Re: How to troubleshoot a faulty 7B92A?


Dan G
 

On Sat, Mar 20, 2021 at 04:19 PM, Thierry Delaitre wrote:

Regarding pin 7 and the TRIG'D light, the light is off when the scope gets
switched-on (with AUTO, AC, INT). The TRIG'D light switches on if i change the
mode to NORM. The light stays on if I revert the mode back to INT.
This is definitely abnormal. When switching from AUTO to NORM, the TRIG'D
light should flash briefly, but then go out. This suggests that the sweep timing
capacitors did not charge, or that sweep end detection circuits are not working.
It also explains why the hold-off and auto ramps are stuck low.

As Mark suggested, if you have not already done so, make sure all mechanical
switches, especially S490, operate correctly.

I would also look at pins AV (Aux Gate signal) and AR (Main Gate signal)
to see whether the plug-in thinks it has started a sweep or not. These
are normally square pulses, but yours are probably stuck. Whether they
are stuck high or low may provide a clue as to what is wrong.

Pin 10 remains at a constant DC high voltage and Pin 8 remains at a constant
DC low voltage. both pin 10 and 8 do not show any visible waveform
This now makes sense given your TRIG'D explanation above.
While this is not what these signals should look like, I think it is the result
of problems earlier in the sequence of events. They may be ok once
the plug-in is able to complete the sweep ramp.

I use a scope probe on the same scope to troubleshoot.
Sounds good.


dan

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