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