Originally posted in December, 2023

Background

The Tektronix CG511/5001 Calibration Generator is a source of calibrated timing signals, voltage and current waveforms, and pulses with low distortion/fast edges. The device can be used to calibrate and check oscilloscope performance characteristics. It has six basic modes and can be remotely programmed via the GPIB interface. It was first produced in 1980 as part of the Tektronix TM5000 instrument family.

A CG5001 unit acquired in 2019 failed with the following errors on power-up self-test:

• 51
• 52
• 54
• 55
• 81
• 83
• 85
• 87
• 17
• Uncal

• Errors “51,52,54, 55” are voltage mode self-test errors.
• Errors “81,83,85,87” are positive or negative edge mode errors.
• Error “17” means that the internal calibration constants have been lost.
• Error “Uncal” indicates a current or voltage overload.

Repair Process

Visual inspection of the unit showed several leaking and open filter capacitors in the power supply:

CG5001 A5 Failed PS OEM Capacitors. Evidence of previous repairs?

These electrolytic capacitors were all replaced:

CG5001 A5 PS Board with new capacitors. Axial lead capacitors of the required value, physical size and rating were not available, so radial capacitors were adapted to fit

In addition, the electrolytic capacitors on the isolator board (A5A1) were replaced:

Continuing with the initial testing, I found that many of the low value carbon composition resistors in the unit had drifted up in value and were out of tolerance. These were replaced as below:

With the power supply stabilized and suspect components replaced, I could now diagnose the error codes shown on power up. The troubleshooting charts in the service manual (V2, Part# 070-2815-01) were very useful in finding the problem. Walking through the troubleshooting charts and eliminating the obvious eventually led me to a pair of failed DFET transistors on the A6 reference board:

These repairs cleared all the power on faults except for the Error “17”. As part of the diagnostic process, I had removed and checked the RAM chips, which erased the calibration data. This resulted in Error “17”. However, I knew that recalibration would be needed in any case, so this was anticipated.

As part of the diagnostic and repair process, I needed to operate various circuit boards outside the instrument. This meant I had to use extender cables:

Conclusion

This CG5001 unit is now repaired and ready to be calibrated. Fortunately, the calibration process can be completed easily with the equipment I have available. I will cover this process in another post.