Distortions in PPP Power Amp (FM/AM Radio TFK Bajazzo TS 301)

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tiefbassuebertr

tiefbassuebertr

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Joined 2005
tiefbassuebertragung.de
#1
This radio I check for a friend. Sounds great.
Service-Manual is here.
TELEFUNKEN BAJAZZO TS 301 SM Service Manual download, schematics, eeprom, repair info for electronics experts
The actually issue is that the rechargeable batteries are too fast empty and in mains operation a loud hum is present.

The cause was quickly found - the quiescent current through the output transistors was set to almost maximum (90mA).

Turning the quiescent current back to the value shown in the circuit diagram (only 5mA) will make the distortion extremely audible (clearly audible below 20mA).
The output transformer does not have an independent winding for the loudspeaker - so it can be assumed that it is a PPP.


From this I conclude that there is a mistake. All measured voltage values ​​are according of those of the schematic diagram.

Where should I start with troubleshooting?

Unfortunately, with germanium transistors and PPP transformers, I have no experience regarding typical errors and also no stock for quick replacement.

Maybe I should buy a defective device of this model, which has a non-failed error and after its repair, different parts interchangeably until the error migrates to the other device.

Maybe someone has another good idea - thanks in advance for advices.
 

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#3
Since the issues seems limited to the OP stage, the number of possible faults/faulty components is rather limited.

My guess is that pushing up the quiescent current drives the OP into SE class A, because one half of the PP doesn't operate.

This could be confirmed by driving the amp section with a signal generator with a level sufficient to reach the ~max theoretical output power (which must be quite substantial given the type of OP transistors) and observing the output signal: if one half is ~missing, the problem is there. Could be one of the transistors, the wire of the driver transformer broken, etc

Just a reasonable guess, but be prepared for any eventuality, incluuding something completely different
 
#6
 
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