HP 4957A Puzzle

[tpolecat]

Hi everyone, this is my first post here.

Bit of a puzzle. For me, anyway. I have an HP 4957A that functions perfectly, with the exception that the CRT does not show half-intensity video, which is a usability hassle because the smart key labels are shown at half-intensity. Interestingly this signal does show up on an attached external monitor:



I have turned every potentiometer with no improvement. Even at the highest contrast/brightness there is no sign of the half-intensity signal. It's just not there. Edit: Actually I misremembered. There is no apparent way to control brightness or contrast. The only pots are focus, vsize, and vpos.

I have also tried both the A0001 and A0002 ROM sets, with no evident change in behavior.

There doesn't seem to be a service manual out there anywhere and I'm really not sure where to look next. Any suggestions would be greatly appreciated. Thanks!

- rob

[ZGoode]

Try digging through the manual for the HP 4952A, they are incredibly similar devices.  The 4952A is the predecessor, so I would not be surprised if there is a lot of reused designs within your machine

[kbrill]

I have available a 4957a manual labeled operating and service
Would have to dig it out of storage

If interested make modest offer & shipping

[tpolecat]

Hi kbrill, yes, I will buy the manual. $20 + shipping? You should be able to mail me directly and we can figure out details. Thanks!

[kbrill]

Located manual


Bad news I only have the operating and reference manual , not an operating and service manual  no schematics or repair info 

in excellent shape except crease and soil on cover

Even worse news my 4952a manual is only an operating manual

Advise if I can be of any assistance

Ken 

[kbrill]

Have PDF's  of the operating and reference manual  & the data and communications test library

If you leave me a message providing your Email address I can forward them to you they exceed the file size I can upload to this site .

 

[tpolecat]

Aha ok, thanks. I have the Op & Ref manual already.

Thank you for looking. I appreciate it.

[edavid]

You could look at the 4951C service manual on this page and see if it's any help:

https://www.curiousmarc.com/instruments/hp-4957a-protocol-analyzer

[tpolecat]

Ok, so looking at the 4951C schematics was helpful, because it showed that the high and low intensity signals are distinct and get mixed together differently for onboard and external video. So once I started tracing it down I noticed that an unpopulated part in the low-intensity video amplifier was actually a broken-off part, and I guessed it was a Zener diode like its high-intensity neighbor. Hacking a new one in fixed the problem. Thanks for you help and encouragement, all.