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.

 

[GeorgeRay]

Nigel. I read 4.6 at the speakers and while I was testing that, which was pretty darn quick since I had my test leads already connected to one channel, a resistor (R62) started smoking. I powered it down immediately. I may sit on this project for a while since the limited research I did on diode replacement looked bleak, like you said, and it looks like the damage is deep.

 

[Nigel Goodwin]

I can't see R62 on the schematic? - or is it just me?.

Anyway, 4.6V is bad - but at least it 'may' mean D6 is OK - but you would really need to change all the transistors I mentioned above, and Q10 could be difficult?. While the other transistors shouldn't be critical, the use of D6 means that it could well be, and using different output transistors could mess up the quiescent current.

If you do choose to do it, I would change C29, C32 and C30, just in case.

 

[augustinetez]

No, I can't see R62 either (nor 60 or 61).

 

[starLED]

On a board view there is R62.

 

[Nigel Goodwin]

This reminds me of a batch of cheap stereo record players we sold back in the early 70's

They were actually made by a small furniture company - as they were contracted to make the (very cheap) wooden case and speaker boxes for a third party who designed them, but who unfortunately went bust before they were delivered or paid for.

This left the furniture company in deep trouble, so they decided to manufacturer record players them selves, and presumably had some one design and build the amplifier boards for them.

The systems were sold to the trade incredibly cheaply (and retailed cheaply as well), so we ordered 100 of them - I presume you got a good price break for 100, or perhaps they only sold them in 100's?. The system was called a 'Style 270'.

Needless to say, no spares or service information was available, and been so cheaply made weren't reliable - so I had to draw the diagram out, and it was still stuck on the radio workshop wall until the workshop was closed in the 2010's

To be fair, this one seems better than those - as they only used AC187K/188K's for the outputs, so probably about 3W - Germanium days as well.

 

[starLED]

Needless to say, no spares or service information was available, and been so cheaply made weren't reliable - so I had to draw the diagram out, and it was still stuck on the radio workshop wall until the workshop was closed in the 2010's

What's different today?

Except, instead of stereo players, it's TV's. today.

 

[GeorgeRay]

Update: Thanks to all for responding and I need to make this update quick. I was able to locate the final transistors but not the diodes (did not order anything yet). And I have not looked into Q10 yet. I want to add some information that came to me after my first post. When I got the mechanical issues ironed out and fired up the record player (Beatles VI) the sound was great but it seemed very loud even at a low volume setting. It was shortly thereafter I notice the sound quality degrade and shut the whole thing down to look further into the board. Sunday night I took a look at the cartridge and discovered someone hacked in a replacement that had exposed wires. was not wired properly and had the potential to short. I fixed that problem...