HP 4957A Puzzle - Page 1
Forum section: electronics / testgear

author: tpolecat
author_rank: Newbie
timestamp: March 25, 2025, 09:02:07 pm
content: 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 thatthe CRT does not show half-intensity video, which is a usability hassle because the smart key labels are shown at half-intensity. Interestingly this signaldoesshow 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

author: ZGoode
author_rank: Frequent Contributor
timestamp: March 25, 2025, 09:43:41 pm
content: 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

author: kbrill
author_rank: Contributor
timestamp: March 25, 2025, 10:04:17 pm
content: I have available a 4957a manual labeled operating and serviceWould have to dig it out of storageIf interested make modest offer & shipping

author: tpolecat
author_rank: Newbie
timestamp: March 25, 2025, 10:14:53 pm
content: 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!

author: kbrill
author_rank: Contributor
timestamp: March 25, 2025, 10:32:02 pm
content: Located manualBad news I only have the operating and reference manual , not an operating and service manual no schematics or repair infoin excellent shape except crease and soil on coverEven worse news my 4952a manual is only an operating manualAdvise if I can be of any assistanceKen

author: kbrill
author_rank: Contributor
timestamp: March 25, 2025, 10:49:51 pm
content: Have PDF's of the operating and reference manual & the data and communications test libraryIf 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 .

author: tpolecat
author_rank: Newbie
timestamp: March 25, 2025, 10:53:52 pm
content: Aha ok, thanks. I have the Op & Ref manual already.Thank you for looking. I appreciate it.

author: edavid
author_rank: Super Contributor
timestamp: March 25, 2025, 11:29:02 pm
content: 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

author: tpolecat
author_rank: Newbie
timestamp: March 28, 2025, 12:33:24 am
content: 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.