1970's Caliphone record player amp

[GeorgeRay]

Greetings : I am fixing up a old record player. Got all the mechanical devices working properly. When played it sounds great but the tone would degrade after a few minutes. I Found a transistor (2N6124) was getting very hot and it's sister (2N6121) was warmer than the other channel after powering up for a couple minutes. I have attached the schematic and the transistors overheating are Q-11 and Q-12 at the output end. I see no blown caps or other visual signs something is wrong. I plan on replacing the electrolytics but am wondering if anyone can suggest other possibilities. The last picture has the heat sinks removed. Finally, that dark matter on the corner of the board is odd but appears to be nothing (?)
Thanks everyone. Standing by.

George

 

[Nigel Goodwin]

Greetings : I am fixing up a old record player. Got all the mechanical devices working properly. When played it sounds great but the tone would degrade after a few minutes. I Found a transistor (2N6124) was getting very hot and it's sister (2N6121) was warmer than the other channel after powering up for a couple minutes. I have attached the schematic and the transistors overheating are Q-11 and Q-12 at the output end. I see no blown caps or other visual signs something is wrong. I plan on replacing the electrolytics but am wondering if anyone can suggest other possibilities. The last picture has the heat sinks removed. Finally, that dark matter on the corner of the board is odd but appears to be nothing (?)
Thanks everyone. Standing by.

George

First off, why are you planning changing all the electrolytics? - this is almost NEVER required, and seems to be a recent fad put about by those with no electronics knowledge or experience. You're far more likely to cause further damage than fix things, although as least this is a very simple design.

Secondly - it's a really cheap and nasty design, a transistor version of the cheap and nasty single valve record players back in the 60's It relies entirely on an excessively high output crystal cartridge.

However, as Q11 and Q12 are getting hot, check the quiescent current - measure the voltage from Q11 emitter (positive) to Q12 emitter (negative) - across R50/R49. And compare it to the other channel. Also, check the voltage at the speaker outputs with the speakers unplugged - both should be very close to zero (well under a volt, positive or negative).

Assuming the speaker voltages are both OK, and quiescent current is high, it's actually set by D6 and R48 (which again is a cheap and nasty method of doing it). You could try reducing R48, but generally the diode needs replacing - which is difficult, as it's a very special and specific type of diode, and unlikely to be available. 'Back in the day' when repairing similar such record players we used to get the exact replacement part from the manufacturer of the record player - there may well even be specific versions of the diode, and you need the exact correct one.

If the speaker voltages are wrong, you have more serious issues, and should probably replace Q9, Q10, Q11, Q12 and D6 - and check the resistors while the transistors are out. Q10 is probably as hard to source as D6 though.