I've ordered all new electrolytic capacitors for my PET 8032. I'm going to recap the digital PCB when they come in (I'm not going to recap the analog board to the CRT at the moment).

The one capacitor I haven't ordered yet is that gigantic screw terminal cap that connects between the transformer and the PCB. I don't have it in front of me at the moment but according to the schematic I found online, it's 23,000 uF. It's the size of a small flashlight (lol). Anyway, I looked up a similar cap (guessing around 25V) on Mouser and they range from $23 to almost $60 for one. Yikes.

I already spent $30 on the other caps (with a few extras) as I prefer to buy Panasonic or Nichicon caps.

So, my question is, how risky is it leaving that giant cap where it sits and just using it (assuming it still works)? Keep in mind I don't plan on using my PET very much. As I just want to clean it up a bit and use it for a few days. It will live most of its life in a climate controlled basement.

If it were to leak, I could see it destroying the metal case it sits on but the PCB is elevated slightly. Unless it were to explode, I think the PCB would be safe.

What are your suggestions?

Thanks

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Cat; the other white meat.

Certainly I'd check it visually for any bulging or corrosion.

Many of the old screw type caps are of the Mallory CGS or Sprague 36DX types (other brands have their own similar series). In general these are very long life caps and intended for commercial grade power supplies. I've never seen one with a bulging case, mainly because they have a vent plug on the terminal mounting disc. I have seen them go open, short and spew out electrolyte from the vent plug before. It would be a good idea to get the full spec from the cap first. They do have common case sizes which specify the case dimensions and mounting distance. And yes... they are expensive these days.

As it is a screw type, you can likely remove it without much difficulty and test it for leakage. If you have a higher function DMM (that handles capacitance) you can check for capacity as well. If it checks out okay and the vent plug looks clean, it's probably fine.

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Regards, KM
https://github.com/floobydust

Can you just leave the whole thing alone? If the computer's not broken, I mean, and the caps seem OK visually. To me it seems there are better uses for a person's time than this recapping stuff -- I never quite understood it. I'll grant that there's probably a feeling of accomplishment after the job is done; maybe that explains it. Myself, I prefer to wait until the thing breaks. Heck, it's not life-support equipment! We can tolerate some down time should a failure eventually does occur -- which it very well may not.

Just my $.02

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

I own nearly 70 vintage computers. Many of them I have are either duplicates or don't have many electrolytic caps all over the place.

Some of my computers are now very expensive and I only have one. Such as my PET 8032.

I'm not one of these people that recap just to recap. But I have seen the horrors of leaked caps and it's not just a matter of "down time". It's a matter of electrolyte leaking all over the board and destroying copper traces. The PET I have is one of my oldest computers. Many IC's in it have a mid-82 date stamp so that puts the caps around 35 years old.

So I felt it was in my best interest to recap some of my computers. Especially this one.

I probably won't replace that 23,000 uF cap as I cannot justify the cost and I think it's far enough away that if it leaks, it won't harm anything other than the sheet metal base it sits on.

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Cat; the other white meat.

In the case of X1 caps which take mains voltage, they fail with a cloud of smelly smoke, so standard practice in Acorn land is to replace those preemptively.

But this isn't that, so I agree.

Except, if you're planning to store a machine, and leakage might cause corrosion, wouldn't it make sense to replace? We see problems with battery leakage.

I don't know from a few pics I took the other day that the cap isn't bulging. In fact, none of them are. There are only 11 electrolytic caps on the PCB and it's all through-hole so it shouldn't be an issue for me to recap. The one I fear the most is the 4700uF cap that's about the size of a C cell battery. It looks fine but there are no vent holes on it that I can see. I fear it might actually go bang. lol

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Cat; the other white meat.

I guess it depends on how much you value your time. Recapping is a nontrivial job (and not without expense, nor risk of creating new trouble). And we balance that against the odds that a cap may fail -- and go unnoticed -- after having first survived 35 years and remaining functional and visually OK?

But, as I've said elsewhere, because this is a hobby we get to set our own goals, and we are perfectly free to define what we consider fun and worth doing!

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In 1988 my 65C02 got six new registers and 44 new full-speed instructions!
https://laughtonelectronics.com/Arcana/ ... mmary.html

This is contradictory to me - you fear electrolytic corrosion AND your heart belongs to this special one BUT you cannot justify the costs ??

Even if you have a fixed monthly hobby budget - then you have to gather some $ over some month and then buy a replacement.


my 2 cents


BTW: 22000µF/25V/105°C instead of 23000µ is much cheaper - ca. 13 $ Nichicon (Mouser: 647-LNT1E223MSE) or ca. 10 $ EPCOS/TDK

Those big electrolytics are very long-lived in my experience.

BTW, I'll second Jeff's comments on the capacitor replacement project. If it ain't broke don't fix it!

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x86?  We ain't got no x86.  We don't NEED no stinking x86!

I wasn't very clear. I fear corrosion on the PCB with the caps that are on it. Hence, I am going to replace them.

As for the large 23k cap, I fear corrosion/damage for the sheet metal base that it sits on. Not so much about the PCB. Plus, I plan on isolating it more when I put it in storage.

The PCB would be expensive and time-consuming to repair or replace. Some sheet metal, not so much.

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Cat; the other white meat.

If the capacitor has not been charged up in years, you can restore / reform it by removing it from the circuit if necessary and bringing the voltage up very slowly. I don't remember if this is supposed to take several minutes, or hours, but you can probably look it up. I seem to remember that the recommended way is to bring it up slowly to, say, 25% of its rated voltage, discharge it, repeat but go somewhat higher, repeat, etc,, until you get to at least as high as it will be operated at.

All the big ones I've seen have a hole with a rubber plug at the contact end, so if pressure builds up, the plug gets pushed out, rather than the whole thing exploding. When I worked at TEAC / Tascam in 1982-83 in repairs, the only electrolytic capacitors that ever went south were the big power supply capacitors whose rated voltage was always too low for the application, to save money, and they would short. Never open. I don't think they ever leaked or exploded either. They'd put a 25V capacitor in a 25V application. The smaller capacitors where for example there'd be a 4.7uF, 50V capacitor in a 12V application, never failed.

In our company which started in 1991 and still has some of the original models flying, the only electrolytic capacitors that have failed were 220uF 16V 6x11mm, all in the same defective batch, and it's always by shorting, and again there's no leakage or rupture. Other 'lytics have never failed, even other brands of 220uF 16V 6x11mm.

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http://WilsonMinesCo.com/ lots of 6502 resources
The "second front page" is http://wilsonminesco.com/links.html .
What's an additional VIA among friends, anyhow?

I have some surface mount caps (electrolytic) in my Amiga 600 that have leaked...even got that nice "fishy" smell.
I've also seen some classic Mac SE's leak everywhere with a nice green fuzz on most of the IC's.

In my experiences, however, I must admit it's almost always the batteries that leak all over the place. I usually go through all of my computers and remove all batteries. Most of the time, the batteries aren't needed anyway as I don't care about TOD with my vintage computers.

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Cat; the other white meat.