Hello Paul,


It's an HP 8754A (4 to 1300Mhz) and luckily old enough to be reasonably
serviceable and repairable!
This is just as well since there is a fault with the rectangular trace,
which just appears to 'flatline' with nothing visible other than the
sweep + markers. So no nice bod plots of filters, I'm afraid. However,
the polar and phase displays seem absolutely fine. If I can just get
that one fault fixed, it'll be a great piece of test gear to have
around. I've never worked on anything this complicated before, though,
so it'll be something of a challenge!


That looks like a problem in the digital section. Just hope that none of
the EPROMs suffered a memory loss. I am not sure whether they'd report a
checksum error in the 8754. I had found one that had become unseated on
another analyzer and the instrument produced only garbage output but
reported nothing on self-test. That disappointed me a bit. The only
reason I found the cause was that I opened it and looked at it long enough.

Complicated they are. However, while on the troubleshooting trail in my
HP4191 I sometimes came upon a large conglomeration of discretes and
banged my head, asking myself "Why didn't they do that with a uA733?".
And on the directional bridge they seem to have left about 10dB of
dynamic range on the table for the test channel versus the ref channel.
I'd have put a wee amp before the sampler.

Regards, Joerg

http://www.analogconsultants.com

Thanks, Joerg.
I think this item pre-dates a lot of this digital stuff; I doubt
there's a single eprom in it - and no custom chips, either!
It was built in about 1980 and doesn't go through any self-testing
routines. One has to carry out manual tests oneself. This wonderful
simplicity should hopefully enable me to keep it running indefinitely,
once I can get it fixed. Serviceability is one of my main criteria when
choosing an item of test equipment.

Hello Paul,


I think this item pre-dates a lot of this digital stuff; I doubt
there's a single eprom in it - and no custom chips, either!
It was built in about 1980 and doesn't go through any self-testing
routines. One has to carry out manual tests oneself. This wonderful
simplicity should hopefully enable me to keep it running indefinitely,
once I can get it fixed. Serviceability is one of my main criteria when
choosing an item of test equipment.


The HP4191A is from that era as well. However, it is one heck of a
complicated machine with a computer board as large as a family-size
pizza. Lots of PROMs and stuff :-(

Regards, Joerg

http://www.analogconsultants.com

Hi Joerg,
Well I'm sorry to hear that. As an analogue guy, I don't much care for
PROMS and stuff. I can fix up anything analogue in good time, given a
schematic and a decent range of test equipment, but am off my territory
with digital stuff. But I'm still not sure what the problem is here.
Are you saying spares for the digital parts are hard to obtain or
rather that the encoding on those chips may be hard to reprogram?

Hello Paul,


Well I'm sorry to hear that. As an analogue guy, I don't much care for
PROMS and stuff. I can fix up anything analogue in good time, given a
schematic and a decent range of test equipment, but am off my territory
with digital stuff. But I'm still not sure what the problem is here.
Are you saying spares for the digital parts are hard to obtain or
rather that the encoding on those chips may be hard to reprogram?


Not sure yet what is broken. The analog sampling stuff and all seems to
work but is reads out garbage. The PROMs are really old style. They look
posh with gold plating and all but I guess that non-programmed spares
would be next to impossible to find. But again, maybe they are still ok.
Hopefully...

Regards, Joerg

http://www.analogconsultants.com