Hello! I am trying to repair Grundig audio amplifier. I changed all transistors, as i had short circuits and improper voltages. I coudn't get all transistors same, so final stage got bd911 and bd912, and instead of bc548c i have b version.Now, i get expected -0.6 at t9;t12 and t8;t11 emiters. But at t14 and t15 collector when turning on it rises from 0.6v to 9v and those transistors gets hot. Dc voltage at output of amplifier is ok.
So my question is- where is fault? Is problem in unsuitable transistors? Or there is another fault in circuit?

 

Welcome to AAC!

You replaced all the transistors without knowing if they were good or bad?
That is not how I would attempt to fix any electronic equipment. I try to determine which area of the unit is malfunctioning and then I would try to isolate the problem to a single faulty component.

The test equipment of choice would be an oscilloscope. Next item would be a sinewave signal generator.
If you do not have access to such test equipment then you can resort to the old school trouble shooting technique using a signal injector and a signal tracer. This can simply be a music source and a powered computer speaker which are both easy to find.

The bottom line is there are 200-500 components shown in that diagram and any single one could be the culprit. It is your task to try and locate and isolate the bad component, not by randomly replacing parts.

 

Model is v4200. I made decision to replace all transistors based on two facts-their cost, and previos experience with soviet electronics(i believe its just chain reaction due to design).
I found bad transistors in both channels, and as they work as team in my mind, i just replaced them.

 

I did a bit of work on amplifier. Found burnt resistors, replaced. Now i get expected voltages, but still those two tranistors T14 and T15 gets pretty hot(too hot for finger to hold). Amplifier is working, havent checked in longer period as being affraid to burn something again. Any ideas?

 

There's something that's confusing me here with this schematic. T407 is a PNP and the emitter is going to positive. T408 is an NPN and the emitter is going to negative. These seem correct. T409 is an NPN but the emitter is going to positive. T411 is a PNP but the emitter is going to negative. Am I missing something here?