Datron 4700 Calibrator Scope Waveforms For Reference.

[TheDefpom]

Hi All,
I just fixed my Datron 4700 again today, the issue I had was on the reference assembly PCB.

I grabbed various screen shots from the scope at various stages and different test points, I thought I would share them here in case they help someone else that is trying to diagnose problems on their reference PCB in the future.

Test points used in the pictures are TP4, TP5, TP3, LK8 and TP12, I also have some on the collectors of Q19 and Q20, refer to the image names to know which is which.

I listed the test points in the channel order.

[TheDefpom]

I was originally thrown by the flip flop output at TP5 being inverted, that was a symptom not a cause (pic 3), this was due to the reference voltage being shorted out and that resulted in the flip flop turning on both halves of the flip flop at the same time, as you can see in the attached image, which shows Q25 and Q27 base's both turning on at the same time ! (pic 1)

I traced this down by (unfortunately) cutting a track that feeds the reference to the flip flops and injecting my own supply to isolate the reference, that instantly corrected the flip flop fault, but the units fault was still present.

I then traced the fault back to the voltages on the collectors of Q19 and Q20, which were very different from each other, despite being mirrored circuits ! it turnout out that Q20 wasn't fully turning on to output -11VDC, and so it was never turning the JFETs off, this is what was shorting out the reference. (pic 1)