Pye 1352 excessive current

[Michael Maurice]

One thing I’ve found is that replacing NKT... transistors with AC... never really works, they are not good substitutes. The originals are available at a price from various suppliers.

[poppydog]

I will try to answer some of the questions asked. The thermistor is right up against the heatsink that’s holding the output pair and is working. As stated previously the variable resistor has been replaced.
Im not using an adaptor or battery to power it, im using a bench power supply. I thought that disconnecting r19 would isolate the driver/output stages from the other stages just to see if the current still increases with the other stages out of circuit, and it does, putting r19 back in circuit adds an extra 3ma approx to the total current draw.
DC voltages are more or less as per the sheet
The emitter current as ukcol suggested measuring, read 9.74ma and does increase slowly.
VT5-B=0.322V E=0.151V C=4.75V
VT6-B=5.11V E=4.93V C=8.98V
VT7=B=4.71V E=4.87V C=0V
C24 the audio coupling cap was the one I replaced first as it was OC, the replacement went in and I had sound, that’s when I realised I had the high current, it was installed correctly and was the right value.
I have reinstalled the original nkt transistors, checked and double checked again, capacitor polarity/ values and checked values of the resistors I have replaced and everything is as it should be.
Current is still very high and sound is
tinny and distorted.
regards poppydog

[buggies]

Straw-clutching suggestions:-
I would lift one leg of both the input and output capacitors so neither can have any effect on the DC conditions and see if any change.

Possibly also try listening on headphones (I know you have tried another speaker) to see if the sound quality is any different.

Freezing spray on output transistors to confirm it is temperature effect.
Soldering iron (briefly) on thermistor to check it reduces current?

[poppydog]

Thanks for the reply Buggies, I will try your suggestions tomorrow. I am still a little reluctant to purchase freezer spray, its so expensive
Regards poppydog

[buggies]

Ouch - just looked at the prices of spray -I withdraw that suggestion.
I have been using a can from about 1987 so had no idea...

[OldTechFan96]

You can use a can of compressed air held upside down as a substitute for freeze spray.

I've bought 'air dusters' for a pound or so a can in some shops.

Edit: It can be found at Poundland. A 200ml can.

[poppydog]

Soldering iron close to thermistor (can’t touch it as it is in a plastic sleeve) and the current drops rapidly. Plugged in a small pair of headphones and sound is just as terrible. The input and output capacitors C24 and C27 have been taken out of circuit and it makes no difference to the excessive current being drawn, voltages more of less the same as well.
I’ve bought some freezer spray, if it helps sort this problem out then it will be worth every penny. Be here middle of week. At the price it is it should be sold on the optic.
regards poppydog

[buggies]

Capacitors proved innocent then.
I can't think why the sound should be bad - need some more opinions on that. Only test I would do is feed signal from volume control into another amplifier or soundcard to see what it sounds like before the transistor output stage is reached.
Seems like the thermistor is capable of doing its job. I wonder what the original correct positioning of it was. I suppose increasing the coupling between the thermistor and the heatsink(?) might help - heatsink compound/silicone grease?
Only a tiny squirt of freezer needed on a suspect component so the spray should last for years.

[poppydog]

Originally Posted by buggies

I wonder what the original correct positioning of it was.

The first pic is of the thermistor and the output pair before I got my hands on it, its close but not touching. I don't think anybody has ever been in here before me.



The second pic is after I have been in there.



I will try feeding the signal into another amp and see what that sounds like.

Regards
Poppydog

[Radio Wrangler]

B&Q sell large cans of freezer spray for freezing pipes!

Plumbers use it to isolate something to work on when there are reasons thy can't turn off the main or drain everything down. Seems to be the same as the electronics spray.

David

[buggies]

I have only seen the radiomuseum circuit which has a drawing in the top left corner. It seems to show VR1 some distance from the bracket. It also has a small drawing above that, showing the output transistors, and on that it looks like only VT7 has a heatsink and this transistor is in a flat package(?).
I have not seen this asymmetrical arrangement before so perhaps I should bow out at this point and become an observer.

[ms660]

Is the blue electrolytic capacitor fitted the correct way round?

Lawrence.

[poppydog]

No its not the right way round, but it was refitted like that earlier on this afternoon after id taken it out of circuit, it wasnt originaly fitted like that. I have replaced it with another 1, the right way round, didnt/doesnt seem to have upset anything. Its been a long week
Regards poppydog

[poppydog]

The freezer spray has arrived, however its not made things any easier/clearer. The output pair got froze first, the current dropped to around 11ma but rose again fairly quickly, the driver was the one that made the real difference. It dropped right down to 8ma so I thought I had found the culprit, it was removed and I was convinced even more when I saw there was grey grease underneath the case. I put a ac121 in and was disappointed that it was no different. I am getting nowhere fast so im going to pull the plug on this one and leave it.
Regards poppydog

[ms660]

All the freezer spray is doing is what it would do to any other conductor or semiconductor,

Freezer is good for helping to pinpoint components that go faulty in a S/C, O/C, breaking down/crackling (noisy) sort of way etc due to thermal expansion and not much else.

Lawrence.

[poppydog]

Which is why it was recommended I get some early on in the thread. Perhaps you should have educated me/others earlier in the thread.

[ms660]

You never indicated that you were going to buy any....Only that you had actually got some....

The drift might be normal, personally I would try adjusting the quiescent current after things have settled down a bit, make sure those resistors you've replaced are connected properly and are of the correct value, especially the output pair emitter resistors and bias resistors if they've been replaced.

Lawrence.

[trickie_dickie]

Sorry to butt in here, but have you tried shorting out R20 47r and seeing what effect that has ? Resetting the Bias pot as necessary.

[poppydog]

I take it you mean bridge with another resistor?
regards poppydog

[ms660]

My guess is he meant shorting it out and measure the quiescent current flowing through the output pair.

Lawrence.