HP 8568B not rendering image properly - Page 1
Forum section: electronics / repair

author: alexturner
author_rank: Contributor
timestamp: October 03, 2021, 11:09:48 am
content: Yesterday I acquired my very first SA and to say I'm excited is a gross understatement. None the less, the past 24 hours have been spent explaining to those who don't want to know and/or don't care what an SA is, what it is, and why this 30 year old piece of equipment was worth as much as a couple of business class domestic flights to one of the state's we're not permitted to visit (from Sydney).The unit is ex ADF and they did a wonderful number on it - they installed an C14 cable dangling out the back with a "115VAC for Fan Only", and sure enough it was only connected to the fan (and only the fan).None the less on delivery, 240V (on the 240V input, mind) let the smoke out which appeared to be a couple of capacitors that had failed - an easy enough replacement given stock in the shed and a handy bargaining point. After tidying it up and cleaning out some of the 20+ year old schmoo inside, it powers on and this time the smoke stays in. However the CRT is a bit janky and it looks as if the signal (the _video_??) isn't being drawn on the CRT.I'm still in my 20's and haven't had to opportunity to be electrocuted by CRT voltages yet - so we're in uncharted territory. What do we think this drawing/rendering problem could be and where should I be looking? Power supply? XY board? What are your thoughts? I should mention that startup doesn't appear reliable however it appears to be related to temperature on the PSU section at the moment given the absence of a fam (I know, I know) - "Inst Check I"Video for effect and perhaps to better explain what 'janky' means. Also some pictures because, well, why not, this thing is beautiful.https://youtu.be/llmjsGeZKuYPics:https://imgur.com/a/MeaxN8k

author: Kerlin
author_rank: Regular Contributor
timestamp: October 03, 2021, 11:42:33 am
content: The basic CRT and its voltages appear to be ok.There is a problem with the image generation. It looks like the video generator or the processor that is running it is continually resetting.Also it could be a problem with the screen memory board, check and clean its board connector.It's supposed to have a start up message, check the manual to see what it is.Because of its high cost its probably been used every day for decades.Had a pile of this stuff from back in the days when I repaired it professionally, threw it all out in the end.Fix one thing another thing goes wrong, past its use by date.

author: alexturner
author_rank: Contributor
timestamp: October 04, 2021, 11:53:10 am
content: The analysis continues - I'm currently debugging the X/Y deflection which has been interesting. I'm able to accurately centre the dot on the monitor which makes me think the deflection amplifiers are ok. Running some of the tests however yield some interesting stats, for example in the image you'll note both X and Y deflection when drawing the test pattern.Not sure if it's related or not, but the video signal (the spectrum?) doesn't appear to be drawing at all - I'm thinking it may make sense to fix this drawing problem first then revert to that.

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 04, 2021, 12:24:45 pm
content: Did the Spectrum analyzer show the proper image before shipping?

author: Kerlin
author_rank: Regular Contributor
timestamp: October 05, 2021, 09:38:12 am
content: Now that is a very very good question and a good diagnostic approach.Try to get an answer to that from seller, see if they can provide you a pic.It's the main key to this fault.

author: Johnny10
author_rank: Frequent Contributor
timestamp: October 05, 2021, 11:33:20 am
content: My second question would be have double checked the interconnect cables haven't been crushed during shippingand you have cleaned and properly seated those cables?I have worked on a couple of these units where re-seating cables has fixed display problems.

author: Runco990
author_rank: Frequent Contributor
timestamp: October 06, 2021, 04:49:45 pm
content: I have 2 of these analyzers. The cables get flaky, so cleaning and proper seating is important. Also, re-seat all the boards whenever these things get moved around a lot.These are the 2 things you SHOULD do before commencing deeper trouble shooting.Although the video almost looks like a power problem??? Did you check how much AC is on the DC rails???I don't recall from memory what pins to jump to put the display into it's diagnostic mode, but I'd start with that to isolate the issue further. It will run stand alone.

author: alexturner
author_rank: Contributor
timestamp: November 03, 2021, 10:45:34 am
content: Thanks for the input guys. After digging around a bit, it turns out that it was one of the larger capacitors powering the display section (47uF @ 200V or something to that effect). She works now and has a wonderfully sharp screen. Turns out that this was part of an EMI detector with Quasi Peak detector and I've since acquired all of the books and floppy's for it.