Sony TA-1150 short circuit tracing

[James Welsh]

How do I figure out where the short is?

I’ve been working on the output board of a SONY TA-1150.
I replaced:
- all the transistors
- capacitors
- main filter caps
- bias trim pots
- Emitter resistors
- Bridge rectifier
- fuse holders

I was trying to set the bias using the Variac I had dialed down the trimmer so 0 mV was showing.

I slowly turned up the voltage checking by touch to see if anything was getting hot.

All of a sudden a tiny puff of “magic smoke” and a blown fuse.

I suppose I should have had the DBT in series with the variac. Because obviously the short was there all along.

If I plug in the output board the dim bulb tester lights up and stays bright.

If I disconnect the output board the bulb does not light up.

I pulled all the power transistors and one NPN was bad. So I replaced that.

I have attached a schematic if that helps.

Thanks!