TRS-80 M1 Monitor rejuvenation - Page 1
Forum section: electronics / general

author: DrG
author_rank: Super Contributor
timestamp: December 06, 2021, 10:05:34 pm
content: So, I have decided to 'set up' my old TRS-80 and now I have to go through and clean up everything so that it can be safely evaluated. I am reasonably comfortable with the motherboard and probably as comfortable with the expansion interface. Less so with the disk drives, but not completely uncomfortable. The monitor is another matter. These were all working in about 1982-1984 (as I recall) and were stored (not necessarily under ideal conditions) since then.I am aware thatthese monitors represent a high voltage hazard. I respect this hazard and I am not interested in exploring the boundaries of this hazard.To cut to the chase, I want some hand holding.I have opened up the monitor - see pics. I am looking for obvious problems like burned portions, burst capacitors, water damage and the like. I am not seeing that at first look over.Next I have to clean. I plan on using air spray and at a distance since it is quite cold. Then a general bath with non-residue cleaner. Are there areas that I should completely avoid?One question concerns the vertical and horizontal holds (back) and the brightness and contrast pots (front). Do I uses a healthy amount of cleaner there as is my inclination? I may have some tuner spray which has a lubricant - I have stayed away from that in the past and am inclined to continue to do so - right?.I want to get to the point where I can plug it in - much like this fellowalthough he did not have a lot to say about it.Do you see anything in the pics of concern? Do you have some general advice/guidance? Why did I schlepp this stuff around for so many years when more normal people would have introduced them to a dumpster at some point? OK, forget that last question

author: james_s
author_rank: Super Contributor
timestamp: December 07, 2021, 12:30:03 am
content: I see nothing alarming in the photos, it's a bog standard mid-70s monochrome CRT display. The lack of an insulator over the anode terminal is unusual but it doesn't appear to have been messed with. What is wrong with the monitor? If it will turn on, then a photo showing the symptoms would help a lot for diagnosis. For cleaning I would just take it outside and blow it out with compressed air, it doesn't even look particularly dirty, just dusty. There's no need to have it sparkling clean, the HV will attract dust in no time anyway.

author: DrG
author_rank: Super Contributor
timestamp: December 07, 2021, 05:45:28 am
content: Quote from: james_s on December 07, 2021, 12:30:03 amI don't know and did not say anything about having a problem with the monitor. I will find out if it turns on after so many years now that I have it cleaned up - will try powering up tomorrow. Thanks.

author: james_s
author_rank: Super Contributor
timestamp: December 07, 2021, 06:07:24 am
content: Just power it on, no reason to mess with it ahead of time as long as it doesn't have a dead rodent or something inside it. It's always good to have a baseline before you do anything anyway.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 07, 2021, 06:22:06 pm
content: It's a "hot chassis" design, so assume mains is everywhere if you power it up with the back off with no isolation transformer.Don't touch the pots if they don't prove problematic.I'd be a bit wary of that big can capacitor by the flyback, they do occasionally pop.Goes without saying but stay far away from that exposed anode connection. I generally leave them alone, but that's one I might discharge and keep a shorting jumper on before working in there.

author: floobydust
author_rank: Super Contributor
timestamp: December 07, 2021, 07:39:29 pm
content: I would not go overboard cleaning it, you can bump into things and cause damage. Don't hit the CRT neck OK? I think compressed air would be good enough.The large aluminum can electrolytic is surely dead, they dry out quickly. I would test it. The one AGC fuse element looks melted, the one in a clip holder.The CRT missing the HV anode cap is odd, usually a bad idea. The corona discharge and hiss would be pretty loud. It might have been rubber, disintegrated and fallen off. I would find a replacement. Note that the HV rectifier is selenium and in-line, inside all that heat-shrink tubing. That whole assembly is very fragile, if you are going in.

author: DrG
author_rank: Super Contributor
timestamp: December 07, 2021, 09:35:45 pm
content: Thanks much for the responses, I appreciate the effort and I read them carefully.I was simply not going to “just turn it on”. I moved carefully and purposefully and only after doing some reading. This is not an area where I have much experience or knowledge and, hence, the hand holding.I especially appreciate the cautions. After schlepping this for so long, the last thing I wanted to do is ruin it by being careless….no wait, the last thing I wanted to do was fry myselfAfter disassembly (the previous pics), I went through an extensive examination of all of it – from the mains plug and cable to everything else. While I didn’t go overboard on cleaning, I did liberally spray the two pots. I have had some experience with them on stored equipment and they are crud collectors. On this point, I admit, I could have done them later, but why go back in if I don’t need to?I was extremely careful not to knock into any parts, but did spray down the electronics that I knew about and did carefully remove dust. No rats inside, dead or otherwise. No burn spots, no excess remnants of stuff - no signs of damage at all. The fuses are intact and there is no evidence of leakage on the big cap (or any others).Finally, I reached the point where I was comfortable applying power. I left the back on, but not screwed in. After powering up, I adjusted brightness and contrast knobs to get an overly bright screen. Turning them back down, I left the power on for a good 15 min and there was no sign (or sound) of a problem. It works…so far.There is no video signal coming in so, while encouraging, I realize that this is not a definitive test. I have a second monitor to do and then on to a CPU unit.Thanks again.

author: james_s
author_rank: Super Contributor
timestamp: December 07, 2021, 09:41:56 pm
content: I've repaired many hundreds of CRT monitors over the past 25 years or so, I've always just given a cursory glance and then plugged it in to see what happens, so far so good. It's not rocket science, especially the old monochrome ones like this. The most important thing is that the flyback transformer and CRT are functional, either of those two parts is going to be difficult to find these days. That monitor looks like it's working perfectly fine, nice clean raster filling the entire screen, no retrace lines, no smoke or sparks, I think you're good to go.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 07, 2021, 10:08:47 pm
content: Looks promising, if none of the caps are running hot you may just go ahead and use it as is. I have a similar vintage TV that I've run extensively on all original parts and it hums along fine.

author: james_s
author_rank: Super Contributor
timestamp: December 07, 2021, 10:12:27 pm
content: A lot of times the original parts are fine. Many times I've seen someone try to "bulletproof" something by going through it and replacing a bunch of original parts, only to end up replacing old high quality stuff with cheap no-name Chinese junk that soon fails. In general I advocate leaving things alone unless a part is defective in some way, unless it's a case where a particular part is known to have a high failure rate or cause damage to other hard to find parts if it fails.

author: BrokenYugo
author_rank: Super Contributor
timestamp: December 07, 2021, 10:25:28 pm
content: Yeah, at this point I don't consider touching anything working well unless perhaps it's life/revenue critical (i.e. failure has some real cost attached) or a known weak point that will lead to additional damage (e.g. early SMD electrolytic caps). Even if you do spec out good parts and do good work replacing them there are sometimes traps encountered when forcing modern parts into ancient designs. I'm even beginning to get wary of shotgunning equipment as a "while I'm in there", change the bad parts and GTFO.

author: DrG
author_rank: Super Contributor
timestamp: December 10, 2021, 04:09:38 pm
content: I'm sorry, but why did I want to do this again?The first CPU has a likely flaky/stuck bit 0 of video RAM [Z48 if you are keeping score]. I did order a couple of 2102s, but I am rethinking this whole project, at least at this time. Brittle plastic. Non-working keys - general electronic malaise.The second CPU is not in as good shape.On the plus side, there is so much documentation on this machine that I am relatively certain I can fix it with enough hours and a little cash. Perhaps, however, some reassembly, and new boxing and storage for another day is in order. We'll see.

author: helius
author_rank: Super Contributor
timestamp: December 11, 2021, 02:32:22 am
content: A CRT tube has a gray paint on it called the aquadag. You should only use mild cleaners such as soap and water (and not too much water!) to avoid damaging this layer.

author: james_s
author_rank: Super Contributor
timestamp: December 11, 2021, 04:07:39 am
content: On a lot of them it's quite durable, I have washed numerous CRTs in the dishwasher, but then I did it once on a 19" monitor and it washed off the aquadag! Thankfully that tube turned out to be the fault with that monitor. I had another where it was stored in a humid environment and the stuff flaked off, I got some graphite paint sold for shielding electric guitar bodies and fixed it with that.