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Capacitor blowout. Is it just me or what?!

T

tezza

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Some of you might have read one of the other threads I posted concerning capacitors blowing up in a newly obtained Osborne 1. Well, it's happened again with a BBC micro I just obtained. It ran for about 15mins then "pop", and smoke. Thankfully, I'd only paid a few bucks for it.

The interesting thing is on opening it up, the capacitors that blew were exactly the same type as those that blew in the Osborne 1! I've attached a picture of a blown one from the Osborne power supply.

From what I've read, these capacitors help prevent RF interference. The specs are:

PME271M capacitor,0.1uF 275Vac
PME271M capacitor,0.01uF 275Vac

Interestingly,. BOTH the larger 0.1uf and smaller 0.01 one blew in BOTH the Osborne and the BBC?? They obviously don't like long periods on non-use.

Has anyone had a similar experience with these sort of capacitors? I've replaced the Osborne ones with modern equivalents, but that didn't work (Maybe some other damage was done). How much collateral damage should be expected when these things do blow?

So now I'll try to replace the BBC ones and hope for the best. The power supply is a bitch to work on with this one, so I'll wait until I have time and motivation. It's tightly packed and it's hard to get to the bottom part of the board.

Maybe I should just stick to collecting emulators!

Anyway, any comments, experiences or wisdom welcome.
 

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Yes, the interesting thing about these blown ones though, is that they are metallised paper capacitors.

The electrolytic ones, which are the ones normally mentioned as leaking or blowing, have been fine!
 
No it's not you!
Both my sirius/victors had those caps in the power supply until 10 seconds after power on, when they let the smoke out. It's the RIFA interference suppressors, I think that they also have a small series resistor built in which provides the heat when the cap starts to go short.
It's probably OK to remove/replace with a similar value "class x" cap (I have, with no obvious problems).
 
Well, I almost feel left out but I don't. My 2 great "Tant" firework shows have been more than enough for me. Both times, I have had the cover off and those little blue bombs have been fairly close to my face. The 2nd one really scared me because for about 2 seconds I thought one of my power supply leads had dropped onto the motherboard. Nope, just a tant, spewing it's guts all over the surrounding area.
 
tezza said:
Yes, the interesting thing about these blown ones though, is that they are metallised paper capacitors.

The electrolytic ones, which are the ones normally mentioned as leaking or blowing, have been fine!
Click to expand...

Yes, but the caps in your picture also contain electrolyte, which defines them as electrolytic cap. You're prob'ly used to seeing the aluminum can type, but the only difference is the packaging. The aluminum ones are rolled up and stuck into the can, which is then sealed with a rubber plug. The cans have stress relief grooves in the top, so if they blow, there is a weak point to release the pressure (at least, that's the hope). The wax & paper ones you have differ only in that they are wrapped with several layers of waxed paper, then dipped into hot wax to seal in the electrolyte. They tend to blow for the same reasons that other caps do.

--T
 
Sorry Terry, I beg to differ, I suspect that although those caps DO contain electrolyte, it's not deliberate, and the electrolyte in this case is water that's accumulated in there through the cracks in the resin.

Excerpt from "oldradios.co.nz" -

"Until about 1960, these were the most common with a range of 1.0 mfd or more down to .001 mfd and consist of two strips of foil separated by layers of thin paper, impregnated with wax or oil. Instead of foil, some had an extremely thin layer of metal deposited on the paper and were known as metallised capacitors.
This construction saved space and rendered the capacitor self-healing in the event of a breakdown, with the metal deposit vapourising around the trouble spot. One synthetic impregnating material used in paper capacitors, PCB, has excellent dielectric characteristics but has been outlawed because it is practically indestructible and can enter the food chain. Be suspicious of metal cased capacitors that leak a clear viscous impregnant. A good paper capacitor will have a resistance measurement of several hundred megohms, but as they were extremely difficult to seal against eventual moisture absorption, many are likely to now measure much less, especially those with cardboard cases. Metal and plastic cased components were used with varying degrees of success. Generally the metal cased type will be found to have survived reasonably well, but there have been some criticisms of plastic cases. In spite of their apparently excellent encapsulation, there are many reports of their failure. Others to watch for are those with thin grey coloured moulded cases, which can develop hairline cracks, which will let in moisture. These were popular at one time as replacements, and if you find any with cracks, don't even bother to test them. "
 
Oh well, anyway, electrolytic or not, here's the sequel...

I was talking to an electronics expert at a family party the night before last about my blown capacitor problems on both the Osborne and the BBC micro. We've both identified them as components to stop stray RF transmission.

The BBC seems to work fine even with these damaged units. I asked him if it would damage the machine if I continue to use it with these blown caps. He said he didn't think it would affect the unit at all, and it should keep running just fine. The only thing affected might be nearby TVs.

Now, it goes against the grain to operate something where I know it's not 100% but I had it running last night for 5 hours and it was fine. No sign of more smoke or overheating and it seemed very stable.

Given the fact that the powerpack is so tightly packed it's almost unrepairable (in fact, it says in the manual it is unrepairable, and you should replace the whole pack if it's faulty), then I think I'll just continue to use it "as is".

Can anyone see any potential dangers with this?
 
No, your resident electronics expert is right. running them without the RF filtering is fine (from the computer's point of view!).
Bear in mind though that if you're into keeping these beasties going, at some point you're going to have to repair some PSUs. The electrolytics on switch mode power supplies do tend to go duff as they're under a lot of stress. first thing you tend to notice are bulges in the cans, or monitors that get "twinkly" as the ripple on the supply lines increases.
The good news is that modern components tend to be smaller than their 1980's equivalent, so no problems stuffing them in.
 
Besides going to KnuckleJunction with the neighbor, the only thing I can think of is possible data loss if your machine doesn't filter out stray RFI during a disk write.

--T
 
Last edited:
nige the hippy said:
The good news is that modern components tend to be smaller than their 1980's equivalent, so no problems stuffing them in.
Click to expand...
Although good to keep in mind stories like the following:

> The origional 1500uF x 50V 85c caps. are cans 1.5" dia. x 3" high my replacements
> are but a fraction of that size. I know this is due to modern caps. being smaller
> but consider this------
> In an IBM 5154 monitor power supply circa 1984 there is a 47uF x 25V 85 cap
> goes low. The size of this cap. is three times bigger than my shelf replacements.
> There are 47uF caps on the deflection boards of the same monitor and they
> are a third the size of the same value cap. that is in the power supply.
>
> Why?

The larger caps are probably high frequency caps, to handle the higher ripple current. I was repairing a monitor that lost deflection (forget
which direction). I found a bad cap. The guy at the parts store found a cap with the same values, but only about one third the size of the bad one. He
said the new one must be smaller due to improved design. I put in, checked polarity, turned monitor on, and the cap exploded. Later I found a high
frequency cap at another store, put it in and it works fine. So, if your day is a little dull, you can try the smaller cap, but you should probably
crouch under your bench when you turn the monitor on. :)
 
thanks for the advice. I'll keep an eye on PSU.

Luckily I live a place with low density housing, so I'm some distance from the neighbours :D

>Uh, the neighbor comes over and punches you in the nose?
>(for messing up his soccer game) chuckle

It wouldn't be soccer as most people don't care a less about that. Rugby, on the other hand...:)
 
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