Damaged HP 9810A

[Tony Duell]

(04-27-2014 01:56 PM)douze-bis Wrote:  Thanks a lot Tony !
I was secretely hoping you would be around

FWIW, I don't mind e-mail (or PMs here) from people repairing old HP desktops :-)

(04-27-2014 01:56 PM)douze-bis Wrote:  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.

Yes I am pretty sure that's the IC. You have the old display board (with separate display devices for each digit, the later one has 3 display modules for each row, those modules are like the ones used in 'Classic' handhelds, but with the pins formed differently), but the circuitry is much the same. If you trace the tracks from the IC pins they seem to go to the right places. And 3 of the gates are even labelled 'X', 'Y', 'Z' in the PCB etch :-). Those are the register display enables.

(04-27-2014 01:56 PM)douze-bis Wrote:  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.

Another repair project for you, then :-)

(04-27-2014 01:56 PM)douze-bis Wrote:  ---
If 7404 eventually proves dead, do you know whether and where I can supply such components nowadays ?

I can't believe they are hard to get. In London (England), I would try Cricklewood Electronics first. I think a 74LS04 or even a 74HCT04 would work, but try to get the original if you can.

(04-27-2014 01:56 PM)douze-bis Wrote:  ---
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 ?

It's probably a lot more than you need, but it will work. 16 channels of logic analyser is a bit on the small side, though.

I use a LogicDart most of the time, that has 3 channels. But I also have larger benchtop analysers with up to 40 channels. You need that if you need to look at the address and data lines in an HP9800 at the same time :-).

Even a simple logic probe -- a thing with LEDs that show 'high', 'low' or 'pulsing' is useful for the display testing. You are really just looking to see if signals are changing at all..

-tony