Damaged HP 9810A

[douze-bis]

Thanks a lot Tony !
I was secretely hoping you would be around
Indeed, it looks like the high bit digit selection and x/y/z enablers are stuck together.

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I could not find the 7404 inverter on the display PCB. Is it the one identified on the attached picture ? It has the marking 1820-0174 on the picture.

I took the machine apart, including the motherboard. I removed the dust. All boards look clean.

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I'm really out of luck... My old scope died on me yesterday...
Without a scope, I still tried a few experiments with a little piece of paper : "cutting" some electrical connections at the edge of the display PCB.

Here are my findings:
a- DO(3) disconnected : all three lines stay black
b- DO(4), DO(5) or DO(6) disconnected : 3 identical lines, the 7 right hand side digits stay black
c- DO(7) disconnected : 3 identical lines, all dots showing, the 7 right hand side digits stay black

Does this confirm 7404 as the likely suspect ?

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I did not try to force the DO(x) to 0V or to 5V. I was afraid I might damage some components.
Would it actually be safe to do such forcing ?
Would it likely tell me anything ? (like if I force DO(4) to 5V, maybe the x register line will stay black ?

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If 7404 eventually proves dead, do you know whether and where I can supply such components nowadays ?

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OK. I need a logic probe if I am to do any further testing. I have found item 281303705304 on TAS : "PC USB 20MHz 2CH Oscilloscope 16Channels Logic Analyzer Data logger 3in1 MDSO-LA".
Do you think this would be useful ?

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Well that's a lot of questions.
Again, thanks a lot for your help,

Frederic