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 > General Vintage Technology > Components and Circuits

Notices

Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

Closed Thread
 
Thread Tools
Old 22nd May 2019, 7:47 pm   #1
John10b
Nonode
 
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
Default Output transformer on Blackbox

I’m doing a bit of background reading in preparation for restoring a Pye Backbox.
I keep coming across reference to the output transformer being “fragile “ and problematic. Could you tell me why this should be? Many thanks.
Cheers
John
John10b is offline  
Old 22nd May 2019, 8:11 pm   #2
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
Default Re: Output transformer on Blackbox

Maybe they were borderline rating. As the amplifier ages and the coupling capacitors start to leak, the output valves get stressed and draw more current. After a while the transformer will just fail on the primary winding.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 22nd May 2019, 8:31 pm   #3
G6Tanuki
Dekatron
 
G6Tanuki's Avatar
 
Join Date: Apr 2012
Location: Wiltshire, UK.
Posts: 13,953
Default Re: Output transformer on Blackbox

I'd be more minded to think of classic "green spot" corrosion rotting through the windings.

If the OPT does fail, don't worry: you can easily/cheaply get a replacement transformer [with much-better insulation] from the likes of Hammond.

Alternatively, a 12-0-12v mains-transformer rated at 10W or so will work. So it's not "HiFi" rated but unless you've got traceably certified, historically-burned-in Golden Ears™ you won't notice the difference.
G6Tanuki is offline  
Old 22nd May 2019, 8:56 pm   #4
Ed_Dinning
Dekatron
 
Join Date: Jan 2004
Location: Newcastle upon Tyne, Tyne & Wear, UK.
Posts: 8,173
Default Re: Output transformer on Blackbox

Hi Gents, there were several different transformers used on the BB.
The early units using the EL42 were simply PP, later units were UL PP.
The later units were UL PP and used the PCL83.
They were all pitch covered to keep out moisture and I have not seen many with "green spot". The majority of them have died from excessive current caused by leaky caps; but, the feedback system (3 different loops) on the PCL83 types has been known to oscillate ultrasonically and cause the valves to draw high currents.
The anode winding on all these units were a bit marginal and subject to getting hot
Note that one of the units had a mains transformer with an unused 60v winding.

Ed
Ed_Dinning is offline  
Old 23rd May 2019, 10:56 am   #5
Sideband
Dekatron
 
Sideband's Avatar
 
Join Date: Oct 2003
Location: Croydon, Surrey, UK.
Posts: 7,549
Default Re: Output transformer on Blackbox

Quote:
Originally Posted by Ed_Dinning View Post
The majority of them have died from excessive current caused by leaky caps; but, the feedback system (3 different loops) on the PCL83 types has been known to oscillate ultrasonically and cause the valves to draw high currents.
Yes that's a good point about HF oscillation....I've had that on the PCL83 version. Fortunately the transformer survived.
__________________
There are lots of brilliant keyboard players and then there is Rick Wakeman.....
Sideband is offline  
Old 23rd May 2019, 2:03 pm   #6
John10b
Nonode
 
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
Default Re: Output transformer on Blackbox

Thank you all for your informative replies. I already had you in mind Ed if the transformer turns out to be faulty, but I’m jumping the gun, I won’t have the BB for a week or so, if the transformer is faulty I’ll consider my options.
Cheers
John
John10b is offline  
Old 30th Jun 2019, 7:56 pm   #7
John10b
Nonode
 
Join Date: Nov 2016
Location: Aberaeron, Ceredigion, Wales, UK.
Posts: 2,869
Default Re: Output transformer on Blackbox

Just an update. Fortunately the output transformer was ok, so far, so this thread can be closed.
Cheers
John
John10b is offline  
Closed Thread

Thread Tools



All times are GMT +1. The time now is 8:26 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.