BBC PSU problem?

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rharper
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BBC PSU problem?

Postby rharper » Mon Jul 17, 2017 4:13 pm

I have a BBC that arrived with a blown C2 capacitor.
I replaced C1 & C2 and the psu gives 4.96v at the leads.
However, when connected to the motherboard multimeter probes to the base of the connectors only gives 0.76v, the beeb shows no signs of life. :(
Is this another psu problem?
Does that voltage indicate the nature of the problem?
Thanks,
Ray :)
Raycomp

cmorley
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Re: BBC PSU problem?

Postby cmorley » Mon Jul 17, 2017 5:21 pm

I'd look for cracked solder joints on the main 5v output filter caps first. If these joints are cracked then the PSU could generate 5v when unloaded but can't maintain the voltage when loaded which matches your symptoms... You can sometimes see the cracks in the solder more easily by wiggling the cap - they can be very fine and easy to overlook.

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hoglet
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Re: BBC PSU problem?

Postby hoglet » Mon Jul 17, 2017 5:45 pm

Next thing to replace would be C9, which can cause this exact symptom.

If that doesn't help then it's probably the chopper transistor.

Dave

duikkie
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Re: BBC PSU problem?

Postby duikkie » Mon Jul 17, 2017 6:09 pm

try simple things like : bicycle lamp 6v 12w or something like that, if volt drops you have a psu problem :) for 12 volt a car lamp 12v 24w ?

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1024MAK
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Re: BBC PSU problem?

Postby 1024MAK » Mon Jul 17, 2017 8:49 pm

Yes, use a suitable lamp as a test load, with the PSU disconnected from the Beeb motherboard.
You only need to load the +5V rail. As the Beeb motherboard does not use the +12V.

That way, it is possible to work out if you have a PSU that needs further attention, or a short circuit on the Beeb motherboard.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...

rharper
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Re: BBC PSU problem?

Postby rharper » Mon Jul 17, 2017 8:57 pm

hoglet wrote:Next thing to replace would be C9, which can cause this exact symptom.

If that doesn't help then it's probably the chopper transistor.

Dave

Replaced the C9 and now all is working fine.
Thanks to everyone for the suggestions.
I have had several C1 & C2's go but never a C9 so I did not bother to do it at the first attempt.
Probably best to do all 3 so as not to waste time later!
Thanks,
Ray :D
Raycomp

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1024MAK
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Re: BBC PSU problem?

Postby 1024MAK » Mon Jul 17, 2017 9:03 pm

Well then, good that it's fixed :D =D>

As C9 is a known cause, it's recommended to renew it when a PSU is serviced regardless of the condition.

Mark
For a "Complete BBC Games Archive" visit www.bbcmicro.co.uk NOW!
BeebWiki‬ - for answers to many questions...


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