Sizzling Model 1 monitor

[syzygy]

First off: I KNOW there are high voltages inside a monitor and there are safety precautions that must be taken very seriously. I also KNOW that even having been unplugged for a while, capacitors can maintain a charge and give you quite a shock - again safety procedures need to be followed.

One of my TRS-80 Model 1 monitors started "sizzling". By that, I mean "sizzling" like bacon. I opened it up, fully expecting to see a capacitor on its last legs....but I didn't.

Otherwise, the monitor works just fine.

This video illustrates what it sounds like (you may need to turn your volume way up

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This video illustrates a milder and intermittent sizzling (more like popping). "

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I cleaned it up, being careful not to bump into things, using canned air. The sizzling and popping seems to have subsided...but I don't know for how long.

I did some reading on the issue and there is a lot out there, but I have very little experience with monitors.



The sound, as best as I can tell, is coming from 'C' or 'B'. I inspected all of the capacitors and none show signs of leakage or inflation.
I know what 'C' is and I can replace that capacitor and, if I do, I will discharge it first. Should I do that?

What is the correct terminology for 'B'? - including the covering and glue.

What is the correct terminology for 'A'?

Although the sound is reminiscent of, what I would call "arcing', I see no such visual activity of that nature - especially at 'A' where is turned the lights off and looked.

Should the connection at 'A' be insulated? Was it common for cheap monitors in the 70s-80s to not do that?

At this point, I am inclined to just put the cover on and carry on. My recollection is that TVs of old would 'sizzle' a bit and it was no big deal. At its worst, in this case, I thought it was excessive. But, again, it appears to have subsided as I have been turning it on every hour or two and hear nothing (and I see the beam contraction on the screen when I turn the power off).

Any sincere education and advice is welcome.

 

[Maikudou]

A is “anode cap”, usually it’s a literal suction cup, my little knowledge suggests it should be there to prevent arching, let’s wait for Hugo to expert opinion

 

[Bruce Tomlin]

B is the flyback transformer. A and B are the dangerous bits, and the CRT itself has capacitance which can zap you hard. A is usually covered with a rubber cap, I can see the outline of the rubber cap, but it looks a bit naked at the connector thingy, like it broke and fell off somehow.

Not sure what C is, but looks like some kind of can capacitor.

 

[Maikudou]

Other photos inside that monitor suggest they did not bother to have a cup, weird

 

[syzygy]

Other photos inside that monitor suggest they did not bother to have a cup, weird

Likely more cheap than weird. AFAIK, they were essentially RCA TVs with as much stuff as they decided they didn't need.

 

[syzygy]

B is the flyback transformer. A and B are the dangerous bits, and the CRT itself has capacitance which can zap you hard. A is usually covered with a rubber cap, I can see the outline of the rubber cap, but it looks a bit naked at the connector thingy, like it broke and fell off somehow.

Not sure what C is, but looks like some kind of can capacitor.

I knew what C was - an electrolytic capacitor.


There is a Sam's on the monitor. I didn't realize that I had it until very recently. I'm not sure I can find a replacement, maybe I can...but at this point, I'm not sure there is a need to do so. It does not get hot, look damaged or leaking.

As @Maikudou noted, and I agree...I don't think there ever was an insulated cup on this. I have two monitors and the other one looks the same. The Sam's pic is very dark, but there doesn't seem to be a cup there either. Maybe on the TV version, it was there, but not on the monitor.

Right now, I am not sure I will do anything else. A little sizzling appears to come with the territory I guess [famous last words].

With more guessing than anything else, I don't think the missing cup is where it is arcing, if that is what I am hearing. I think it is in the flyback, at least that is what it sounds like. Wouldn't I be able to actually see the arcing?

I hate working inside TVs...I think they are booby traps

 

[Bruce Tomlin]

One silly suggestion is to turn off the lights in the room to see if there are any sparks to go along with the sizzling.

 

[syzygy]

One silly suggestion is to turn off the lights in the room to see if there are any sparks to go along with the sizzling.

Not to be too critical because I can be pretty verbose sometimes, and often too lazy to proof read, but from the 1st post...

Although the sound is reminiscent of, what I would call "arcing', I see no such visual activity of that nature - especially at 'A' where is turned the lights off and looked.

 

[Maikudou]

If I was very curious and had two identical or very similar shematic monitors like you do, I would start to switch components one by one until the issue is transplanted from unit to unit.

 

[mark0x01]

The sizzle is common for CRT's, usually caused by a combination of dust/dirt build up and moisture.
The static charge attracts the particles, and when a bit of moisture builds up, sizzle, which lessens when it has run for a while and dried out.

Discharge from point A to the metal band on the CRT and then give it a clean.
Isopropol on a cloth, clean the red area point A, and maybe the cable and the top of the transformer - B as well. Usually a black residue comes off.
Don't overdo the cleaning around A on the CRT and there is a conductive coating that will come off.

 

[tech58761]

The long cylinder between the flyback and the CRT anode button is a high-voltage diode. That coating mentioned is because the CRT envelope doubles as the filter capacitor for the flyback circuit.

 

[hornbetw]

Service Manual for your particular revision of Model I monitor.

Tom

 

[NeXT]

I'm not sure I can find a replacement, maybe I can...but at this point, I'm not sure there is a need to do so. It does not get hot, look damaged or leaking.

Those are multi-stage capacitors. They are in fact multiple capacitors in one can sharing a common negative. Note how the printing on the can shows different shapes which match the holes cut around each of the terminals. These were incredibly common in TV's at the time. You can replace them with individual lytic capacitors. Likewise the weird values. It's not unheard of to fudge the capacitance a little bit. In tube sets I've wandered as far off as 25% of the specified value and it's fine.

A word of caution about these sets. While the cheap-out of not using an anode cup has its own dangers, this is what is called a "hot chassis" set. If you intend to ever poke at this with an oscilloscope you will either need an isolation transformer or isolate the ground on the scope.

 

[Maikudou]

Service manual tells the opposite though, I don’t know anything in this area, just mentioning the discrepancy

 

[retrogear]

One way to check a filter capacitor is using a DMM. Set the DMM to AC volts and while the set is running measure AC readings from the ground can to each terminal of the cap.
If you get readings over 1V AC or so then that cap section could be open. You are measuring the ripple voltage which the cap is supposed to remove.

Larry G

 

[NeXT]

Service manual tells the opposite though, I don’t know anything in this area, just mentioning the discrepancy

In a nutshell, one side of the unpolarized AC plug is the ground for the entire TV chassis. If the plug has been inserted "upside down", when you attach a scope's signal ground to the chassis (or you use something like a test pattern generator that has a ground tap for shielding) it's a direct short across line/neutral and things go explody. Since (usually) AC neutral is bonded to ground in all modern American homes, it's asking you to measure the voltage between chassis to ground (either it's going to be 120v or nearly nothing) before attaching any test leads or grounded probes.

Normally I also replace those multi-stage capacitors but in this instance they have nothing in relation to your HV corona. As others have said it just needs the area around the anode and flyback cleaned.