A3000 PSU capacitors

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Tills
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A3000 PSU capacitors

Post by Tills » Wed Sep 25, 2019 1:22 pm

While troubleshooting another problem concerning a crackling noise through the right speaker since last week (and occasional dipping in screen brightness, which is new), it occurs to me there may be something also up with the PSU. I am concentrating on replacing the electrolytics on the board at the moment, but am now wondering whether the two larger caps in the PSU are in fact damaged. On the other hand, I am reading a stable 4.92 volts on my multimeter.

I know about old glue being mistaken for a leaking cap, but on reflection I think the glue under two caps looks distinctly different from the other set. It could of course be due to heat on the higher voltage ones.

Here's are some pictures.

Compare the 200 volt and the 10 volt 'glue'.
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anightin
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Re: A3000 PSU capacitors

Post by anightin » Wed Sep 25, 2019 2:05 pm

I'd be inclined to replace the electrolyics as you mention.

There does seem to be some definite corrosion on the resistor legs which might suggest what you're seeing is a mix of glue+cap-guts.

Would be interesting to check the consistency of that compound compared to that around the 10v caps to see if it is softer or more brittle etc.

I'd be also tempted to check the resistances too after cleaning to ensure that the corrosion was just on their surface.


If you're feeling flush there is an alternative to DIY it would appear:
https://www.ebay.co.uk/itm/New-Acorn-A3 ... 1429216454

Cheers

Andy

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Re: A3000 PSU capacitors

Post by Tills » Wed Sep 25, 2019 2:36 pm

Thanks. I agree, the resistor+green stuff isn't a good sign.

I think I'll replace the caps and the resistors too. 220k ohms, I assume, but I wonder what the wattage is. 2 watts? I'll get the calipers out and measure the width, if that's a reasonable indicator.
Last edited by Tills on Wed Sep 25, 2019 5:10 pm, edited 7 times in total.

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Re: A3000 PSU capacitors

Post by markdryan » Wed Sep 25, 2019 6:47 pm

anightin wrote:
Wed Sep 25, 2019 2:05 pm
If you're feeling flush there is an alternative to DIY it would appear:
https://www.ebay.co.uk/itm/New-Acorn-A3 ... 1429216454
That's quite tempting. It says new old stock though. So presumably the capcitors in these power supplies are almost as old as the capacitors in my A3000. Are they likely to be in better condition, if they've never been used?

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Re: A3000 PSU capacitors

Post by markdryan » Wed Sep 25, 2019 6:51 pm

markdryan wrote:
Wed Sep 25, 2019 6:47 pm
That's quite tempting. It says new old stock though.
Actually, that's not quite what it says. The description reads "A new unused 22W power supply unit originally designed for the Acorn A3000."

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IanS
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Re: A3000 PSU capacitors

Post by IanS » Wed Sep 25, 2019 6:55 pm

a3k_psu.PNG
"New Unused but old stock"

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Re: A3000 PSU capacitors

Post by markdryan » Wed Sep 25, 2019 7:00 pm

IanS wrote:
Wed Sep 25, 2019 6:55 pm
a3k_psu.PNG
"New Unused but old stock"
Ah, so that is what is says. So what does that mean for the capacitors on these things?

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Re: A3000 PSU capacitors

Post by IanS » Wed Sep 25, 2019 7:24 pm

markdryan wrote:
Wed Sep 25, 2019 7:00 pm
Ah, so that is what is says. So what does that mean for the capacitors on these things?
Components age, used or unused - viewtopic.php?f=8&t=17861&p=248958#p248958

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Re: A3000 PSU capacitors

Post by anightin » Wed Sep 25, 2019 7:41 pm

Ah, should have read the Ebay Ad more thoroughly before posting earlier.

Apparently idleness is the worst thing for an electrolytic.

https://www.nmr.mgh.harvard.edu/~reese/electrolytics/

I'd be inclined in that case to stick to plan A and replace the caps, and the corroded resistors on your existing supply.


Crystals in my limited experience degrade over time too. If you have the machine apart you might also want to consider replacing.

On another thread for example I had a 16Mhz crystal on a Beeb that was contributing to a glitching display. Replacing it with a NOS version exhibited the same issue. Only when I replaced it with a new component was everything happy again.

Best

Andy
Last edited by anightin on Wed Sep 25, 2019 7:41 pm, edited 1 time in total.

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Re: A3000 PSU capacitors

Post by Tills » Wed Sep 25, 2019 9:06 pm

On the cusp of ordering some new Nichicon caps, and the resistors too (I think they're 1 watt, according to my measurements, but I might go for 2 watt to be on the safe side).

The caps (4 biggies, 2 small) and resistors come to about £5 with free postage, so that's an acceptable repair.
Last edited by Tills on Wed Sep 25, 2019 9:55 pm, edited 2 times in total.

arkleyjoe
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Re: A3000 PSU capacitors

Post by arkleyjoe » Thu Sep 26, 2019 8:39 pm

I do not think you can ever really argue against replacing old electrolytic caps in something this old. It is going to either be needed now or saving you an inevitable job in the future.

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Re: A3000 PSU capacitors

Post by Tills » Fri Sep 27, 2019 7:24 pm

arkleyjoe wrote:
Thu Sep 26, 2019 8:39 pm
I do not think you can ever really argue against replacing old electrolytic caps in something this old. It is going to either be needed now or saving you an inevitable job in the future.
Indeed. I have now changed them and the voltage now reads a more healthy 4.98, rather than 4.92, which may be significant (and 5 volts without load). On to the next item on the agenda ...
Last edited by Tills on Fri Sep 27, 2019 10:03 pm, edited 2 times in total.

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