[BBC-Micro] [OT-ish] A3020 won't turn on

Robin Commander robincommander at blueyonder.co.uk
Wed Dec 6 10:44:50 GMT 2006


Hi,

Dried up caps tend to have a high internal resistance. This leads to
internal heating, which long term just accelerates the decline of the part.
They can sometimes short but usually you'll get smoke and blown fuses.

There's a measurement known as effective series resistance (ESR) which is
sometimes the only way to say if a cap is u/s. In many cases a cap will show
the correct capacitance value when tested but can be completely useless in
circuit. Great device for testing by comparison (with a known good device)
is this one: http://www.peakelec.co.uk/acatalog/jz_esr60.html though as most
caps are inexpensive it's often easier to just blanket replace anything
that's suspect. At this age - everything ;-)

On a linear psu with a dried out cap, if you remove the load you'll normally
see something close to the expected voltage, but this will drop and there'll
be AC ripple present as soon as you reconnect the load. Whilst the rectifier
diodes are non conductive (as they will be for part of an AC cycle) it's the
reservoir capacitor which keeps the output voltage present by supplying
energy stored in the capacitor.

I'll probably not sleep until I've got this straight in my head :-) What's
the number printed on the top of the integrated cicuit, near to the toroid ?

Regards,
Robin


-----Original Message-----
From: bbc-micro-bounces+robincommander=blueyonder.co.uk at lists.cloud9.co.uk
[mailto:bbc-micro-bounces+robincommander=blueyonder.co.uk at lists.cloud9.co.uk
] On Behalf Of Richard Gellman
Sent: 06 December 2006 10:16
To: bbc-micro at lists.cloud9.co.uk
Subject: Re: [BBC-Micro] [OT-ish] A3020 won't turn on

Hi,

> I was curious so went looking for some info on the web and found this
> picture : http://acorn.chriswhy.co.uk/Pics/A3020C-9.html
>
> The PSU is built into the main pcb on the left hand side. The circuitry on
> the left hand bottom corner does look like it is a switched mode power
> supply. I can't see the number on the top of the integrated circuit there
> as
> the picture isn't close up enough but if you google the part number you
> can
> confirm what it's doing from the manufacturer info.

I'm inclined to disagree, but it would be the short-sighted leading the
blind. My reasoning being the apparent simplicity of the circuit and the
presence of 4 discrete rectifier diodes adjacent to the transformer. The
transformer itself also seems to have 5 output lines, which I imagine to
be +12V, +5V, 0V, -5V, -12V (even though -5V is not used).

> It's quite unusual to have a transformer along with a switcher, normally
> switchers just rectify AC mains in and use a reservoir capacitor to
> provide
> a 400v DC feed which is chopped by the switcher. Possibly Acorn wanted to
> they drop the mains voltage down first then use the switcher to provide
> multiple outputs at high currents.
>
> Maybe the transformer is providing plus and minus 12v supplies and the
> switcher is providing (or maybe not in your case) a +5 volt supply.
>
> My money's still on a dried up capacitor though if there's no blown fuses
> or
> smoke.

If it were a linear supply, I would expect the output voltage to still be
around 5/12V, but with more ripple. Or does a dry capacitor have a
shorting out effect? This would cause a current drain possibly leading to
a weird voltage level.

On the up side, I've managed to bodge a PC ATX PSU into running it, so the
the onboard PSU is completely unrepairable, the machine is still usuable
with the application of a suitable replacement PSU.


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