The service manual states that the turntable has a supply voltage of 9v, but upon measurement it's sitting at 9.7v. A little high for my tastes... and I'd be fearful of this causing damage over time. It also states that this rail can be adjusted by turning R472, which is a 1k ohm trimpot. It was replaced somewhere down the line with a modern multiturn Bourns type. I started off making small adjustments to it, but this is actually increasing the supply voltage! So I turned it back to as low as it can go (measuring the resistance in-circuit it appears to be at 0 ohms...) and the lowest voltage I've achieved is again 9.7v.

I'm guessing there is an issue elsewhere - but I don't have an idea of where to start to troubleshoot this. If someone could point me in the right direction, it would be greatly appreciated.

Service manual with schematic can be found here (I would add this as an attachment but I'm unsure if this would violate any site rules)

 

You need to register for service manual s.

Take pictures of the circuit board.

 

You need to register for service manual s.

Take pictures of the circuit board.

Hopefully these will be helpful. I've also attached the service manual + schematic.

 

Please measure the base, collector and emitter voltages of TS441.

 

Please measure the base, collector and emitter voltages of TS441.

Emitter: -0.82v
Base: -0.73v
Collector: +5.27v

Looks like the collector is 2v lower than it should be... would this be a fault of that transistor, or something else?

 

Am I correct in saying that the blue dot on these germanium transistors indicates the collector lead? If so I think whoever last worked on it changed the transistor and installed it the wrong way round. The resistor that you see immediately beside one of the legs of the transistor is R583, which is meant to be directly connected with the emitter lead.

 

I can't remember blue dots but below is the view from underneath the transistor.

 

Looks like the collector is 2v lower than it should be... would this be a fault of that transistor, or something else?

I expect that is just because the output voltage is wrong.
Measure the voltage across D458 - should be 7.5V.

 

I expect that is just because the output voltage is wrong.
Measure the voltage across D458 - should be 7.5V.

I'm getting exactly 9.0v across D458.

 

I'm getting exactly 9.0v across D458.

So that's why the output voltage is too big.
Can you get hold of a BZX79C7V5?

 

Hello,

Here is a nice page with the transistor data:
https://sound-au.com/trans.htm

Bertus

 

So that's why the output voltage is too big.
Can you get hold of a BZX79C7V5?

Would this be suitable?

 

Would this be suitable?

The very thing.

 

Hello,

Here is a nice page with the transistor data:
https://sound-au.com/trans.htm

Bertus

Do you know about the colour dot?

 

Ah! The good old AC127. Brings back memories.
I have a copy of Philips Transistors Theory and Applications if there is any technical information that you need.

 

Blue dot marks the collector.

 

Do you know about the colour dot?

Yes the dot marks the Collector for these germanium types, if it's got 4 legs the extra one is a Shield..

 

Yes the dot marks the Collector for these germanium types, if it's got 4 legs the extra one is a Shield..

I think it's strange, but I don't remeber those dots at all.

 

Blue dot marks the collector.

Do you remember these babies with 4 pins?

 

Oh, yes. Along with 114 and 117.
And converting an OC71 into a phototransistor with the help of a spin drier. Oh, what fun we had.