Hi all
I have this electronic circuitry (pre-amp from the 70s) would like to find out which power (yellow) wire is the positive and which is the negative Thanks!

 

Welcome to AAC!

It would help a lot if you told us what is the device (make and model).
Also a larger view of the board will also help.

Having said that, the yellow wires are AC inputs going to four diodes in a bridge rectifier configuration.
There is no positive and negative polarity that you need to worry about.

 

It's an AC input.

 

Here's a picture of the board. It's the pre-amp from an old turtable from the 70's (a LLoyds). The transformer is shot, no voltage output. I would try to replace it with a DC power adapter, would it be possible to connect it after the bridge ? If so, is it possible to identify the polarity ?

 

I would try to replace it with another power adapter, but if the circuitry is indeed AC then I'll have to give up

If you can determine the DC voltage being generated from the rectified AC, you could apply an appropriate DC voltage to the bridge rectifier. Or bypass the rectifier and save the 2 diode drops.

 

Thanks. I though the transformer was DC and since didn't get any voltage I though it was shot. I went back to measure after the bridge and I'm getting 18V so it's actually working. I appreciate the help!

 

I though the transformer was DC and since didn't get any voltage I though it was shot.

Transformers don't work well with DC.

 

Measure the transformer on AC volts, it's probably 14v if you're getting 18V dc..

 

That’s not a transformer, it’s a shaded pole motor missing the rotor.