Pye black box PCL83.

[Kennoone]

Hello all, I've just managed to get hold of a Pye black box, it's the plc83 version with the 4 speakers, I've replaced all the hunts paper capacitors and I'm in the process of replacing the resistors purely as a matter of course, I've also replaced the metal rectifier, I'm getting 311v DC out to T2, I've measured 14.65v AC on the pentodes from T1, I've connected everything up but only getting the slightest low level hum out of speakers, this doesn't change when I increase the volume
, What output should I be getting out of T2, I'm getting 311V DC on all, I'm a complete novice at all this so any help would be really appreciated, thanks in advance

[Station X]

What DC voltages are you seeing on the anodes, screen grids, control grids and cathodes of the pentodes?

[Heatercathodeshort]

Did it work even poorly before you did the mass change of capacitors?
Please don't replace the resistors. They are probably all within tolerance and you will get in a right mess changing so many components unnecessarily.

The PCL83 version is difficult to work on and very easy to make mistakes when replacing components. The little chassis is very cramped. It needs to be worked back from the push pull output stage back to the pick up input socket.

Grahams suggestion is a good starter but treat the voltage readings as a general guide especially if using a DMM. Good luck with it. John.

[Ed_Dinning]

Hi Ken, known problems on this amp with poor valve holder sockets and changing can be a pain, much better to swap a complete pin if possible
Anode and screen voltages should be similar on each side, output transformer often suffers a burnt out section. This can be caused by instability in the feedback circuit, often evidenced by background noise and higher than normal currents being drawn.
Check with a scope on the output for this if you have one

Ed

[Kennoone]

Thanks for the heads up, I've only changed a couple of resistors but all the hunts capacitors, I haven't got a scope so will have to rely on a dmm I'm afraid, I'm awaiting delivery of another black box, same amp so I have something to cross reference against, on another note I can't find any evidence of r30, some kind of resistance wire to the speakers in series with the 25uf capacitor any idea what could be used in place, thanks

[Station X]

Circuit here:-

https://www.vintage-radio.net/forum/...1&postcount=11

and here:-

https://www.radiomuseum.org/r/pye_black_box_bba.html

[Edward Huggins]

This is not an easy amplifier to work on. I would advise against replacing all the low voltage caps en-bloc in future, even if eventually you do need to change them all. In this circuit, do make sure the resistors are as close tolerance as posible - minimum 5%. This circuit is very quirky and component sensiitive!

[Michael Maurice]

There is a hunts capacitor that is ‘hidden’ from view and is under the tag strip. Did you replace that one as well?

[Ed_Dinning]

Hi Ken, the hidden resistor is a piece of resistance wire inside , i think, red sleeving and is part of the current feedback circuit. It is run close to the chassis

Ed

[lesmw0sec]

Originally Posted by Station X

Circuit here:-

https://www.vintage-radio.net/forum/...1&postcount=11

and here:-

https://www.radiomuseum.org/r/pye_black_box_bba.html

Out of curiosity I took a gander at that circuit - what a frightening combination of feedback paths...

[Half a Mullard]

It's certainly not the easiest chassis to work on. The critical resistors are R13 and R14 which must be a matched pair. The actual value itself isn't so critical but they must be very near the same. I would also agree to keep component changing to a minimum as work progresses (been there, got the T shirt) although I appreciate this has been done. As others have mentioned the feedback arrangement is complex and critical to the stability of the amplifier. Good luck!