Thanks for the details Pat. I wouldn't dream of fixing these complex beasts without the service manual, that's for sure...
Now that you have the adequate extender board you will be able to work much more comfortably but from what you say indeed it looks like it's a done deal sadly
I am starting to get the impression that this particular model is well known, and not just in the US, so maybe at some point, as people collect them, there will be enough people affected by this problem to warrant a collective effort in designing a replacement. This RF stuff is a specialist job of course but maybe there are enough collective knowledge in this forum or out on the web, to allow one to embark into designing a replacement...
That would be a nice little project for RF design aficionados ! Maybe we could ask Shariar/ The Signal Path, and Alan / w2aew what they think of it from a design perspective, if it's feasible or not, realistically... that could make for an interesting video for them to do !
I think I might contact them about it... both being in the US, I guess it would not cost too much for someone in the US, owing on of these machines, to ship it to them (along with the much required extended card of course, and a copy of the printed service manual if not available in PDF form for them to download). They have the academic and practical knowledge to tackle this, or at least provide us with a well informed insight, as well as the expensive equipment required to play with this thing at will.
Hell... the two of them know each other, why not combine their expertise in a joint video ?! A co-produced video... I guess they live too far apart from each other to actually meet physically, the US being a vast place, but they could still communicate via e-mail to discuss this.
I just downloaded a copy of the service manual... sadly 99% of it is impossible to read, the PDF is massively corrupted somehow. I could only figure out a few snippets here and there, but OMG I see that this instrument came with GPIB ?!!
And the manual is date 1975 ! Sure, HP invented this bus so I understand that they were the very first to implement it on their instruments, before other manufacturers adopted it, but still, 1975 ?!! There was not even an 8 bit micro to run such a thing back then, I think ! Had a quick look on Google to refresh my memory, found an article from 1974 announcing a super duper 4 bit MCU...
The PDF was really too fucked up so I couldn't see how they did it... my assumption is that they did not use an 8 micro which did not even exist probably, or elese they might have used it to control the front panel and give their instrument a competitive edge in terms of user interface.... so my guess is that they designed their own stand-alone bus controller, a big state machine, which would not necessitate a micro ti run it ? I am intrigued !
Also saw that this beast is really complex, found a snippet about a PLL ?!
That machine was indeed a marvel of engineering compared to my French nixie counters or the same era !
I want one of those, definitely, working or not !!!