Pye 1352 excessive current

[poppydog]

I have been having real trouble setting the bias on my Pye 1352. It should draw around 12ma but starts out at 14ma and rises within 5mins to 21ma at idle. It’s had all the electrolytics replaced as they were all way out of spec and I had hoped that that would have sorted this but it’s only made a little difference, several out of spec resistors were replaced as well. At first I thought it may have something to do with the fact that is has some af117s in there so I lifted r19 so isolating the driver and output stages but the current only dropped around 3ma. The output pair were warm to the touch so I replaced them and the driver transistor in the hope that 1 or all transistors were leaky but they are still warm. Voltage readings were taken before and after replacement and they were fine.
If I try to adjust the emitter voltage to get 4.85v at the junction of r23/24 I get odd readings ie- if I adjust the 5k variable resistor (which I have now also replaced) to 4.50v the current increases dramatically , adjust it back to 4.85v and the current comes back down but not to the specified current. Clutching at straws now (inexperience), I replaced the rest of the resistors around the output stage and its not made any difference. I have checked and double checked I have got the elec caps the correct polarity/values and have checked the resistors are of the correct value and for dry joints/ bridging.
regards poppydog

[Boater Sam]

One vital piece of info missing,,,,,,does it work?

[poppydog]

Yes it does Sam, it works very well, however the sound is a little sharp and ever so slightly distorted

[Boater Sam]

I don't know the set and have no details of it.

Is there an output transformer that could have shorted turns?

Are there any resistors whose value is changing due to warming up?

You say the quiescent current increases with time, so its thermal. Try a cooling spray on selected parts and see what happens.

[Cobaltblue]

Is the Thermistor VR1 in good condition?

Often they disintegrate, are the heatsink clips fitted to the output transistors which are hidden under the socket plate.

Cheers

Mike T

[buggies]

I don't know it either but a line of text on the radio museum site says:-

"Output is "matched" complementary push pull with a thermistor to stabilise bias.
DC feedback to driver with bias preset."

Don't know if the thermistor should be in free air to work from ambient or coupled to the output pair?

[poppydog]

As stated previously the resistors around the output/driver stage have been replaced (with good quality Vishay ones). The current increase from when first switched on is very quick, it goes from 14ma up to around 21ma very quickly then I switch it off.

The thermistor vr1 lifted measures approx. 240r, held between my fingers the value decreases quite rapidly. The output pair are still in their heatsink, when touched between fingers you can feel they are warm not hot, they get warm very quickly.

Here is a snippet from the driver/output stage.



Regards



Poppydog

[ajgriff]

Does the set still have its original 25 Ohm speaker?

Alan

[poppydog]

Yes it does have its small original elac speaker, I have tried another in its place.

Transistor voltages taken quiescent state:
VT5 - B=0.290v, E=152.5mv, C=4.60v
VT6 - B=4.97v, E=4.79v, C=8.94v
VT7 - B=4.59v, E=4.74v, C=00.6mv

regards poppydog

[Herald1360]

Why not set the quiescent current after it's "warmed up"?

With the thermistor in circuit, does the current go down when you warm it?

Remember that even germanium transistors can get "ouch" hot ~70C before they are in danger.

[poppydog]

It has been adjusted after being on for a while and it doesn’t really make much difference. Very early this morning it was switched on and the current draw was around 12ma, E bias voltage was 4.93v.
Within 5 mins the current drawn was up to 18ma and the E bias voltage had dropped to 4.84v, after 10 mins the current is nearly up to 20ma and the bias voltage is back down to around 4.83v. which I think is acceptable apart from the current being drawn. Surely the output pair shouldn’t be warm after 10 mins of no signal etc? I dont have any other small radios that will draw nearly 8-9ma more than their stated quiescent after being on for 10 mins, most might draw a 1/2 ma extra at most after warm up. There is something not right.
regards poppydog

[poppydog]

Warming the thermistor slows the current increase but doesnt stop it

[brightsparkey]

What an odd circuit! the bias for the base of the output stage relies on the speaker being connected... peculiar.

Have you checked that the trimmer pot is working OK? If the wiper wasn't contacting reliably then the bias current would be upset. Just a guess.

Have you checked that the DC is right everywhere, particularly the output voltage. The DC should give it away..

good luck,

Kevin.

[jonnybear]

Are you using a 9 volt battery or some other means?. if you are using an adapter remember these sets were never designed for use other than with a battery, so would need better smoothing, I would not disconnect R19 as this could alter biasing of VT5.
Good luck
John

[ukcol]

As Mike T says, the first suspect for this symptom after replacing the output transistors is the thermistor VR1. It sounds from your tests that you have a good one but George's point is worth following up. If possible arrange that the thermistor can sense the temperature of one of the output transistors. A blob of heat sink compound between the transistor and the thermistor will do the job. Make sure though that the thermistor is electrically isolated from the transistor, you want a good thermal path but you don't want an electrical path.

What did you replace the original NKT output transistors with?
AC128 and AC127 would be suitable substitutes. AC128 for the PNP TR6 and AC127 for the NPN TR7. It is desirable that the hfe of the output pair should be similar to keep distortion to a minimum.

The quiescent current of the output pair should be about 8mA.
This set doesn't have a set Q-current pot.
Set the mid-point balance pot (RV2) as in the manual. Connect your current meter in series with the collector of TR6 or TR7 (not the emitter) and note the Q current after the set has been on a few minutes. If it is a bit high (with your replacement transistors) try reducing the value of R20 and/or R21. Germanium output transistors are very sensitive to thermal effects so make sure your replacements are not leaky and make sure all is well with the heat sinks.

Finally, run the set for a short while with the volume up and make sure the output stage doesn't go into thermal run away. (A destructive increase in leakage current.)

[poppydog]

Thanks for the replies. Before I do anything else, these are the replacement transistors-
Driver- AC153Y
Output pair-AC187/AC151VII
Will post more replies tonight
regards poppydog

[brightsparkey]

Colin beat me to it, but I would say the thermistor should be in thermal contact with the output transistor cases to do its job.

Kevin.

[GeoffK]

The AC187, AC151 are not a good combination, the AC151 is an earlier driver transistor and much lower power than the AC187 which is an output transistor for capacitor coupling to the speaker in conjunction with its matched AC188.

[poppydog]

Not quite sure what was going on when I decided to install those transistor types... I have however replaced them with quite a well matched ac187/188 pair and its worse Current rises even higher and quicker so I need to go over everything I have done again, although it was like this in the very beginning after the replacement of the audio coupling cap in this thread.
https://www.vintage-radio.net/forum/...d.php?t=149362

Thanks for all the replies.
regards poppydog

[ukcol]

Originally Posted by poppydog

..........although it was like this in the very beginning after the replacement of the audio coupling cap......

Which audio coupling capacitor did you change? If it was C24 and the replacement is leaky or connected the wrong way round it will upset the DC conditions in the output stage.