Polytone Baby Taurus - Bad Hum and High Current

[misterc57]

Unlike a tube amp, a SS amp doesn't need a load (for stability or OT protection).
Troubleshooting a SS amp can and should be done without a load. You need to find and eliminate the root cause for the DCV offset.

Ok. Tried again without the load. Seeing the negative DCV on the output.

So, what would cause this, and does the offset being negative instead of positive mean anything?

Is this caused by the output transistors not matched or one of them not working?

Thank you

[Helmholtz]

Ok. Tried again without the load. Seeing the negative DCV on the output.

So, what would cause this, and does the offset being negative instead of positive mean anything?

It may indicate a shorted semiconductor in the lower half of the amp.

[g1]

Have you replaced that dual transistor with it's head blown off? (post #9)

[Jon Snell]

As I wrote in post 7, mains down the input line ... not good news.
Just replacing the resistor will not repair a damaged amplifier.
The dual transistor has a hole in it with a bad headache no doubt, (2SC1583), and the coupling capacitor will be suspect. So as g1 suggests, replace that and then look for other issues.
With the differential front end damaged, there will be no control over the DC offset.

[misterc57]

I can follow a tube signal path and understand most of what is going on. SS is another story. I hope to learn a lot from this one. I want to understand what each of the transistors in this circuit is doing. I am reading up on anything I can find as time permits.

Is the function of 2SC1583 and the other transistors to amplify the signal?

As far as replacing 2SC1583, I saw one source posted here and I see many on eBay. I wonder if I will find one in my parts bin. Does not look like this is available on mouser or digikey. I suppose this dual transistor can not be replaced with two NPN transistors having the emitter leg joined?

Thank you all for your comments and patience as I learn. You all are very helpful.

MC

[g1]

I suppose this dual transistor can not be replaced with two NPN transistors having the emitter leg joined?

Yes you can using 2N5210's (which are standard single transistors), as shown on schematic and mentioned in the notes.

[misterc57]

Good news, the amp is working now!

I installed a pair of 2N5210. Managed to squeeze both emittter legs into the same hole.

No hum, no DC offset. Parts replaced were R1, Q1 and Q1A.

The amp pops when turned or or off, as many do. I have yet to put the reverb back in and test that function. Will get to that tomorrow.

Thank you to everyone for all the help and guidance. Learned a bit on this one.

MC