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bugman2112
November 11th, 2010, 09:36 PM
I have been building and collecting vintage computers for a number of years now. I have always been surprised at how few equipment failures that I encounter. The majority of my collection is Apple ii, which I consider to be generally realiable (of course, a few bad ram chips here and there). I had a surprise failure tonight that made me chuckle a bit and I thought I would share. I was re-working some older text games this late night (wumpus II) on one of my old apple ii+ computers and heard a stout POP noise, followed by a fizzle. I quickly suspected a capacitor failure in the power supply and pulled the power cord. Nice rancid plumes of smoke filled my office. Its not that big of a deal, since I have a couple of stacks of these computers, as well as, extra power supplies. But it did make me wonder what would have happended if I was wandering downstairs to get another beer when this happened. I might tear into it just to see the damage and attempt a repair. The computer was still functioning as I shut it down, so I'm hopefull of limited damage. The only comparable experience I have had was with a tantulum capacitor that exploded in a Northstar upon first power up. That was replaced and worked fine.

Moonferret
November 12th, 2010, 09:23 AM
I had the same thing happen to me a couple of days ago with a II Europlus. Machine was on for about 5mins before I got a fizzling noise with some smoke. (No Pop) Powered off right away and opened up the psu but I couldn't find the source. Nothing looked damaged or burnt out. Re-connected it to the machine and ran it with the lid off expecting to see a cap fail but after another 30mins there was nothing. Very strange.

barythrin
November 12th, 2010, 11:47 AM
http://www.history.navy.mil/photos/images/h96000/h96566kt.jpg (http://www.history.navy.mil/photos/images/h96000/h96566kc.htm)Computer bug maybe? ;-)

Actually, my Apple IIe pop noise was me putting my led flashlight face down on the table and forgetting to turn it off. I had the top off my A2e and had swapped around that resistor pack on the serial card to do some null modem transfers with ADTPro, which naturally having a pop and piece of plastic hit me in the face I assumed I blew something on the A2e.. but fortunately? (at the time the flashlight was worth more than the A2e) the system was fine.

camdude
November 13th, 2010, 03:51 PM
Check the AC Mains filter capacitors. If they blow, your computer will still work but can be destroyed at even the slightest electrical ripple.

bugman2112
November 15th, 2010, 02:53 AM
I need to find and double check the schematic. It wasnt as easy as thought to find the bad components. With all the smoke, I thought it would be easy. But the tight lay out of the components hid the damage. After further inspection and dissassembly, I found the damage. It is a square componet close to the AC cource. It has a 0,1 uF rating on it, and also a Mhzt rating. I'm assuming that this is the AC filter capacitor?

Chuckster_in_Jax
November 15th, 2010, 07:29 AM
I need to find and double check the schematic. It wasnt as easy as thought to find the bad components. With all the smoke, I thought it would be easy. But the tight lay out of the components hid the damage. After further inspection and dissassembly, I found the damage. It is a square componet close to the AC cource. It has a 0,1 uF rating on it, and also a Mhzt rating. I'm assuming that this is the AC filter capacitor?

Sounds like a Metalized Paper Capacitor. Does it look something like this (damaged)?

http://i909.photobucket.com/albums/ac299/cledford22/MetalizedPaperCapacitor001.jpg

I replaced mine with a Polyester Film Capacitor like this one from Mouser P/N 539-160104J1000N-F. Reliability is much higher than the Paper type.

http://i909.photobucket.com/albums/ac299/cledford22/MetalizedPaperCapacitor042.jpg

Chuck(G)
November 15th, 2010, 08:12 AM
Looks to me like a polystyrene capacitor. At any rate, it's part of the AC line filter and really doesn't have a lot to do with the basic functioning of the power supply.

And, yes, polyester is a good substitute. Polypropylene will also work well.

Polystyrene caps aren't made any more as they had a nasty property of catching on fire--that's why you'll occasionally see polypro caps labeled "safety".

NOS polystyrene caps are still sought after by audiophiles.

bugman2112
November 15th, 2010, 11:22 AM
Yes, it looks exactly like your damaged picture. Even down to the cracked casing....

arjoll
November 16th, 2010, 01:29 AM
You should be replacing anything that goes across the mains with an X2 rated polyester cap. I do that as a matter of course with old power supplies after having a mains filter cap explode in a Sord M23! Here we'd use a 275V version for 230V mains, but I guess you use smaller ones there.

Lorne
November 16th, 2010, 06:26 AM
Yes, it looks exactly like your damaged picture. Even down to the cracked casing....

And I recently had the exact same cap go poof on an Astec power supply for an Osborne1. They smell real nice when they fry - just like a fire cracker smells.
I replaced it with a polypro cap.
Tezza had the same cap go on one of his machines as well.

Chuck(G)
November 16th, 2010, 07:37 AM
You should be replacing anything that goes across the mains with an X2 rated polyester cap. I do that as a matter of course with old power supplies after having a mains filter cap explode in a Sord M23! Here we'd use a 275V version for 230V mains, but I guess you use smaller ones there.

Not really--many SMPSUs will operate at either voltage with at most a jumper change, so the same part is used. Very often you'll see these as 400V parts. Even on the old gear, much of the time these were rated at 250V. It isn't over-voltage that kills them most of the time; the dielectric ages, a tiny arc gets started and it's all downhill from there.

The bad news is that there's not much evidence that polyester caps won't eventually suffer the same end.

tezza
November 16th, 2010, 09:01 AM
Tezza had the same cap go on one of his machines as well.

Actually I've had them go in three of my machines, the Osborne, Kaypro II and BBC.

One of the things on the "to do' list is to work through my collection at some stage and replace all these PSU filter caps in the machines that still have them. I exercise all my machines every few months. These caps tend to fizz and burn when they go and I'm concerned if one of them blew when I wasn't present, the computer itself might go up in flames, taking the rest of my collection with it.

Tez

donutty
November 16th, 2010, 09:27 AM
Replaced all that type of cap in a Lisa after one went fizz. Same with every Apple II and BBC I've got. One of my old Yamaha electric organs too. I now replace them as default as they will go one day if not when you first power them up.

The casing cracks with age and then they meet a smoky end.

bugman2112
February 15th, 2011, 07:10 AM
I thought I would mention that I had this same capacitor blow on another apple ][+ that I just bought. I obtained the suggested Mouser replacement in this thread and everything works fine. I wanted to post ths again because it really does seem like a growing problem with this type of old power supply and may be a safety concern. I wonder what would happen if this went unchecked if it was left unattended?

camdude
March 12th, 2011, 11:26 PM
Most probably, there would be some noticeable damage to the more-sensitive electrical components (CPU, RAM, any expansion cards). However, I'll take one of my old PSU's (that I have yet to retrofit), and check it out with my oscilloscope and multi-meter, and get back to you on that.

camdude
March 15th, 2011, 07:08 AM
I wonder what would happen if this went unchecked if it was left unattended?

Here's the verbal description of the damage: Within digital circuits, it reduces the threshold, as does any form of supply rail noise, at which logic circuits give incorrect outputs and data is corrupted.

and here's a picture of the difference in the current ripple differences: http://upload.wikimedia.org/wikipedia/en/7/7e/Smoothed_ripple.svg

I hope this is an accurate description.