BBC Model B takes up smoking

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philb
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Re: BBC Model B takes up smoking

Postby philb » Mon Oct 16, 2017 11:59 am

KayjayUK wrote:That is so true. I stripped and repaired 6 psu's at one time about 2 years ago, every one of them has now popped. Oh and I used RS parts, I have always believed in their quality but I assume this isn't a quality issue.


That is a bit strange. Do you remember exactly what part you used?

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1024MAK
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Re: BBC Model B takes up smoking

Postby 1024MAK » Mon Oct 16, 2017 1:31 pm

vanpeebles wrote:Ah when I did the Master, it was left for a number of days but I was still a bit weary :) So you touch the points of a single capacitor together?

Yes, you short the pair of two pads/leads of the first capacitor to each other. Then do the same for the other capacitor.

Try to short across the bits of wire rather than just the flatter part of the solder, and rather than just touching the connections with the screwdriver, rub it across from one connection and onto the other connection. This helps if the connection is a bit dirty.

Oh, and, I forgot to say earlier, hold the screwdriver via the plastic handle and the metal shaft of the screwdriver needs to be clean.

Mark
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KayjayUK
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Re: BBC Model B takes up smoking

Postby KayjayUK » Tue Oct 17, 2017 7:03 am

I don't remember the actual part no. but it was the RS numbers given by Mark Haysman in his BBC repair manual. They were all the correct values both X2 caps and the electrolytic startup cap.

Keith
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1024MAK
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Re: BBC Model B takes up smoking

Postby 1024MAK » Wed Oct 18, 2017 2:48 am

As I understand it, moisture getting in to the capacitors, and/or large numbers of high voltage mains spikes, helps to kill them.

The plastic package should stop moisture getting in, but if it becomes damaged...

If you do suspect mains spikes, use a good quality extension socket that contains VDRs. You may not want to feed a CRT colour TV or monitor from the same extension socket if it uses a mains SMPSU.

It's also possible that there was a bad batch that were manufactured. I bought some from RS last year, but only one of each value is currently in a PSU. And this Beeb is not used that often.

I have some ex-school machines, and the two Master 128 machines have a different make of capacitor for C1 and C2. I don't know how long ago they were changed, but they are currently fine.

Mark
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crj
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Re: BBC Model B takes up smoking

Postby crj » Wed Oct 18, 2017 3:02 am

I gather it's preferable to short the capacitor with a well insulated 1kOhm-or-so resistor to bleed it first, and only then poke at it with a screwdriver to be doubly sure.

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danielj
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Re: BBC Model B takes up smoking

Postby danielj » Wed Oct 18, 2017 7:24 am

Given that Mark works on big industrial electrical systems for a living, if he says its OK to poke it with a screwdriver, I'm happy to poke it with a screwdriver :)

d.

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1024MAK
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Re: BBC Model B takes up smoking

Postby 1024MAK » Wed Oct 18, 2017 7:26 am

crj wrote:I gather it's preferable to short the capacitor with a well insulated 1kOhm-or-so resistor to bleed it first, and only then poke at it with a screwdriver to be doubly sure.

If you are in a workshop and have just powered it up to test, and now wish to work on it, then yes, using a suitable resistor to discharge capacitors is best practice. However, the capacitors that are being talked about should discharge via the resistors already on the board. So shorting out with a screwdriver is just to make sure that they have discharged.

Of course if vanpeebles has a suitable resistor (the value is not that critical, anywhere between 560 ohms and 4.7k ohms, but it should be rated at 1 Watt or greater and be sleeved in insulated heatshrink tube with insulated wires connected to suitable insulated test probes) then he could use that. But it's unlikely he has such an item.

Mark
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vanpeebles
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Re: BBC Model B takes up smoking

Postby vanpeebles » Wed Oct 18, 2017 9:02 am

I was purely asking as a noggin who didn't want to end up with curly teeth :mrgreen:

crj
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Re: BBC Model B takes up smoking

Postby crj » Wed Oct 18, 2017 12:01 pm

1024MAK wrote:Of course if vanpeebles has a suitable resistor (the value is not that critical, anywhere between 560 ohms and 4.7k ohms, but it should be rated at 1 Watt or greater and be sleeved in insulated heatshrink tube with insulated wires connected to suitable insulated test probes) then he could use that. But it's unlikely he has such an item.

The simple option is to stick the resistor in a project box with a couple of banana sockets on the front, then use the test probes you already have.

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richmond62
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Re: BBC Model B takes up smoking

Postby richmond62 » Wed Oct 18, 2017 6:26 pm

My packet from RetroClub took donkey's ages to get out here in Bulgaria: just ran it round with my Model B to a chap who is
capable of sorting out these capacitors for me . . .

. . . I hope I can use this as a "way into his heart" as he has 2 Pravetz computers and a (very rare) Puldin sitting on his shelves and has always been evasive every time I've "wiggled my bum" at him with respect to those computers . . . even if my wife will kill me when/if I bring them home.


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