[BBC-Micro] [OT-ish] A3020 won't turn on
julesrichardsonuk at yahoo.co.uk
Wed Dec 6 15:37:09 GMT 2006
Richard Gellman wrote:
> 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...
Yep, that's probably just the meter picking up noise...
>> 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
>> 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.
Hmm, you made me go dig out my machine... right you are (on the A3010 that is
a heatsink with a regulator IC bolted to it), and it does look to be a linear
PSU. Surprising on a machine so recent (relatively), I would think.
There's a L4974A regulator IC for the +5V rail, driven via a diode bridge
(D5/D6/D7/D8) and capacitor (probably C200) for the smoothing.
The +12V and -12V rails are totally unregulated and use diodes D1/D2/D3/D4 for
rectification (and probably capacitor C201 for initial smoothing); they're
fused via FS2 and FS3.
The datasheet for the L4974A is online if needed. As a start, I'd measure the
DC voltage at pin 11 (relative to the 0V contact pad on the PCB) as that
should be the unregulated DC from the transformer. On my A3020 it's +26V,
although it'll vary a little between machines. If you've got 0V there then
there's something up with the rectifiers / transformer / smoothing cap /
solder joints / power switch (as you've tested fuses you know there's not a
dead short within the chip or elsewhere as the main fuse would have gone)
Pin 13 of the chip is the voltage reference, and should be a little over +5V.
Pin 20 is the output, and should also be around +5V, but you already know it's
Don't rule out bad solder joints to the transformer; it's a big heavy lump to
mount on a PCB and no doubt introduces all sorts of thermal stresses as it
warms up / cools. Unfortunately separating the PCB from all that shielding is
a right pig (hence my 'probably' regarding which smoothing caps are which
above as I didn't bother!), but it may be worth you checking.
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