Electronics > Repair

Signal Measurement in Op Amp Amplifier circuit

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rodd:
[attachimg=1][attach=2]Dear all,
I am trying to troubleshoot the circuit below.
I can detect a staircase signal from pin 6 of U350B up to pin 3 of U380A, as expected.
However, I can't detect any signal at U380A output, pin 1 or the base of the Darlington transistors ( I am using a scope for that).
What I can't understand is that the signal appears again at either transistor collectors.
why this happens?
This has happened before in similar circuits, so my understanding of this topology is nonexistent and I decided to ask for help.
Thank you in advance,
Roger
 
 [attachimg=1]

CounterEMF:
So what exactly is the problem (or are you just trying to understand the circuit)? By the looks of it I'm assuming this is the step generator output for something like an old Tek curve tracer. Since this circuit works in both current and voltage modes you have to pay attention to probe reference. For example have you measured across R381 rather than from there to common? Is there an output at P429? Have you verified power supply voltages and both relay and E/I switch operation? I guess I really need to know what you're looking for (troubleshooting help or topology theory).

rodd:
Thank you for your reply CounterEMF.

You are right. This is the step generator circuit of a TEK 577 curve tracer I am trying to fix.

I have two objectives with this question.
 1 - Understand the circuit
      I have tried to follow signals in circuits with this topology: one op amp followed by two transistors in "push pull" and it never "works" - No signal at the op amp output. Why?

  2- Fix the strange behavior of this particular circuit is showing
     There is a "bump" on the staircase signal at the output of Q386.

  I will post a photo of the signal as soon as I fix the power supply that was damaged due to a short that happened while trying to to some measurements

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