Troubleshooting sampling scopes

Sampling scopes are quite different from regular analog scopes. When something is wrong with a sampling scope, the instrument is often completely unusable, which makes repairing it more difficult. Following these steps can make the repair process more manageable. These steps apply to Tektronix modular sampling scopes such as the 661, 567, and the other 560-series scopes fitted with sampling vertical and horizontal plug-ins. If you need to do work on plug-ins, extenders are highly desirable.

First, make sure the power supply is working properly. Power up the scope with the plug-ins installed and check all of the power supply voltages for DC voltage and for ripple. If there is ripple, there is a good chance that the power supply electrolytic capacitors need replacing. You can attach another equivalent in parallel with the suspect capacitor. If that fixes the ripple, replace the capacitor. If not, there are other problems. Don't connect capacitors while the scope is on. Be careful of polarity an voltage ratings. Be safe and think before turning on the power. When all of the power supply voltages meet the DC voltage and ripple specifications given in the manual, this first step is complete.

The second step is to get the timing (horizontal) unit working. The best technique here is to apply a pulse train (100 mV, 1 MHz, for example) waveform to the external trigger input of the timing unit while observing its outputs on a second oscilloscope.

Some timing plug-ins such as the 3T77 provide a trigger pulse output on the front panel. Other plug-ins such as the 5T1A and 5T3 do not provide this signal on the front panel, so for those you have to probe the signal inside the plug-in. The timing plug-in is supposed to produce strobe pulses that are sent to the vertical sampling plug-in. If the timing plug-in isn't producing strobe pulses, typically you won't see anything on the screen. If it is producing strobe pulses, but they are not correct, you will get garbage on the screen. The two most common problems are that the timing plug-in either doesn't produce and strobe pulses, or the strobe pulses are not synchronous with the trigger signal (free-running). There should be some range of settings of the trigger sensitivity control where the plug-in produces strobe pulses when the trigger signal is applied and does not produce strobe pulses when the trigger signal is removed. When you have the timing plug-in producing proper strobe pulses, check the blanking pulse and horizontal deflection outputs.

The third step is to get the vertical sampling unit working. Now simultaneously apply the 100 mV, 1 MHz squarewave to the signal input of the sampling unit and to the external trigger input of the timing unit. You can use a BNC T-adapter. It will corrupt the signal integrity, but at this stage that doesn't matter. Confirm that the strobe pulses are getting to the sampling plug-in.

The fourth step (if the scope is a 567 or 568/230) is to get the digital unit working.