PET 8032 - CRT Help

[cbmeeks]

OK, so I decided to take a closer look at my PET 8032. From a previous thread, I mentioned that I was using it and I heard a sizzle sound and saw some smoke. The picture didn't turn off immediately but I quickly turned it off via the power strip.

Today I opened up the CRT back to look at the analog board. I was hoping to see a blown capacitor that could easily be replaced. But I found nothing of the sort. In fact, nothing appears to be wrong with it on the outside.

Which makes me think the problem is internal to the CRT itself.

I have attached some pictures below.

I haven't attempted to power it up. I'm pretty nervous about the high voltages.

Any ideas on what it could be? I've tried to find a local technician that works on CRT's but have had no luck.

Thanks.

[cbmeeks]

**EDIT**

I just noticed this. It's the only thing that sort of looks damaged. Not sure what it is and I don't see any debris around it. But it may have slid off the board when I was carrying the computer.

[GARTHWILSON]

It looks like it could be a potted inductor with the potting scraped on the top. If that's the case, it's probably not related to the burning smell.

[cbmeeks]

GARTHWILSON wrote:

It looks like it could be a potted inductor with the potting scraped on the top. If that's the case, it's probably not related to the burning smell.

Shoot. I really wished I could run down to my local electronics store and buy a new CRT. lol

[Dr Jefyll]

cbmeeks wrote:

I heard a sizzle sound and saw some smoke. The picture didn't turn off immediately but I quickly turned it off via the power strip. [...]

I haven't attempted to power it up. I'm pretty nervous about the high voltages.

Well, you need to collect more information, and IMO it's time to power the machine up. So, what steps are necessary to make you comfortable with that? (You shouldn't proceed if you're not comfortable.)

Shock hazard is easy to manage. Simply don't touch the thing. Use the power strip (and an extension cord, if you like) to power it up -- that way you can keep your distance.

Safety glasses would be a good idea, as there's a slight chance you'll get more sizzle or even a pop. And work in a well ventilated area if you're concerned about toxicity of any smoke. I think that about covers it regarding hazards. Did I miss anything?

BTW I doubt the CRT could have produced the smoke you observed, as it's a sealed unit (nothing outside the glass to smoke).

To be honest, it won't surprise me if you find the machine to be in reasonably good working order. Not every part is absolutely essential -- there are quite a few which could fail without bringing the machine down. Your comment above suggests that may be what happened.

let us know how you make out!

-- Jeff

[cbmeeks]

Dr Jefyll wrote:

cbmeeks wrote:

I heard a sizzle sound and saw some smoke. The picture didn't turn off immediately but I quickly turned it off via the power strip. [...]

I haven't attempted to power it up. I'm pretty nervous about the high voltages.

Well, you need to collect more information, and IMO it's time to power the machine up. So, what steps are necessary to make you comfortable with that? (You shouldn't proceed if you're not comfortable.)

Shock hazard is easy to manage. Simply don't touch the thing. Use the power strip (and an extension cord, if you like) to power it up -- that way you can keep your distance.

Safety glasses would be a good idea, as there's a slight chance you'll get more sizzle or even a pop. And work in a well ventilated area if you're concerned about toxicity of any smoke. I think that about covers it regarding hazards. Did I miss anything?

BTW I doubt the CRT could have produced the smoke you observed, as it's a sealed, metal-and-glass unit.

To be honest, it won't surprise me if you find the machine to be in reasonably good working order. Not every part is absolutely essential -- there are quite a few which could fail without bringing the machine down. Your comment above suggests that's indeed what happened.

let us know how you make out!

-- Jeff

Those are some very good points.

I'm usually pretty safe with electronics so I'm probably being a drama queen. It's late tonight but I have a long weekend coming and I might do that. In fact, I may power the machine up outside this weekend so that I don't stink up my house again. lol

Now that you mentioned it, the CRT is sealed so the smoke shouldn't have come from that. That's a good idea!

OK, so assuming I power this on and nothing explodes or catches fire, then what? If it works as normal should I just package it up and continue using it?

Obviously, I will never leave this thing un-attended. But any advice on repairing and keeping safe?

Thanks.

[Dr Jefyll]

cbmeeks wrote:

OK, so assuming I power this on and nothing explodes or catches fire, then what?

I don't have a master plan -- I'm just hoping for more information.

If the machine appears to work then have a closer look for symptoms. For example, take a look at the video display to see if its size and position are normal, and whether there's any distortion apparent. Or maybe there's something else you'll notice. But you have to turn the power on first!

[floobydust]

In general, when troubleshooting old gear without a schematic, some things to note;

1- electrolytic caps get leaky or go open over time and cause many problems, even if they don't look bad. Leaky ones can get warm or hot as well.
2- plug-in connections oxidize over time, so un-plugging them, cleaning them and re-plugging them is a good idea.
3- heat sink compound dries out over time, so a good thermal contact is lost, remove the heat sinks, clean up the surfaces and apply fresh compound and reassemble. NOTE: in some cases, plastic cased transistors (like the TO-220 pieces with the simple alloy piece sinks) can burn and give off a smell without being that visible. They might also be toast afterwards.
4- cold solder joints simply happen over the decades... look at the underside of the PCB for any bad looking solder joints. Retouch as needed.

There's also the possibility that the sizzle and smoke came from somewhere else, perhaps the power supply board? Helps to check all of the PCBs for any visible damage. Also, if something did smoke, the sniff test (literally getting close enough to the PCB to sniff it) might actually allow you to zero in on a component that still has a burnt smell.

Nothing in the pics look obvious, but I've not yet seen a CRT that smokes and sizzles. If you look at the underside of the neck around 8 o'clock, it looks like the getter is still visible. It's usually a grayish or silver color on the inside of the glass (cleaning the neck with some windex will reveal it). If the CRT looses vacuum, the getter goes chalky white and of course the CRT is dead. If you can see the filaments in the neck glow orange when lit, you still have vacuum in the CRT.

Granted, no silver bullet in all of this but some hints and suggestions on troubleshooting it.

[whartung]

Isn't there a way to safely discharge this thing? Isn't the high voltage risk in some large capacitor someplace?

Then, there's this: http://lowendmac.com/2007/the-truth-abo ... ck-danger/

Take that with what you paid for it. IANATVRM I am not a TV Repair Man.

[BigEd]

There's a resistor on the RHS of the board with a black patch - Rtwenty-something - just above the legend Q76something.
See this photo: 6 49 17 PM

To discharge a tube, make a solid ground to the chassis, and then, using a screwdriver and wearing rubber gloves, and rubber boots, and with any spare hands in your pockets, dig under the sucker which attaches to the side of the tube to connect ground to the little metal clip inside it - without popping it off.

(I haven't done this, although I did once investigate the popper and I think I even punctured the tube by hitting a screwdriver through the popper's socket. That was destructive and dangerous, but I was young and inquisitive.)

[ttlworks]

It's been more than 20 years that I had been into TV repairs,
and I'm not familiar with the PET monitors.

Commodore 8032 tech manual, monitor is on page 56:
http://www.zimmers.net/anonftp/pub/cbm/ ... Manual.pdf

;---

Nothing obvious to see in the pictures...
but IMHO a "sizzle sound" has something to do with high voltage.

That diode split flyback transformer...
Please check, if a pin of that transformer has a bad soldering joint at the bottom of the PCB.

A defective diode split transformer could make such a "sizzle sound".
Hmm... there is a big transistor (2SC681A), Q721, that seems to drive this transformer.

If you would disconnect the collector of this transistor (unfortunately collector is the casing of the transistor),
power up the PET, and there would be no "sizzle sound", chances are good that the transformer might be defective...
...but don't ask me from where to get a transformer that might fit as a spare part.

[DavidL]

BigEd wrote:

There's a resistor on the RHS of the board with a black patch - Rtwenty-something - just above the legend Q76something.
See this photo: 6 49 17 PM
...

I believe that's just a shadow. I'm much more concerned about the electrolytics, especially the largest one.

[BigEd]

I know electrolytics fail, but do they fail by smoking? (X1 capacitors certainly can, but I didn't see any of those in the photos - they are large and rectangular.)

[floobydust]

BigEd wrote:

I know electrolytics fail, but do they fail by smoking? (X1 capacitors certainly can, but I didn't see any of those in the photos - they are large and rectangular.)

Over the decades, I've seen many fail. In general, the electrical failure ranges from losing capacitance to going open or becoming leaky up to a near short. As for physical failures, I've seen zero indications (usually an open cap) or in some cases the guts spewing out the top (which is why they have scribe marks in the case tops which are uncovered, i.e., these are cheaper caps). I've also heard them pop, usually preceded by a sizzle type sound or spew electrolyte out the vent cap (Mallory CGS failures). On rare occasions I've seen a few explode, but that's generally a minor prototyping error involving reverse polarity or too high a voltage In some cases they just get very hot (leaky). A sizzling sounds usually precedes either the big pop or electrolyte coming out somewhere. I've even seen some cheap ones where the electrolyte started coming out the bottom rubber plug at the leads, which became corroded and you could just pull it out by hand.

In any case, all are worth checking for age related issues.

[BigEd]

Maybe if there was a sizzle but nothing can be seen, it could be a cap which leaked at the bottom instead of the top. A bigger cap is more likely to hide its leaked electrolyte?