Finding datasheets and service manuals is a nightmare. So many websites claiming to have the right file, only to end up being a scam and not having any files. Having files they have no right to and are publicly available behind a pay wall. Having weird online viewers instead of just giving the file. Padding the file with extra pages of nonsense so they can claim more pages and a larger file size. Having the wrong file mislabeled. Etc. It goes on and on. And then there’s the sites that redirect a thousand times and then crash the browser. I hope I didn’t just get a virus or something.
All I want is to fix this old CRT from 1981 so I can enjoy it for a few more years, is that too much to ask? And back in those days they actually cared about repairability. Especially the services manuals with scope traces for the test points save so much time troubleshooting.
Archive.org is a good place luckily. If it isn’t down because shit heads can’t behave on the internet. As a species we really like to get in our own way all of the time.
And no I still haven’t fixed up that CRT. It is working now after replacing two weirdly behaving transistors, a new power cord and a new power button (old one worked but didn’t stay on unless you held it on). Replaced a few caps but most tested fine, good quality caps. Even the once I replaced were working, but marginal on the ESR. Cosmetics are also good, but there is still an intermittent fault with it losing horizontal size adjustment. It goes from fine and perfectly working to a little too narrow without any adjustment. 90% of the time it’s fine, 10% of the time it’s faulty and it switches random. I’ve been going mad tracing where the issue is, but I will fix it one day.
Pretty sad that a technical manual for the monitor was most likely created but just not digitized in a way you can find. Intermittent faults are very hard to diagnose. What I would try:
- I assume you’ve cleaned the potentiometer, which is also easy to check with an ohmmeter (mind the polarity or desolder it to protect the rest of the circuit).
- Poke the circuit board with a non-conductive object to find loose solder joints or components with bad contacts inside. If the fault is not mechanical, it might be an overheating component.
- Try adding a fan temporarily to see if the fault appears later, or use a thermal camera to find semiconductors that might go near their threshold temperature (150 °C for the silicon die).
- Find points in the horizontal deflection circuit where voltage or waveform changes as the fault manifests.
Yeah the pot is fine, that was the first suspect. Cleaned it and even thought of replacing it, but it measures just fine.
I’ve found several versions of the service manual and combined them to get the info I want. Both seem to be parts of a larger manual, which I can’t find.
I’ve been probing and scoping for hours over the past months. But when the CRT is apart that’s kinda hard, since it has a lot of high voltage I want to avoid. No worries, I have the tools and the experience to work with these things. Over my professional and hobby lifetime I’ve fixed over a hundred CRTs and worked on/designed/built hundreds of other electronics.
I took out a couple of transistors (this thing is almost all discrete components, no integrated stuff) in the horizontal deflection path and tested them and they seem fine. I hooked them up to a function gen and a scope and tested them within parameters. That’s how I found and replaced two other transistors that were dodgy and bringing the thing back to life to start with (it had fully collapsed vertically).
I checked every component in the horizontal deflection path and they all seem fine. And since the thing works most of the time, I suspect they are fine. It might be mechanical, but I’ve tapped all around and that does nothing. If the issue shows up, it stays like that for a while and then randomly disappears again. Only for it to randomly come back.
I’m pretty sure it isn’t a thermal issue, it’s a small CRT which doesn’t use a lot of power and doesn’t really get hot. The issue also appears and goes away randomly. And with the parts open on my bench it still happens. I’ve blasted the entire horizontal path with hot air in case of some cracked solder or something like that. But it still happens.
I suspect it’s actually one of the other circuits that throws off the horizontal deflection. Probably something shorting or close to somewhere. The weird thing is, it doesn’t collapse, it just loses horizontal adjustment. It’s as if the pot is bad, but the pot is perfectly fine. I’ll have to get out my notebook, print out the schematic and start drawing it out. That usually does the trick to get my brain to figure out what the actual issue is. Somehow even after using computers for so long, I need to revert back to how I learnt it in school back in the days to fully engage my brain. Computers probably are too easy and make me lazy.
Well, you have a lot more experience than I do. I don’t think I can provide good advice but the idea of another circuit borking it is interesting, I’d put voltmeter(s) on the supply voltage(s) of the deflection circuit and check for changes.
Please don’t call the unit “CRT”, it’s weird to read phrases like “when the CRT is apart” because you can’t really make a tube work again if it’s been apart.
However, it makes me think if one could smash all tubes in a vacuum tube TV in the vacuum of space or one big glass chamber and have it still working. Or build a monitor into a tube that looks like an overly long CRT but just needs power and video. Maybe even include an IR remote receiver for digital picture adjustment. This is way beyond what I will ever be able to do, though, and there is little reason to make this gimmicky thing that is worse than Philco Predicta (TV model line with CRTs outside cabinets) in almost every way.
I assume datasheet websites just have pages for every combination of 3-12 alphanumerics to appear in search results, and then use shitty fuzzy string matching tactics to find “most relevant” items. It would help if they managed to extract package marking codes from datasheets so you can find SOT-23 parts by their 2-3 character codes, or whatever obscure system used by individual IC manufacturers. I think they have resources to make the experience way better but they prefer to turn high profits. Personally, I would not mind trying AI (not neccessarily a LLM) for the data extraction but I’d be cautious and only release it if it is decently reliable (but I know they wouldn’t bother with that).
