAG: switch with the following positions: AC: Channel | input signal is capacitively coupled; de component is blocked. When pulled out, increases vertical sensitivity by a GND: factor of five. Effectively provides two extra sensi- Opens signal path and grounds input to vertical tivity settings: 2 mV/div and 1 mV/div. In X-Y amplifier. This provides a zero-volt base line, the mode, increases X-sensitivity by a factor of five. position of which can be used as a reference when 17. CHI 4 POSition/PULL ALT TRIGger Control: performing dc measurements. a De: POSition: Direct coupling of channel | input signal; both ac Adjusts vertical position of channel | trace. and de components of signal produce vertical de- PULL ALT: flection. Used in conjunction with the Trigger SOURCE 14. CHI (X) Input Jack. Vertical input for channel 1. switch to activate alternate triggering. See the de- scription under the Trigger SOURCE switch. 18. CH2 V POSition/PULL INVert Control: X-axis input for X-Y operation. 15. CH1 (X) VOLTS/DIV Control. Vertical attenuator for channel 1. Provides step adjustment of vertical # POSition: sensitivity. When channel 1 VARiable control is set Adjusts vertical position of channel 2 trace. In X-Y to CAL, vertical sensitivity is calibrated in 10 steps operation, rotation adjusts vertical position of X-Y from 5 mV/div to 5 V/div in a 1-2-5 sequence. When display.CONTROLS AND INDICATORS 19, 20. 21. 22. PULL INVert: When pushed in, the polarity of the channel 2 signal is normal. When pulled out, the polarity of the channel 2 signal is reversed, thus inverting the waveform, CH2 VOLTS/DIV Control. Vertical attenuator for channel 2. Provides step adjustment of vertical sensi- tivity. When channel 2 WARiable control is set to CAL, vertical sensitivity is calibrated in 10 steps from 5 mV/div to 5 V/div in a 1-2-5 sequence. When the X-Y mode of operation is selected, this control pro- vides step adjustment of Y-axis sensitivity. CH2 VARiable/PULL X5 MAG Control: VARiable: Rotation provides vernier adjustment of channel 2 vertical sensitivity. In the fully-clockwise (CAL) position, the vertical attenuator is calibrated. Coun- terclockwise rotation decreases gain sensitivity. In X-Y operation, this control becomes the vernier Y-axis sensitivity control. PULL X5 MAG: When pulled out, increases vertical sensitivity by a factor of five. Effectively provides two extra sensi- tivity settings: 2 mV/div and | mV/div. In X-Y mode, increases Y-sensitivity by a factor of five. CH2 (Y) Input Jack. Vertical input for channel 2. Y-axis input for X-Y operation. CH2 AC-GND-DC Switch. Three-position lever switch with the following positions: AC: Channel 2 input signal is capacitively coupled; dc component is blocked. GND: Opens signal path and grounds input to vertical amplifier. This provides a zero-volt base line, the position of which can be used as a reference when performing dc measurements. DC: Direct coupling of channel 2 input signal; both ac and dc components of signal produce vertical de- flection. HORIZONTAL CONTROLS 23. 24, Main Time Base TIME/DIV Control. Provides step selection of sweep rate for the main time base. When the VARiable Sweep control is set to CAL, sweep rate is calibrated. This control has 23 steps, from 0.1 S/div to 2 S/div, in a 1-2-5 sequence. 2125A Only. DELAY Time Base TIME/DIV Con- trol. Provides step selection of sweep rate for delayed 25. 26. 27. 28. 29. sweep time base. This control has 23 steps, from 0.1 uS/div to 2 S/div, in a 1-2-5 sequence. 2125A Only. DELAY TIME POSition Control. Sets starting point of delayed sweep. Clockwise rotation causes delayed sweep to begin earlier. VARiable Sweep Control. Rotation of control is ver- nier adjustment for sweep rate. In fully clockwise (CAL) position, sweep rate is calibrated. On the Model 2125A, this control is the vernier adjustment for both the main and delayed time bases. <> pOsition/PULL X10 MAG Control. <> Position: Horizontal (X) position control. PULL X10 MAG: Selects ten times sweep magnification when pulled out, normal when pushed in. Increases maximum sweep rate to 10 nS/div. . 2125A Only. Sweep Mode Switch. Selects sweep (horizontal) mode. Four-position rotary switch with the following positions: MAIN: Only the main sweep operates, with the delayed sweep inactive. MIX: The main and delayed sweep share a single trace; main sweep occupies the left portion of the display; delayed sweep occupies the right portion of the display. The DELAY TIME POSition control de- termines the percentage of display that is main sweep and the percentage of display that is delayed sweep (main sweep is usually brighter than the delayed sweep). Delayed sweep speed cannot be slower than main sweep speed. DELAY: Only delayed sweep operates, while main sweep stays inactive. DELAY TIME POSition control determines the starting point of the delayed sweep. X-Y: Used with the VERTical MODE switch and Trig- ger SOURCE switch to select X-Y operating mode. The channel | input becomes the X-axis and the channel 2 input becomes the Y-axis. Trigger source and coupling are disabled in this mode. 2120B Only. X-Y Switch. Used with the VERTical MODE switch and Trigger SOURCE switch to se- lect X-Y operating mode. The channel | input be- comes the X-axis and the channel 2 input becomes the Y-axis. Trigger source and coupling are disabled in this mode.CONTROLS AND INDICATORS TRIGGERING CONTROLS 30. HOLDOFF/PULL CHOP Control. 31, HOLDOFF: Rotation adjusts holdoff time (trigger inhibit period beyond sweep duration). When control is rotated fully counterclockwise, the holdoff period is MIN- inum (normal). The holdoff period increases pro- gressively with clockwise rotation. PULL CHOP: When this switch is pulled out im the dual-trace mode, the channel 1 and channel 2 sweeps are chopped and displayed simultaneously (normally used at slower sweep speeds). When it is pushed in, the two sweeps are alternately displayed, one after the other (normally used at higher sweep speeds). Tri gger SOURCE Switch. Selects source of sweep trigger. Four-position lever switch with the following positions: CHYUX-Y/ALT CHi: Causes the channel | input signal to become the sweep trigger, regardless of the VERTical MODE switch setting. X-Y: . Used with two other switches to enable the X-Y mode see the Operating Instructions under XY Operation. ALT: Used with the channel 1 POSition/PULL ALTernate TRIGger control to enable alternate triggering. Alternate triggering, used in dual- trace mode, permits each waveform viewed to become its own trigger source. CH?2: The channel 2 signal becomes the sweep trigger, regardless of the VERTical MODE switch setting. LINE: Signal derived from input line voltage (50/60 Hz) becomes trigger. EXT: Signal from EXTernal TRIGger jack becomes sweep trigger. 32. Tri gger COUPLING Switch. Selects trigger cou- pling. Four-position lever switch with the following positions: AUTO: Selects automatic triggering mode. In this mode, the oscilloscope generates sweep (free runs) in absence of an adequate trigger; it automatically reverts to triggered sweep operation when an adequate trigger signal is present. On the Model 2125A, automatic triggering is applicable to both the main sweep and delayed sweep. 10 NORM: Selects normal triggered sweep operation. A sweep is generated only when an adequate trigger signal is present. TV-V: Used for triggering from television vertical sync pulses. Also serves as lo-pass/dc (high frequency reject) trigger coupling. TV-H: Used for triggering from television horizontal sync pulses. Also serves as hi-pass (low frequency reject) trigger coupling. 33. TRIGger LEVEL/PULL (-) SLOPE Control.: TRIGger LEVEL: Trigger level adjustment; determines the point on the triggering waveform where the sweep is trig- gered. Rotation in the (-) direction (counterclock- wise) selects more negative triggering point; rotation in the (+) direction (clockwise) selects more positive triggering point. - PULL () SLOPE: Two-position push-pull switch. The in position selects a positive-going slope and the out position selects a negative-going slope as triggering point for main sweep. 34. EXTernal TRIGger Jack. External trigger input for single- and duai-trace operation. REAR PANEL CONTROLS (not shown) 35. Fuse Holder/Line Voltage Selector. Contains fuse and selects line voltage. 36. Power Cord Receptacle. 37. 2125A Only. CH 2 (Y) SIGNAL OUTPUT Jack. Output terminal where sample of channel 2 signal is available. Amplitude of output is nominally 50 mV per division of vertical deflection seen on CRT when terminated into 50 Q. Output impedance is 50 Q. 38. 2125A Only. Z-Axis Input Jack. Input jack for inten- * sity modulation of CRT electron beam. TTL compat- ible (5 V p-p sensitivity). Positive levels increase intensity. 39. Handle/Tilt Stand. 40. Feet/Cord Wrap.OPERATING INSTRUCTIONS NOTE All operating instructions in this chapter EQUIPMENT PROTECTION PRECAUTIONS apply equally to both Models 2125A and 2120B, except for the sections on Delayed Sweep Operation and Component Test, which apply only to the Model 2125A. Other differences are noted when nec- essary. SAFETY PRECAUTIONS The following precautions must be ob- served to help prevent electric shock. When the oscilloscope is used to make measurements in equipment that contains high voltage, there is al- ways a certain amount of danger from electrical shock. The person using the oscilloscope in such conditions should be a qualified electronics technician or other- wise trained and qualified to work in such circum- stances. Observe the TEST INSTRUMENT SAFETY recommendations listed on the inside front cover of this manual. Do not operate this oscilloscope with the case removed unless you are a qualified service technician. High voltage up to 2100 volts is present when the unit is operating with the case removed. The ground wire of the 3-wire ac power plug places the chassis and housing of the oscilloscope at earth ground. Use only a 3-wire outlet, and do not attempt to defeat the ground wire connection or float the oscil- loscope; to do so may pose a great safety hazard. Special precautions are required to measure or observe line voltage waveforms with any oscilloscope. Use the following procedure: a. Do not connect the ground clip of the probe to either side of the line. The clip is already at earth ground and touching it to the hot side of the line may weld or disintegrate the probe tip and cause possible injury, plus possible damage to the scope or probe. b. Insert the probe tip into one side of the line voltage receptacle, then the other. One side of the recepta- cle should be hot and produce the waveform. The other side of the receptacle is the ac return and no waveform should result. The following precautions will help avoid damage to the oscilloscope. Never allow a small spot of high brilliance to remain stationary on the screen for more than a few seconds. The screen may become permanently burned. A spot will occur when the scope is set up for X-Y operation and no signal is applied. Either reduce the intensity so the spot is barely visible, apply signal, or switch back to normal sweep operation. It is also advisable to use low intensity with AUTO triggering and no signal applied for long periods. A high intensity trace at the same position could cause a line to become perma- nently burned onto the screen. Do not obstruct the ventilating holes in the case, as this will increase the scopes internal temperature. Excessive voltage applied to the input jacks may dam- age the oscilloscope, The maximum ratings of the inputs are as follows: CH 1 and CH 2: 400 V dc + ac peak. EXT TRIG: 1 300 V de + ac peak. * Z-AXIS INPUT (Model 2125A): Ll 30 V ( de and ac peak). 4. Always connect a cable from the ground terminal of the oscilloscope to the chassis of the equipment under test. Without this precaution, the entire current for the equipment under test may be drawn through the probe clip leads under certain circumstances. Such condi- tions could also pose a safety hazard, which the ground cable will prevent. The probe ground clips are at oscilloscope and earth ground and should be connected only to the earth ground or isolated common of the equipment under test. To measure with respect to any point other than the common, use CH 2 CH | subtract operation (ADD mode and INV 1), with the channel 2 probe to the point of measurement and the channel | probe to the point of reference. Use this method even if the reference point is a dc voltage with no signal.OPERATING INSTRUCTIONS OPERATING TIPS The following recommendations will help obtain the best performance from the oscilloscope. 1. Always use the probe ground clips for best results, attached to a circuit ground point near the point of measurement. Do not rely solely on an external ground wire in lieu of the probe ground clips as undesired signals may be introduced. 2. Avoid the following operating conditions: a. Direct sunlight. b. High temperature and humidity. c. Mechanical vibration. d. Electrical noise and strong magnetic fields, such as near large motors, power supplies, transformers, etc. 3. Occasionally check trace rotation, probe compensa- tion, and calibration accuracy of the oscilloscope using the procedures found in the MAINTENANCE section of this manual. 4. Terminate the output of a signal generator into its characteristic impedance to minimize ringing, espe- cially if the signal has fast edges such as square waves or pulses. For example, the typical 50 Q output of a square wave generator should be terminated into an external 50 terminating load and connected to the oscilloscope with 50 coaxial cable. 5. Probe compensation adjustment matches the probe to the input of the scope. For best results, compensation should be adjusted initially, then the same probe al- ways used with the same channel. Probe compensation should be readjusted when a probe from a different oscilloscope is used. INITIAL STARTING PROCEDURE Until you familiarize yourself with the use of all controls, the settings given here can be used as a reference point to obtain a trace on the CRT in preparation for waveform observation. 1. Set these controls as follows: On both models: VERTical MODE to CH1. CH1 AC/GND/DC to GND. Trigger COUPLING to AUTO. Trigger SOURCE to CH1. All POSition controls and INTENSITY control cen- tered (pointers facing up). Main Time Base control to 1 mS/div. On the Model 2125A: Sweep Mode switch to MAIN. 2. Press the red POWER pushbutton (Model 2120B), or rotate the POWER control clockwise away from OFF (Model 2125A). 3. A trace should appear on the CRT. Adjust the trace brightness with the INTENSITY control, and the trace sharpness with the FOCUS control. NOTE On the Model 2125A, you can use the BEAM FINDER pushbutton to locate a trace that has been moved off the screen by the POSition controls. When the button is pushed, a compressed version of the trace is brought into view which indicates the location of the trace. SINGLE TRACE DISPLAY Either channel | or channel 2 may be used for single-trace operation. To observe a waveform on channel |: 1. Perform the steps of the Initial Starting Procedure. 2. Connect the probe to the CH 1 (X) input jack. 3. Connect the probe ground clip to the chassis or com- _ mon of the equipment under test. Connect the probe tip to the point of measurement. 4. Move the CH1 AC/GND/DC switch out of the GND position to either DC or AC. 5. If no waveforms appear, increase the sensitivity by turning the CH 1 VOLTS/DIV control clockwise to a position that gives 2 to 6 divisions vertical deflection. 6. Position the waveform vertically as desired using the CH1 POSition control. 7. The display on the CRT may be unsynchronized. Refer to the Triggering paragraphs in this section for pro- cedures on setting triggering and sweep time controls to obtain a stable display showing the desired number of waveforms. DUAL TRACE DISPLAY In observing simultaneous waveforms on channel | and 2, the waveforms are usually related in frequency, or one of the waveforms is synchronized to the other, although the basic frequencies are different. To observe two such related waveforms simultaneously, perform the following: 1. Connect probes to both the CH 1 (X) and CH 2 (Y) input jacks. 2. Connect the ground clips of the probes to the chassis or common of the equipment under test. Connect the tips of the probes to the two points in the circuit where waveforms are to be measured.OPERATING INSTRUCTIONS To view both waveforms simultaneously, set the VERTical MODE switch to DUAL and select either ALT (alternate) or CHOP with the PULL CHOP switch. In the ALT sweep mode (PULL CHOP switch pushed in), one sweep displays the channel 1 signal and the next sweep displays the channel 2 signal in an alternating sequence. Alternate sweep is normally used for viewing high-frequency or high-speed wave- forms at sweep times of 1 ms/div and faster, but may be selected at any sweep time. In the CHOP sweep mode (PULL CHOP switch pulled out), the sweep is chopped (switched) between channel | and channel 2. Using CHOP, one channel does not have to wait for a complete swept display of the other channel. Therefore, portions of both chan- nels waveforms are displayed with the phase relation- ship between the two waveforms unaltered. Chop sweep is normally used for low-frequency or low- speed waveforms at sweep times of | ms/div and slower; or where the phase relationship between chan- nel | and channel 2 requires measurement. If chop sweep is used at sweep times of 0.2 ms/div and faster, the chop rate becomes a significant portion of the sweep and may become visible in the displayed waveform. However, you may select chop sweep at any sweep time for special applications. Adjust the channel 1 and 2 4 POSition controls to place the channel 1 trace above the channel 2 trace. Set the CH 1 and CH 2 VOLTS/DIV controls to a position that gives 2 to 3 divisions of vertical deflec- tion for each trace. If the display on the screen is unsynchronized, refer to the Triggering paragraphs in this section of the manual for procedures for setting triggering and sweep time controls to obtain a stable display showing the desired number of waveforms. When the VERTical MODE switch is set to ADD, the algebraic sum of CH 1 + CH 2 is displayed as a single trace. When the PULL INV switch is pulled out, the algebraic difference of CH 1 CH 2 is displayed. If two waveforms have no phase or frequency relation- ship, there is seldom reason to observe both wave- forms simultaneously. However, these oscilloscopes do permit the simultaneous viewing of two such unre- lated waveforms, using alternate triggering. Refer to the paragraphs on Triggering - Trigger SOURCE Switch, for details on alternate triggering. 13 TRIGGERING The Models 2125A and 2120B Oscilloscopes provide versatility in sync triggering for ability to obtain a stable, jitter-free display in single-trace, or dual-trace operation. The proper settings depend upon the type of waveforms being observed and the type of measurement desired. An explanation of the various controls which affect synchroni- zation is given to help you select the proper setting over a wide range of conditions. Trigger COUPLING Switch 1. In the AUTO position, automatic sweep operation is selected. In automatic sweep operation, the sweep generator free-runs to generate a sweep without a trigger signal. However, it automatically switches to triggered sweep operation if an acceptable trigger source signal is present. The AUTO position is handy when first setting up the scope to observe a waveform; it provides sweep for waveform observation until other controls can be properly set. Once the controls are set, operation is often switched back to the normal trigger- ing mode, since it is more sensitive. Automatic sweep must be used for dc measurements and signals of such low amplitude that they will not trigger the sweep. The NORM position provides normal triggered sweep operation. The sweep remains at rest until the selected trigger source signal crosses the threshold level set by the TRIG LEVEL control. The trigger causes one sweep to be generated, after which the sweep again remains at rest until triggered. In the normal triggering mode, there will be no trace unless an adequate trigger signal is present. In the ALT VERTICAL MODE of dual trace operation with the SOURCE switch also set to ALT, there will be no trace unless both channel | and channel 2 signals are adequate for triggering. Typically, signals that pro- duce even one division of vertical deflection are ade- quate for normal triggered sweep operation. The TV H and TV V. positions are primarily for viewing composite video waveforms. Horizontal sync pulses are selected as trigger when the trigger COU- PLING switch is set to the TV H position, and vertical sync pulses are selected as trigger when the trigger COUPLING switch is set to the TV V position. The TV H and TV V positions may also be used as low frequency reject and high frequency reject coupling, respectively. Additional procedures for observing video waveforms are given later in this section of the manual.OPERATING INSTRUCTIONS Trigger SOURCE Switch The trigger SOURCE switch (CH 1, CH 2, etc.) selects the signal to be used as the sync trigger. 1. If the SOURCE switch is set to CH 1 (or CH 2) the channel | (or channel 2) signal becomes the trigger source regardless of the VERTICAL MODE selec- tion. CH 1, or CH 2 are often used as the trigger source for phase or timing comparison measurements. 2. By setting the SOURCE switch to ALT (same as - CH1) and PULL ALT TRIG pulled, alternating trig- gering mode is activated. In this mode, the trigger source alternates between CH 1 and CH 2 with each sweep. This is convenient for checking amplitudes, waveshape, or waveform period measurements, and even permits simultaneous observation of two wave- forms which are not related in frequency or period. However, this setting is not suitable for phase or timing comparison measurements. For such measurements, both traces must be triggered by the same sync signal. Alternate triggering can only be used in dual-trace mode (VERT MODE set to DUAL), and with alter- nate sweep only (PULL CHOP not engaged). 3. In the LINE position, triggering is derived from the input line voltage (50/60 Hz) and the trigger SOURCE switch is disabled. This is useful for meas- urements that are related to line frequency. 4. In the EXT position, the signal applied to the EXT TRIG jack becomes the trigger source. This signal must have a timing relationship to the displayed wave- forms for a synchronized display. TRIG LEVEL/PULL (-) SLOPE Control (Refer to Fig. 3) A sweep trigger is developed when the trigger source signal crosses a preset threshold level. Rotation of the TRIG LEVEL control varies the threshold level. In the + direction (clockwise), the triggering threshold shifts to a more posi- tive value, and in the direction (counterclockwise), the triggering threshold shifts to a more negative value. When Slope - Range Slope + Range Level Fig. 3. Function of Slope and Level Controls. 14 the control is centered, the threshold level is set at the approximate average of the signal used as the triggering source. Proper adjustment of this control usually synchro- nizes the display. The TRIG LEVEL control adjusts the start of the sweep to almost any desired point on a waveform. On sine wave signals, the phase at which sweep begins is variable. Note that if the TRIG LEVEL control is rotated toward its extreme + or setting, no sweep will be developed in the normal trigger mode because the triggering threshold ex- ceeds the peak amplitude of the sync signal. When the PULL (~) SLOPE control is set to the + (in) position, the sweep is developed from the trigger source waveform as it crosses a threshold level in a positive-going direction. When the PULL (~) SLOPE control is set to the (out) position, a sweep trigger is developed from the trigger source waveform as it crosses the threshold level in a negative-going direction. MAIN TIME BASE Control Set the Main Time Base TIME/DIV control to display the desired number of cycles of the waveform. If there are too many cycles displayed for good resolution, switch to a faster sweep time. If only a line is displayed, try a slower sweep time. When the sweep time is faster than the wave- form being observed, only part of it will be displayed, which may appear as a straight line for a square wave or pulse waveform. HOLDOFF Control (Refer to Fig. 4) A holdoff period occurs immediately after the comple- tion of each sweep, and is a period during which triggering of the next sweep is inhibited. The normal holdoff period varies with sweep rate, but is adequate to assure complete retrace and stabilization before the next sweep trigger is permitted. The HOLDOFF control allows this period to be extended by a variable amount if desired. A. Holdoff not used B. Holdoff used Fig. 4. Use of HOLDOFF Control.OPERATING INSTRUCTIONS This control is usually set to the MIN position (fully counterclockwise) because no additional holdoff period is necessary. The HOLDOFF control is useful when a com- plex series of pulses appear periodically such as in Fig. 4B. Improper sync may produce a double image as in Fig. 4A. Such a display could be synchronized with the VAR SWEEP control, but this is impractical because time meas- urements are then uncalibrated. An alternate method of synchronizing the display is with the HOLDOFF control. The sweep speed remains the same, but the triggering of the next sweep is held off for the duration selected by the HOLDOFF control. Turn the HOLDOFF control clock- wise from the MIN position until the sweep starts at the same point of the waveform each time. MAGNIFIED SWEEP OPERATION Since merely shortening the sweep time to magnify a portion of an observed waveform can result in the desired portion disappearing off the screen, magnified display should be performed using magnified sweep. Using the <P POSition control, move the desired portion of waveform to the center of the CRT. Pull out the PULL X10 knob to magnify the display ten times. For this type of display the sweep time is the Main Time Base TIME/DIV control setting divided by 10. Rotation of the <P POSition control can then be used to select the desired portion of the waveforms. X-Y OPERATION X-Y operation permits the oscilloscope to perform many measurements not possible with conventional sweep opera- tion. The CRT display becomes an electronic graph of two instantaneous voltages. The display may be a direct com- parison of the two voltages such as stereoscope display of stereo signal outputs. However, the X-Y mode can be used to graph almost any dynamic characteristic if a transducer is used to change the characteristic (frequency, temperature, velocity, etc.) into a voltage. One common application is fre- quency response measurements, where the Y axis corresponds to signal amplitude and the X axis corresponds to frequency. 1. On the Model 2125A, set the SWEEP MODE switch to the XY position. On the Model 2120B, depress the X-Y switch. On both models, set the Trigger Source and VERTical MODE switches to X-Y. 2. In this mode, channel | becomes the X axis input and channel! 2 becomes the Y axis input. The X and Y positions are now adjusted using the <> POSition and the channel 2 YW POSition controls respectively. 3. Adjust the amount of vertical (Y axis) deflection with the CH 2 VOLTS/DIV and VARIABLE controls. 4. Adjust the amount of horizontal (X axis) deflection with the CH 1 VOLTS/DIV and VARIABLE con- trols. VIDEO SIGNAL OBSERVATION Setting the COUPLING switch to the TV-H or TV-V position permits selection of horizontal or vertical sync pulses for sweep triggering when viewing composite video waveforms. When the TV-H mode is selected, horizontal sync pulses are selected as triggers to permit viewing of horizontal lines of video. A sweep time of about 10 1s/div is appropriate for displaying lines of video. The VAR SWEEP control can be set to display the exact number of waveforms desired. When the TV-V mode is selected, vertical sync pulses are selected as triggers to permit viewing of vertical fields and frames of video. A sweep time of 2 ms/div is appropriate for viewing fields of video and 5 ms/div for complete frames (two interlaced fields) of video. At most points of measurement, a composite video signal is of the () polarity, that is, the sync pulses are negative and the video is positive. In this case, use (- ) SLOPE. If the waveform is taken at a circuit point where the video wave- form is inverted, the sync pulses are positive and the video is negative. In this case, use (+) SLOPE. APPLICATIONS GUIDEBOOK B+K Precision offers a Guidebook to Oscilloscopes which describes numerous applications for this instrument and important considerations about probes. It includes a glossary of oscilloscope terminology and an understanding of how oscilloscopes operate. It may be downloaded free of charge from our Web site, www.bkprecision.com. DELAYED SWEEP OPERATION (Model 2125A) (Refer to Fig. 5) 15 Delayed sweep operation is achieved by use of both the main sweep and the delayed sweep and allows any portion of a waveform to be magnified for observation. Unlike X10 magnification, delayed sweep allows selectable steps of magnification. Lt. Set the Sweep Mode switch to the MAIN position and adjust the oscilloscope for a normal display. 2. Set the Sweep Mode switch to the MIX position. The display will show the main sweep on the left portion (representing the MAIN Time Base control setting) and the delayed sweep on the right portion (repre- senting the DELAY Time Base control setting). The MAIN Time Base portion of the trace usually will be brighter than the delayed time base portion. Fig. 5 shows a typical display for the MIX display mode. 3. Shift the percentage of the display that is occupied by the main sweep by adjusting the DELAY TIME POSition control. Counterclockwise rotation causes more of the display to be occupied by the main sweepOPERATING INSTRUCTIONS Delayed Sweep 100 +4 QO f-----f-----f----- ef EC foenebeceeefec eee [a TT t . Main + 410 f-----f---- Sweep .--{-----35-----}----4------p-----fe---- 0 + Fig. 5. MIX SWEEP MODE Display. and clockwise rotation causes more of the display to be occupied by the delayed sweep. 4. Set the Sweep Mode switch to the DELAY position to display only the magnified delayed sweep portion of the display. NOTE In order to obtain meaningful results with delayed sweep, the DELAY Time Base control must set be set to a faster sweep speed than the MAIN Time Base control. Because of this, the oscilloscope automat- ically prevents (electrically) the DELAY Time Base from being set to a slower sweep speed than the MAIN Time Base. For example, if the MAIN Time Base is set to 0.1 ms/div, the slowest possible DELAY Time Base sweep speed is also 0.1 ms/div, even if the control is set slower. COMPONENT TEST OPERATION (Model 2125A) Do not apply an external voltage to the COMP TEST jacks. Only non-powered circuits should be tested with this unit. Testing powered circuits could damage the instrument and increase the risk of electrical shock. The component test function produces a component sig- nature on the CRT by applying an ac signal across the device and measuring the resulting ac current. The display represents a graph of voltage (X) versus current (Y). The 16 component test function can be used to view the signatures of resistors, capacitors, inductors, diodes, and other semi- conductor devices. Devices may be analyzed in-circuit or out-of-circuit and combinations of two or more devices may be displayed simultaneously. Each component produces a different signature and the components can be analyzed as outlined below. Component Test mode is activated by depressing the COMPonent TEST switch. The SWEEP MODE switch must not be in the DELAY position. Resistors A purely resistive impedance produces a signature that is a straight line. A short circuit produces a vertical line and an open circuit causes a horizontal line. Therefore, the higher the resistance, the closer to horizontal the trace will be. Values from 10 Q to about 5 kQ are within measurement range. Values below 10 Q will appear to be a dead short while values above 5 kQ will appear to be an open circuit. Fig. 6 shows some typical resistance signatures. To test a resistor, insert one of the resistors leads into the white COMP TEST jack, and the other into the GND jack (make sure that the leads touch the metal walls inside the jacks). To test in-circuit, a pair of test leads can be used to connect the COMP TEST and GND jacks to the compo- nent(s). Be sure to discharge capacitors (by short- ing the leads together) before connecting to the COMP TEST jack. Some capaci- tors can retain a voltage high enough to damage the instrument. Capacitors A purely capacitive impedance produces a signature that is an ellipse or circle. Value is determined by the size and shape of the ellipse. A very low capacitance causes the ellipse to flatten out horizontally and become closer to a straight horizontal line and a very high capacitance causes the ellipse to flatten out vertically and become closer to a straight vertical line. Values from about 0.33 UF to about 330 pF are within measurable range. Values below 0.33 pF will be hard to distinguish from an open circuit and values above 330 uF will be hard to distinguish from a short circuit. Fig. 7 shows several typical capacitance signatures. To test a capacitor, insert the capacitors positive lead into the white COMP TEST jack, and the negative lead into the GND jack (make sure that the leads touch the metal wails inside the jacks). To test in-circuit or to test a capacitor with leads that are too short to fit into the COMP TEST and GND jacks, a pair of test leads can be used to connect the COMP TEST and GND jacks to the component(s).OPERATING INSTRUCTIONS 100 90 f= 100 QO b----4----- feeceopenee fee eden fee }----4-----f----4 por tia Pett Ppp TIT Tyor T TT a Tepe te 10 b--.~4-----}----+-L--+- woe eP--~- 4-----b----4-----}----4 py dy Open Circuit Short Circuit 100 + 90 |----4-----fe---ben ee foee eB een denen pene fee p ee petted TT TET ttt yet 0p fe pt TTT TTT 10 Q Resistor 100 E 100 QO b----4e---- forceps bed 90 dept ph a 10 provers pore pores Bonen onneopenennnmno pone 10 200 Q Resistor 5.1 kQ Resistor Fig. 6. Typical Resistive Signatures. 17OPERATING INSTRUCTIONS 0.33 uF Capacitor io ! 90 b---- 4-----}-----b ewes fone Bee fence pene eee fe | | p----4-----[-.---b---- mee. || de ---b----4 10 t 0 + 4.7 uF Capadtor 10 : 90 b----4-----f-----p----f eo Beoe fee ff VO Prono ports prone penn grep der re pa rng 0 + 300 iF Capacitor inductors Like capacitance, a purely inductive impedance produces a signature that is an ellipse or circle and value is determined by the size and shape of the ellipse. A very high inductance causes the ellipse to flatten out horizontally and a very low inductance causes the ellipse to flatten out vertically. Values from about 0.05 H to about 5 H are within measurement range. Values below 0.05 H will be hard to distinguish from a short circuit and values above 5 H will be hard to distin- guish from an open. Fig. 8 shows several typical inductance signatures. To test an inductor, insert one of the inductors leads into the white COMP TEST jack, and the other into the GND jack (make sure that the leads touch the metal walls inside the jacks). To test in-circuit or to test an inductor with leads that are too short to be inserted into the COMP TEST and GND jacks, a pair of test leads can be used to connect the COMP TEST and GND jacks to the component(s). 1 Henry Inductor 5 Henry Inductor Fig. 7. Typical Capacitive Signatures. 18 Fig. 8. Typical Inductive Signatures.OPERATING INSTRUCTIONS Semiconductors Purely semiconductor devices (such as diodes and transis- tors) will produce signatures with straight lines and bends. Typical diode junctions produce a single bend with a hori- zontal and vertical line as shown in Fig. 9. Zener diodes produce a double bend with two vertical and one horizontal line as shown in Fig. 10 (value is determined by the distance of the leftmost vertical component from the center gradu- ation on the CRT). The maximum Zener voltage observable on this feature is about 15 V. It is also possible to test transistors and ICs by testing one pair of pins at a time. NOTE When testing diodes it is important to connect the diodes cathode to the white COMP TEST jack and the anode to the GND jack. Reversing the polarity will not damage the device but the horizontal and vertical components of the signature will appear in different quadrants of the display. To test semiconductors, insert the diodes or transistors leads (only two at a time) into the COMP TEST and GND jacks (make sure that the leads touch the metal walls inside the jacks). To test in-circuit or to test ICs or devices with leads too short to insert into the COMP TESTand GND jacks, a pair of test leads can be used to connect the COMP TEST and GND jacks to the component(s). Combinations of Components Using the component test feature it is also possible to observe the signatures of combinations of components. Combinations cause signatures that are a combination of the individual signatures for each component. For example, a signature for a resistor and capacitor in parallel will produce a signature with the ellipse of the capacitor but the resistor would cause the ellipse to be at an angle (determined by the value of the resistor). When testing combinations of compo- nents it is important to make sure that all the components being connected are within measurement range. In-Circuit Testing The component test feature can be very effective in locat- ing defective components in-circuit, especially if a known good piece of equipment is available for reference. Com- pare the signatures from the equipment under test with signatures from identical points in the reference unit. When 19 Silicon Diode Fig.9. Typical P-N Junction Signature. 10 V Zener Diode Fig. 10. Typical Zener Signature. signatures are identical or very similar, the tested component is good. When signatures are distinctively different, the tested component is probably defective.MAINTENANCE The following instructions are for use by qualified service personnel only. To avoid electrical shock, do not perform any serv- icing other than contained in the operat- ing instructions unless you are qualified to do so. High voltage up to 2000 V is present when covers are removed and the unit is oper- ating. Remember that high voltage may be retained indefinitely on high voltage capacitors. Also remember that ac line voltage is present on line voltage input circuits any time the instrument is plugged into an ac outlet, even if turned off. Unplug the oscilloscope and dis- charge high voltage capacitors before performing service procedures. FUSE REPLACEMENT If the fuse blows, the ON indicator will not light and the oscilloscope will not operate. The fuse should not normally open unless a problem has developed in the unit. Try to determine and correct the cause of the blown fuse, then replace only with the correct value fuse. For 110/125 V line voltage operation, use an 800 mA, 250 V fuse. For 220/240 V line voltage operation, use a 600 mA, 250 V fuse. The fuse is located on the rear panel adjacent to the power cord receptacle. Remove the fuseholder assembly as follows: 1. Unplug the power cord from rear of scope. 2. Insert a small screwdriver in fuseholder slot (located between fuseholder and receptacle). Pry fusehoider away from receptacle. 3. When reinstalling fuseholder, be sure that the fuse is installed so that the correct line voltage is selected (see LINE VOLTAGE SELECTION). LINE VOLTAGE SELECTION To select the desired line voltage, simply insert the fuse and fuse holder so that the appropriate voltage is pointed to by the arrow. Be sure to use the proper value fuse (see label on rear panel). 20 PERIODIC ADJUSTMENTS Probe compensation and trace rotation adjustments should be checked periodically and adjusted if required. These procedures are given below. Probe Compensation 1. Connect probes to CH 1 and CH 2 input jacks. Per- form procedure for each probe, one probe at a time. 2. Set the probe to X10 (compensation adjustment is not possible in the X1 position). Touch tip of probe to CAL terminal. 4. Adjust oscilloscope controls to display 3 or 4 cycles of CAL square wave at 5 or 6 divisions amplitude. 5. Adjust compensation trimmer on probe for optimum square wave (minimum overshoot, rounding off, and tilt). Refer to Fig. 11. Correct Compensation Over Compensation Insufficient Compensation Fig. 11. Probe Compensation Adjustment. Trace Rotation Adjustment 1. Set oscilloscope controls for a single trace display in CH 1 mode, and with the channel 1 AC-GND-DC switch set to GND. 2. Use the channel 1 POSition control to position the trace over the center horizontal line on the graticule scale. The trace should be exactly parallel with the horizontal line. 3. Usethe TRACE ROTATION adjustment on the front panel to eliminate any trace tilt.MAINTENANCE CALIBRATION CHECK A general check of calibration accuracy may be made by displaying the output of the CAL terminal on the screen. This terminal provides a square wave of 2 V p-p. This signal should produce a displayed waveform amplitude of four divisions at .5 V/div sensitivity for both channel | and 2 (with probes set for direct). With probes set for X10, there should be four divisions amplitude at 50 mV/div sensitivity. The VARIABLE controls must be set to CAL during this check. NOTE The CAL signal should be used only as a general check of calibration accuracy, not as a signal source for performing recali- bration adjustments; a voltage standard calibrated at several steps and of 0.3% or better accuracy is required for calibration adjustments. The CAL signal should not be used as a time base standard. 21 INSTRUMENT REPAIR SERVICE Because of the specialized skills and test equipment re- quired for instrument repair and calibration, many custom- ers prefer to rely upon B+K Precision for this service. To use this service, even if the oscilloscope is no longer under warranty, follow the instructions given in the SERVICE INFORMATION portion of this manual. There is a flat rate charge for instruments out of warranty.APPENDIX IMPORTANT CONSIDERATIONS FOR RISE TIME AND FALL TIME MEASUREMENTS Error in Observed Measurement The observed rise time (or fall time) as seen on the CRT is actually the cascaded rise time of the pusle being meas- ured and the oscilloscopes own risetime. The two rise times are combined in square law addition as follows: Tobserved = V (Toutse y + (Tscope ? The effect of the oscilloscopes rise time is almost negli- gible when its rise time is at least 3 times as fast as that of the pulse being measured. Thus, slower rise times may be measured directly from the CRT. However, for faster rise time pulses, an error is introduced that increases progres- sively as the pulse rise time approaches that of the oscillo- scope. Accurate measurements can still be obtained by calculation as described below. Direct Measurements The Models 2125A and 2120B oscilloscopes have a rated rise time of 12 ns. Thus, pulse rise times of about 36ns or greater can be measured directly. Most fast rise times are measured at the fastest sweep speed and using X10 magni- fication. For the Models 2125A and 2120B, this sweep rate is 10 ns/div. A rise time of less than about four divisions at this sweep speed should be calculated. 22 Calculated Measurements For observed rise times of less than 36 ns, the pulse rise time should be caluclated to eliminate the error introduced by the cascaded oscilloscope rise time. Calculate pulse rise time as follows: 2 2 Tpulse = (Tobserved ) + (T scope ) Limits of Measurement Measurements of pulse rise times that are faster than the scopes rated rise time are not recommended because a very small reading error introduces significant error into the calculation. This limit is reached when the observed rise time is about 1.3 times greater than the scopes rated rise time, about 16 ns minimum for the Models 2125A and 2120B. Probe Considerations For fast rise time measurements which approach the limits of measurement, direct connection via 50 Q coaxial cable and 50 Q termination is recommended where possible. When a probe is used, its rise time is also cascaded in square law addition. Thus the probe rating should be considerably faster than the oscilloscope if it is to be disregarded in the measurement.