Amiga 1200 12v Short Problem

[piggz]

Hi


I was having trouble with my A1200 and ended up making it worse. I had a problem with a 12v line and audio ... i think the +12v line was reading a -ve voltage.


Anyway, what happened next made it worse, I decided to try my A500 PSU. On switch on it instantly blew up.


Now, on the A1200, diagrom will boot up, but a normal kickstart wont, and ive tried 2.


The interesting thing, is that the + and - 12v lines are shorted together, not to ground, and check of the voltage is around 0 on either line.


I removed the coil at the start of the circuit, as thats pretty much all I can see that is common to them, but the short persists.


Any ideas what might be wrong, what to check, or what to replace?


Thanks


Adam

[Daedalus]

There are a couple of possibilities: U28 is the serial driver, and uses the +/-12V rails for power. However, they shouldn't be a dead short if that is the issue as both rails are passed through diodes, so you should see various results depending on your meter. Similarly, the audio op amps (U15) uses both rails, but both rails are also connected via resistors, so there would be a 20 ohm resistance between the rails if the op amp shorted out.

Probably more likely is a short on the motherboard itself. Near the points mentioned above, both rails run relatively close to each other and also pass through vias that are commonly corroded by capacitor leakage. It might be worth checking those areas for any sign of greasy residue or corrosion, and clean it up if necessary. It's a more common fault on the A600 which has the rails closer together, but not impossible on the 1200.

[solarmon]

Which revision is it. Check on amigapcb.org to see where those two power rails are close to each other, or which components are connected to both.

Check C803 and C802 (on bottom of board) which have pads on those rails right next to each other.

[pandy71]

You need to inspect every component connected to +12 and -12 lines - schematics on desk and multimeter with buzzer... you may try to isolate place of issue by unsoldering suspicious elements and verifying this elements off the PCB.

[cpiac64]

remove lf347, 1488 and 1489.

[piggz]

Thanks all, ill aim to do this.

Its a rev 1D.4, which isnt on the explorer, but i guess it pretty close to 1D.1 ? I made a crap job of C303 and C304 which are beside U15, so ill initially concentrate there

[Lamaman71]

I agree with Pandy71 - get those 12v lines checked out.