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

Jules Richardson 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
>> 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.

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 
not :-)

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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