34401a front panel reset

[Andrejka]

Hello everyone,

Another lucky owner of non working 34401a;) Popular problem - no display. VFD shows all segments at power on, then goes off. Shift can enable/disable "shift "mark.
What I inspected, power voltages are ok without significant ripples, U553 / LM2925T reset line is toggling at 7khz rate, so maybe micro trying to boot, but unsuccessfully. Now I checking communication with front panel, and there is no communcation via serial bus.
Can anyone explain how front panel reset works?

Several scoping points.


 IGFPRES signal without FP (referenced to AGND)


1. IGFPRES signal with  FP connected (referenced to AGND)


2. after bypassing C601 (-18V referenced)


3. at Q601 base (-18V referenced)


4. Q601 collector  (-18V referenced)



Schematic


Why IGFPRES signal drops significantly after connecting FP?
At  Q601 collector is always active high level, and why I not see any reset pulses from main CPU?

P.S. FP CPU looks like working, there is DISP_SCLK pulses and keyboard scanning also works.

Thanks Andrej

[Samogon]

I would check if central CPU is working

[picburner]

I would try to connect to the IGFPRES line a 6,8Kohm resistor as load without connecting the FP and I would watch what happens with the oscilloscope.
If the signal collapses as in the second picture there could be a problem on the main CPU side.
Otherwise I would check C601, CR601 and Q601.

[Andrejka]

Thanks. Will check.