Hello Gary,
I am facing same issue on A8 CPU board, where I can find the Roms data for HP8903A?
Would you consider to share the Roms image?
Last week, got a non-working HP8903A from local seller,
before plug in, checked the Main capacitors, C02 (for -15V) was short.
replace C02/03 with new same type,
feed-in power through VARIAC, the unit start to respond around 80~90VAC,
once over 90VAC, it shut down.
seems the U2 -15V regulator does not work, the output increase when main AC increased.
then triggered the over voltage protection.
since difficult to find a TO3 replacement locally in short time, i tried to mod a TO220 L7915 into TO3 socket. ugly but works,
now the unit can output correct voltage (+5V/+12V/+15V/-15V) within VAC 85~120V
Now I can run the unit happily,
the oscillator seems within spec. low distortion and correct output voltage checked by Panasonic VP7722a
feeding OSC to input BNC, gave THD 0.003~4% (1kHz,2V) , seems not so bad.
then I tried to setup the GPIB connection to PC by Pete Millett's software, (NI USB GPIB)
through hours of trial, still can't get connections.
then suddenly, HP8903A was frozen, no response to any of keypad.
after reboot, only strange pattern shown on left LED section (similar to Gary's descriptions)
tried to reset, not work,
checked the voltage at A8 (+5V/+12V are good), Tantalum caps are checked OK by ESR meter.
what I can guessed is one of those ROMs might damaged after long time no powered.......
any comments and showing the location of HP8903A Roms would be super appreciated.
Thanks again.
Katdesu
2016kat211@gmail.com