Date   : Wed, 06 Dec 2006 13:52:11 -0000 (GMT)
From   : splodge@... (Richard Gellman)
Subject: [OT-ish] A3020 won't turn on

Hi,

> The two most common culprits in any failure though are dried-out filter
> caps
> (usually on the mains side) and a blown chopper transistor. However, a
> shorted
> chopper should take out fuses too unless there's some sort of additional
> failsafe circuitry present (such as fusable resistors). I've seen input
> rectifier diodes go too, and also faults in any thermal protection
> circuitry
> (a typical 'home' machine may be built to a price and not have that,
> though!)
>
> If you've got a multimeter, check for shorted caps, shorted chopper
> transistor, and the presence of rectified mains downstream of the diodes
> as a
> start. As someone else said though, caps often fail high-ESR rather than
> shorted, and you'll need an ESR meter to detect that (or just do some
> random
> replacement of most likely culprits).

I don't know if this'll help at all, but putting a voltmeter on the 5 tap
outputs of the transformer read as 0.01 V. This may be a red herring
however, as I forgot to change my multimeter from DC to AC reading...

> I don't have a photo from the right angle handy, but in the one that I
> have it
> looks like there's probably a chopper transistor mounted on the heatsink
> which
> separates the transformer from the bulk of the machine.

Negative. There are no heatsinks in the whole machine. The metal upright
to the right of the transformer is merely a seperator. In its normal
configuration the whole box is shrouded in metal, save for the podule area
at the back, and a small area for adding extra RAM under the keyboard.

> Typically though this transformer's pretty
> small (compared to a linear supply) - I'm still surprised at how large the
> one
> is for the A3020.

This is what led me to believe it was a linear PSU.

>> 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.
>
> Hmm, so you know the machine's OK. Note that switchers will often do odd
> things if there's no load present (even tear themselves to bits in some
> designs :) - so it's worth checking for any dead fuses between the PSU and
> the
> computer itself (just in case you're feeding the PC's PSU lines in
> downstream
> of any damaged fuses).

There are only five fuses in the A3020, one on the mains line, and 4 on
the other side of the transformer. All five fuses are intact.

Cheers.

-- Richard