UK Vintage Radio Repair and Restoration Powered By Google Custom Search Vintage Radio and TV Service Data

Go Back   UK Vintage Radio Repair and Restoration Discussion Forum > Specific Vintage Equipment > Vintage Audio (record players, hi-fi etc)

Notices

Vintage Audio (record players, hi-fi etc) Amplifiers, speakers, gramophones and other audio equipment.

Closed Thread
 
Thread Tools
Old 15th Jan 2022, 8:07 pm   #1
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
Default Pye Black Box.

Hi Folks, for my sins I see quite a few of these amps for repair. The later ones using PCL83's are noted for their feedback instability, but the earlier EL 42 types are normally OK.

There appears to have been occasionally factory problems with the early triode connected EL42 types in that the cap across the tone control was sometimes increased from 0.04uF to 0.14uF. (This appeared to be a final test mod)

The later UL connected EL42 amps also had stability problems, which I recently cured by reducing the value of the resistor in series with the bottom end of the vol control from 39K to 18K.

In most cases these are ultrasonic oscillations at about 50 to 80KHz, easily visible on a scope and also increasing the output valve cathode voltage from about 17 to 19 volts. I'm sure this increased current then leads (with contributions from leaky caps) to transformer burn out

Ed
Ed_Dinning is online now  
Old 16th Jan 2022, 7:04 pm   #2
Techman
Dekatron
 
Techman's Avatar
 
Join Date: Nov 2009
Location: Lincolnshire, UK.
Posts: 4,985
Default Re: Pye Black Box.

I thought this rang a bell and you'd recently posted on this, so I just found it and added a link to it, as it's an interesting topic:-
https://www.vintage-radio.net/forum/...d.php?t=187281

In the PCL83 version I've replaced the four critical capacitors C6,7,9 & 10 @ 0.04uf (or is it 0.05uf in some models?) with 0.1uf - works well!
Techman is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 7:57 am.


All information and advice on this forum is subject to the WARNING AND DISCLAIMER located at https://www.vintage-radio.net/rules.html.
Failure to heed this warning may result in death or serious injury to yourself and/or others.


Powered by vBulletin®
Copyright ©2000 - 2024, vBulletin Solutions, Inc.
Copyright ©2002 - 2023, Paul Stenning.