BBC micro: Astec PSU recap

for bbc micro/electron hardware, peripherals & programming issues (NOT emulators!)
Post Reply
JimmyWinch
Posts: 3
Joined: Sat Oct 12, 2019 6:26 am
Contact:

BBC micro: Astec PSU recap

Post by JimmyWinch » Sat Oct 12, 2019 7:03 am

Hi. We've decided to get the various BBCs which my dad has lurking around the house back into service (at least 4, plus a Master!). We're investing in a range of test equipment, the boys are honing their soldering skills and we are about to work through the various problem-solving guides - thanks to all who's produced them!
We've bought the obligatory re-cap kits for the PSUs. On the latest, we are encountering a surprise. The capacitor in the (unlabelled) C2 position is, instead of being a .1uF, is a .22uF polypropylene capacitor.
Is this because it's an early model of PSU? It's an Astec Rev 00, with 894VO and 04202032000 inscribed on the underside.
And what's the best guidance to deal with this: replace or leave alone? And if it is one of the earlier PSUs, would it be best practice to replace all electrolytic capacitors at the same time?
Thanks!

User avatar
1024MAK
Posts: 9236
Joined: Mon Apr 18, 2011 4:46 pm
Location: Looking forward to summer in Somerset, UK...
Contact:

Re: BBC micro: Astec PSU recap

Post by 1024MAK » Sat Oct 12, 2019 7:25 am

The Rifa X2 capacitors are the only ones that normally self destruct (with the smelly smoke).
Metallized Polypropylene Film Capacitors do degrade over time (their capacity drops), but are not known to self destruct like the Rifa type.

Have a look at the soldering. It may be that someone else has already replaced this capacitor in the past.

The exact value is not critical, so you can leave the 220nF capacitor alone. Or you can replace it with a new 100nF. It will not affect the operation of the PSU. The different value will just slightly change the frequency of the filter that this capacitor is part of, designed to reduce leakage of the PSU’s switching signal getting out onto the mains wiring.

C9 should be replaced as it is a known cause of faults. The others are normally only replaced when symptoms occur. All electrolytic type capacitors have a limited life, but the actual life depends on many factors. Including how much use the machine has had and the temperature the capacitor has been running at.

But there is nothing to stop you from replacing all of the electrolytic type capacitors if you wish.

You could carry out some tests if you have an oscilloscope and something to use as a test load (I use 12V 5W and 12V 21W lamps as test loads).

Mark

JimmyWinch
Posts: 3
Joined: Sat Oct 12, 2019 6:26 am
Contact:

Re: BBC micro: Astec PSU recap

Post by JimmyWinch » Sat Oct 12, 2019 7:25 pm

Thanks for the ultra-fast reply. We decided to replace all caps and now have a beautiful 5.08V output... Thanks also for including the links to material that we've not come across before in your email footer- we're more confident we'll get things working. Though just have to work out why the PSU we re-capped a day or two ago and worked, now doesn't!

User avatar
danielj
Posts: 7513
Joined: Thu Oct 02, 2008 4:51 pm
Location: Manchester
Contact:

Re: BBC micro: Astec PSU recap

Post by danielj » Sun Oct 13, 2019 5:58 am

Check the polarity of the startup capacitor c9 :D

JimmyWinch
Posts: 3
Joined: Sat Oct 12, 2019 6:26 am
Contact:

Re: BBC micro: Astec PSU recap

Post by JimmyWinch » Sun Oct 20, 2019 10:40 am

Thanks for the ongoing advice. C9 was right in the other PSU. However, the TO3 transistor had packed up. PSU now works - just need to see if this makes the Beeb work!

Post Reply