BBC Master PSU capacitors - watchful waiting?

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aqarius
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BBC Master PSU capacitors - watchful waiting?

Postby aqarius » Mon Nov 27, 2017 8:45 pm

Hello,

I've just recently bought myself a BBC Master 128. Like many (most?) of them, it's a retired classroom computer, and was sold to me in working condition. And it is working, albeit in need of some attention - the CMOS battery's dead, the keyboard is temperamental and in need of some servicing, and theres a general fuzz from the speaker that seems to be relaying processor activity. Importantly, the PSU seems to be ok for the moment. I don't know if it's possibly been serviced at some stage, I haven't looked inside it yet and even then I'm not sure if I'd be able to tell. I had been planning on doing the capacitor replacement myself, but to be honest I've no real soldering/desoldering experience, and I've had second thoughts about whether I want to risk damaging the PSU or myself in the process.

So as it's working at the moment, do you think it's ok to just wait and see how it goes, and defer replacing capacitors until I either get some experience or have the spare cash to get it serviced by an expert? Or would you always get it sorted ASAP for peace of mind? As a side question, is it possible the speaker noise will reduce after the PSU is serviced?

I can investigate inside the machine if anyone thinks there'd be something useful to see.

Thanks for reading :)

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1024MAK
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Re: BBC Master PSU capacitors - watchful waiting?

Postby 1024MAK » Mon Nov 27, 2017 9:51 pm

Hello aqarius. Welcome to StarDot :D

Sometimes school computers were serviced by good engineers, and so the problem capacitors may well have already been replaced. I have two Master 128 machines where the problem capacitors are not originals. But without opening the case of the computer, then removing the PSU section and removing the PSU metal cover, it is hard to tell.

The two capacitors that make a nasty smell and produce smoke are X2 type interference suppression capacitors. They do not affect the operation of the power supply. If you are able to travel to an ABUG meeting, someone can have a look, and if needed, we can renew the two X2 capacitors plus renew the "start-up" capacitor (this is for reliably reasons).
If not, is it safe to continue using it? The answer is yes. As long as you are aware of the possible risk of smelly smoke if the original X2 capacitors are still fitted. The reason for the recommendation to replace them before they fail, is to prevent the smelly smoke and because it's easier and quicker to change them before they splatter gunk within the power supply.

All BBC Micros and BBC Master machines produce some noise from the loudspeaker. This is normal. And yes, the sound does change a bit depending on what the machine is doing. If it is too annoying, there are some things that can be done to reduce the noise level (assuming the modification has not already been done).

The "CMOS" (actually the Real Time Clock or RTC chip) battery is easy to replace. Just buy a new one from RetroClinic if you are not inclined to build one yourself.

Most of the time, non-working keyboard keys can recover if you exercise them. Yes, keep pressing them for upto 80 to 100 times. This cleans the contacts. If this does not work, there are other things you can do such as spray contact cleaner inside them.

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hicks
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Re: BBC Master PSU capacitors - watchful waiting?

Postby hicks » Mon Nov 27, 2017 10:44 pm

Pressing the keys is good advice, a few keys on an Electron I bought came back to life with that. A couple more needed a more involved clean and I found some useful images of how on memotech. Half way down for the part about Electron/BBC switches.

philb
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Re: BBC Master PSU capacitors - watchful waiting?

Postby philb » Mon Nov 27, 2017 10:59 pm

aqarius wrote:So as it's working at the moment, do you think it's ok to just wait and see how it goes, and defer replacing capacitors until I either get some experience or have the spare cash to get it serviced by an expert? Or would you always get it sorted ASAP for peace of mind? As a side question, is it possible the speaker noise will reduce after the PSU is serviced?


Yes, it's fine to wait. As Mark already mentioned, about the worst that will happen is that the suppression caps break down and produce a load of evil smoke. It's also possible that you might find the machine becomes unreliable / hard to start if the other capacitors have dried up. But none of this will have any permanent consequences and you can just fix the PSU if and when any of these problems show up.

If the output filter capacitors on the PSU have gone bad then it's possible that this might be making the speaker noise worse and, if that's indeed the case, servicing the PSU might indeed make it better again. I don't recall Masters being particularly noisy so if yours is noticeably bad then it possibly does mean that some or other component is getting a bit marginal.

aqarius
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Re: BBC Master PSU capacitors - watchful waiting?

Postby aqarius » Tue Nov 28, 2017 2:18 pm

Thanks for the comprehensive advice guys, that's reassuring. I don't relish finding out what the smoke's like as its always described vividly as unpleasant...

1024MAK wrote:Most of the time, non-working keyboard keys can recover if you exercise them. Yes, keep pressing them for upto 80 to 100 times. This cleans the contacts. If this does not work, there are other things you can do such as spray contact cleaner inside them.


hicks wrote:Pressing the keys is good advice, a few keys on an Electron I bought came back to life with that. A couple more needed a more involved clean and I found some useful images of how on memotech. Half way down for the part about Electron/BBC switches.


I've also had success fixing a few keys on an Electron keyboard with repeated firm pressing (and removing the key cap and blowing - not sure if that did anything). I gave this a go with the H key on my Master and it did improve, it went from seemingly unresponsive to about 75% responsive, but I've found more since then that are intermittent. I'll probably invest some more time in exercising the keys, and then look at using contact cleaner if they're still troublesome.

philb wrote:I don't recall Masters being particularly noisy so if yours is noticeably bad then it possibly does mean that some or other component is getting a bit marginal.


I don't have much to compare it against, but I'm guessing it's noisier than average. The sound chip is coming through nice and clear though. It reminds me of a ZX Spectrum + my school used to have, it sounded like a cat purring when it was plugged in, I thought this was normal until I got a Spectrum 128k and it was completely silent to the point I'd often leave it on by accident (no LEDs and no on/off switch. The only giveaway was the radiator!) The Master's not nearly as noisy as that Spectrum anyway, it doesn't interfere with the enjoyment of using the computer, I only worried that it was symptomatic of something else.

Thanks again for the info! Mark, I'll go to the introduction threads soon :)

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1024MAK
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Re: BBC Master PSU capacitors - watchful waiting?

Postby 1024MAK » Tue Nov 28, 2017 11:04 pm

ZX Spectrum 16k, 48k and plus models use boards that have small transformers (the "coil"). On some boards, it is this that people can hear "buzz" and not the speaker. It affects mostly the issue 1 through to some issue 4 boards. It is the noise of the transformer vibrating at the frequency used by the on board DC/DC converter/inverter power circuitry. Some early issue boards can be very noisy!

The 128k toastrack also has a DC/DC converter, but as this computer uses more modern RAM chips, the DC/DC converter has a lot less work to do. And by then the manufacturing of the coil had improved.

Mark
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