HP 83480A (aka 54750A) not booting

[Miek]

I recently bought an HP 83480A apparently in working condition. Upon receiving it I booted it a few times and ran some basic tests, and it all seemed to be working fine. Then I went to boot it again that evening, and only got this on the screen:


(ignore the hint of the UI in the background, it's just the screen burn-in - the only thing being drawn is the red junk & a purple-ish background)

I've been through the service manual troubleshooting steps: I've reseated everything, checked the voltages and they're OK, voltage on the RAM battery looks OK (3.05V unloaded, ~2.9V with a couple hundred ohms load), and I checked the signal going to the CRT and it matches what's on screen so I think the CRT is fine.

The other thing I notice is that there's no floppy disk activity or any response from the front panel when it's in this state, so it seems like the firmware is getting hung up somewhere.

If I flip the DIP switch to force a firmware upgrade on boot, it does seem to go through the process: it reads from the floppy disk for a while, thinks for a bit, then takes the flash Vpp high for a while before turning off the disk light. So I think the boot ROM is present and working, but it just goes back to the same junk on screen after that.

Then yesterday I reassembled it after debugging session and it magically started booting, albeit with some lingering weird patterns on some of the screen. I went through the self tests and they all passed except for the "video board decrementer" test. I also tried booting it without the video board connected, which didn't work before. It worked this time and I could query the device over GPIB. So I'm suspicious that the video card test fail could be a red herring, or a separate issue.

Alas, this morning it's back to the original symptom of not booting at all. So, I'm wondering if anyone else has seen anything like this? Or maybe has ideas on what parts might be likely to fail and cause intermittent symptoms like this? It's a challenge to debug since the cards need to be installed in the frame to run and there's no probing access, so I'm hoping to narrow down what to check.

[Musent]

Hello Miek

Just some late information if you not successful to repair this HP product.
I had similar problem and it run again with a good battery with a voltage near 3.3V.
May be You have the same issue ...
BR
Bruno

[Gerhard_dk4xp]

Weird. I had set this "thread" on notify because I had a 54750,
but in working state. Last autumn it ceased booting. It turned out
it was the 3.3V lithium battery on the CPU card. After replacement
it insisted into a mainframe calibration, which was easy.
It just takes a signal generator.

Gerhard