TRS-80 repair, those damn filter caps

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TRS-80 repair, those damn filter caps

Postby matsondawson on Mon Apr 18, 2011 2:59 pm

I picked up a free TRS-80 Model III, all it needed was a filter cap replaced.
So I replaced the filter caps on the board that was broken, and didn't bother with the other one as I'd have to remove the drives.
30 seconds after successful power up *KABOOM*, there goes the filter caps in the other PSU.
Never seen one of these explode before, just crackle and smoke.
Cool thing is, it uses the same caps as the BBC range of computers, and I have a bunch lying around.

Image
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Re: TRS-80 repair, those damn filter caps

Postby tezza on Mon Apr 18, 2011 3:16 pm

Yes, I've seen lots of these PSU filter caps go now. In fact, I now EXPECT them to explode within a minute of turning on a machine that's been sitting for a few years. If one fails, I now redo all similar caps on the board as a precautionary measure. I've had them go on..

Three apple II/IIe PSUs
A BBC PSU
A Kaypro PSU
An Osborne PSU
A Lisa PSU

Luckily replacement caps are cheap and readily available. I'm just waiting for my Model IV ones to go. :)

P.S. You did well to get a Model III for free! :shock:
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Re: TRS-80 repair, those damn filter caps

Postby nama on Tue Apr 19, 2011 3:13 am

When I get any new(old) machine. I always turn it on for about 5 seconds, then turn it off, wait about 5 mins and repeat, but this time for about 20 seconds..repeat again. This way the caps aren't suddenly flush with too much power all at once, its more af a gradual process of refilling them...I read this somewhere, and so far it's worked for me...not a single explosion yet. Fingers crossed.

Phil
Retro computing at: http://www.neoncluster.com
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Re: TRS-80 repair, those damn filter caps

Postby lizardb0y on Wed Apr 20, 2011 4:41 pm

nama wrote:When I get any new(old) machine. I always turn it on for about 5 seconds, then turn it off, wait about 5 mins and repeat, but this time for about 20 seconds..repeat again. This way the caps aren't suddenly flush with too much power all at once, its more af a gradual process of refilling them...I read this somewhere, and so far it's worked for me...not a single explosion yet. Fingers crossed.


This process is called "reforming" and I've generally had few problems following a similar procedure. Sometimes I receive equipment with bulging electrolytic caps, and these need replacing regardless. I was lulled into a false sense of security with the Mac, having been told it was working fine!

A great article, and circuit diagram for a proper reforming tool, and be found here: http://www.vmars.org.uk/capacitor_reforming.htm
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Re: TRS-80 repair, those damn filter caps

Postby matsondawson on Thu Apr 21, 2011 10:00 am

*Kaboom*, I bought the BBC Master off trade me with the tube co-processor.
2 minutes in, there goes the filter caps.
I've had the following pop on me:
1 BBC B
3 BBC Masters
1 TRS80

I think from now on, any time I get a BBC I'll just replace the caps before turning it on.

I wonder if that sweet smelling smoke is carcinogenic?
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Re: TRS-80 repair, those damn filter caps

Postby ZL2AOX on Thu Apr 21, 2011 10:30 am

nama wrote:When I get any new(old) machine. I always turn it on for about 5 seconds, then turn it off, wait about 5 mins and repeat, but this time for about 20 seconds..repeat again. This way the caps aren't suddenly flush with too much power all at once, its more af a gradual process of refilling them...I read this somewhere, and so far it's worked for me...not a single explosion yet. Fingers crossed.

Phil

I can't see how this can possibly work with electrolytics in situ since there's no current limiting in a standard power supply. If they were going to blow, they'd do so on initial power-up. Further, I'd expect 5 minutes power-down to be more than adequate for complete discharge.

In order to "reform" them, you'd need to remove and process each individually in a custom reforming circuit. But meh, why bother.. you may just as well replace them outright.
Last edited by ZL2AOX on Thu Apr 21, 2011 10:42 am, edited 1 time in total.
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Re: TRS-80 repair, those damn filter caps

Postby tezza on Thu Apr 21, 2011 10:42 am

When my first lot of caps went on me (the Osborne 1) I read up about reforming etc.

With old microcomputers it seems to be the paper line filter caps that blow more than any thing else. I have read of electrolytics blowing up but I haven't experienced this myself. In the end, I decided with a new acquisition that I'd just check the innards for any obvious damage (or inhabitants), and if none was seen I'd just stand back and switch it on (making sure I'm close to the power socket switch on the wall for a quick switch off). If a cap is weak or faulty, I'd rather that it did blow, so I could identify it and replace it.
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Re: TRS-80 repair, those damn filter caps

Postby lilliann on Tue Apr 26, 2011 2:17 am

I think it is not in that bad condition and if it is working good then it is gonna be great. i also have a old computer not that old but ya its old but it is my first one.

cheers!!
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Re: TRS-80 repair, those damn filter caps

Postby coogie on Tue Nov 15, 2011 6:54 pm

tezza wrote:Yes, I've seen lots of these PSU filter caps go now. In fact, I now EXPECT them to explode within a minute of turning on a machine that's been sitting for a few years. If one fails, I now redo all similar caps on the board as a precautionary measure. I've had them go on..

Three apple II/IIe PSUs
A BBC PSU
A Kaypro PSU
An Osborne PSU
A Lisa PSU

Luckily replacement caps are cheap and readily available. I'm just waiting for my Model IV ones to go. :)


Ok guys I now have a small mountain of PSU's that have suffered the same fate :| and it looks like I am in need of quite a few replacements, as I intend to replace any that looked cracked (blown or not) (and no gradual power introduction did not help me...)

Now for the million dollar question :wink: , where would you recommend I go to get these at a good price?

Specifically I am looking for direct replacements for the following:
0.1uF 250V 4.9MHz X Series capacitor
0.01uF 250V 15MHz X Series capacitor
0.47uF 250V 1.9MHz X Series capacitor

Gotta add... YES they do stink something terrible! :!: one of the power supplies was still smoking 30 minutes after pulling the plug!I seriously was contemplating going for a fire extinguisher!
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Re: TRS-80 repair, those damn filter caps

Postby matsondawson on Tue Nov 15, 2011 9:10 pm

I usually pick mine up from Jaycar.
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Re: TRS-80 repair, those damn filter caps

Postby tezza on Tue Nov 15, 2011 10:47 pm

matsondawson wrote:I usually pick mine up from Jaycar.

Me too!
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Re: TRS-80 repair, those damn filter caps

Postby lizardb0y on Wed Nov 16, 2011 10:07 am

tezza wrote:
matsondawson wrote:I usually pick mine up from Jaycar.

Me too!


Elecment 14 is often cheapest, but can be a bit hit & miss with component availability. Jaycar always has what they say they have, but can be a bit more expensive. RS seems pretty pricey.

Jaycar http://www.jaycar.co.nz/
Element 14 (formerly Farnell) http://nz.element14.com/
RS Components http://newzealand.rs-online.com/web/
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Re: TRS-80 repair, those damn filter caps

Postby coogie on Thu Nov 24, 2011 7:57 pm

Thank to your prompt replies, I now have my components (via Jaycar) and the games have begun - hopefully nothing more is toasted....

I will post back when I have completed the job to let you know my successful resurrection rate ;-)
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