QED Vintage Amp ... - diyAudio
Go Back   Home > Forums > Amplifiers > Solid State

Solid State Talk all about solid state amplification.

Please consider donating to help us continue to serve you.

Ads on/off / Custom Title / More PMs / More album space / Advanced printing & mass image saving
Reply
 
Thread Tools Search this Thread
Old 5th July 2012, 12:39 AM   #1
diyAudio Member
 
Join Date: Jul 2012
Default QED Vintage Amp ...

Hi all

Anyone help me out with shematics for a QED A270 vintage 1983 ?

TIA Arfa
  Reply With Quote
Old 5th July 2012, 02:26 AM   #2
49 - for the 18th time - again!
diyAudio Member
 
c2cthomas's Avatar
 
Join Date: Oct 2006
Location: Near "Music City" (Nashville Tennessee)
That's a hard one Arfa - I looked around and all that I could find was info on the A240
__________________
DIY audio can be expensive but getting to see things go up in smoke - that's priceless!!!! ..... "whatever - call it brainfart of Mighty ZM"
  Reply With Quote
Old 5th July 2012, 03:29 PM   #3
diyAudio Member
 
Join Date: Apr 2010
Location: Coffs Harbour
According to #16 of this thread, A240 & A270 are similar amplifiers. It may be worth the chance of help if nothing else is available.
__________________
Ian
  Reply With Quote
Old 5th July 2012, 05:58 PM   #4
lohk is offline lohk  Europe
diyAudio Member
 
lohk's Avatar
 
Join Date: Apr 2001
Location: Northern Noricum, near the Limes
I do have the service manual for the A240 - you will also find a partial schematic here on diyaudio if you do a search. Do you need it to service the amp? Can you post details?

(I am often asked about schematics from british amps from the 80ies and 90ies - I get a little bit hesitant giving it all away without further infos about the individual project...)
__________________
/lohk/
  Reply With Quote
Old 6th July 2012, 01:56 AM   #5
diyAudio Member
 
Join Date: Jul 2012
Hi all. Thanks for the info. Yes, I need the schematics for a repair that's on the bench at the moment. It had a transistor about half way down the strip that was leaky all round. It looks like it's half of a long-tailed pair. I replaced it with an identical one, but that channel produces no audio. It has equal levels of signal to the working channel up to somewhere near the stage that had the bad transistor in, then it all disappears. There is no sign of any other component being faulty, and cold resistance checks between channels are pretty much identical. There are some slight discrepencies in voltages, but nothing really significant. I'm a bit suspicious about the relative pinouts of the original and replacement transistors. Although both are BC546's, I'm sure that I have in the past come across plastic TO92 BC series transistors where the emitter and collector leads are reversed. I could verify this if I had schematics. I'm loathe to try just reversing the replacement trnasistor, as I'm sure any of you that repair DC coupled amps are aware of the potential consequences of getting it wrong - if it's not already ... If you see what I mean ...

So if anyone has amanual. or anything close for another model in the series, I would greatly appreciate getting a squint.

TIA

Arfa
  Reply With Quote
Old 6th July 2012, 04:04 AM   #6
49 - for the 18th time - again!
diyAudio Member
 
c2cthomas's Avatar
 
Join Date: Oct 2006
Location: Near "Music City" (Nashville Tennessee)
Quote:
Originally Posted by Arfa Daily View Post
So if anyone has amanual. or anything close for another model in the series, I would greatly appreciate getting a squint.

TIA

Arfa
All that I have been able to find is owner manuals - and this old thread that has some information....

QED A240CD Blows Fuse on Powerup
__________________
DIY audio can be expensive but getting to see things go up in smoke - that's priceless!!!! ..... "whatever - call it brainfart of Mighty ZM"
  Reply With Quote
Old 6th July 2012, 10:45 AM   #7
lohk is offline lohk  Europe
diyAudio Member
 
lohk's Avatar
 
Join Date: Apr 2001
Location: Northern Noricum, near the Limes
I have scanned the pages what I found for a more or less complete service manual for the QED A240 Ser.2. It is scanned from copies from faxes, so do not expect a very good quality. But it should be enough for repairing purposes.
The PDF is too big to upload here, so please PM me for the complete one.

Additional here two images of the powerstage and the line buffer of the A240 (without components values):
Attached Images
File Type: gif QED_A240_pwr.gif (48.4 KB, 267 views)
File Type: gif QED_A240_line_buffer.gif (28.1 KB, 257 views)
__________________
/lohk/
  Reply With Quote
Old 9th July 2012, 03:45 PM   #8
diyAudio Member
 
Join Date: Jul 2012
Ok. In case it helps anyone in the future, the schematics for the A240 do seem to match the A270 in terms of both circuitry and component identifiers. My thanks to lohk for being kind enough to scan them and send them to me.

The problem turned out to be R33, 18k. It is one of a pair of 18ks, the other being R32 which feed a bias supply to the transistor in the bottom end of the longtailed pair Ta & b or Tc & d depending on which channel. This transistor is T4 in one channel, and T12 in the other. This bias supply comes from the small relay that is time delayed after switch on. I guess that it's an anti thump muting circuit.

The resistor was completely open, but showed no sign of distress at all, In fact it is quite a 'chunky' device compared to all the others in there, and I would guess it to be at least 0.6 W. Most probably a voltage stress thing, as it has around 76 volts across it when working. This is because it is fed via the relay from the +38 v rail, and the pair of diodes that it feeds to develop the 1.3v bias for T4 (12) are returned to the -38v rail, along with T4's emitter resistor.

So, a nice fault in the end, and I'm sure the owner will be delighted to get it back. Thanks all for the comments and help.

Arfa
  Reply With Quote
Old 9th July 2012, 04:14 PM   #9
49 - for the 18th time - again!
diyAudio Member
 
c2cthomas's Avatar
 
Join Date: Oct 2006
Location: Near "Music City" (Nashville Tennessee)
Quote:
Originally Posted by Arfa Daily View Post
So, a nice fault in the end, and I'm sure the owner will be delighted to get it back. Thanks all for the comments and help.

Arfa
That is some mighty fine troubleshooting there Arfa - Well done!!!
__________________
DIY audio can be expensive but getting to see things go up in smoke - that's priceless!!!! ..... "whatever - call it brainfart of Mighty ZM"
  Reply With Quote
Old 9th July 2012, 04:49 PM   #10
diyAudio Member
 
Join Date: Jul 2012
Why thank you, kind sir !! All in a day's work, here at the sharp end ...

Arfa
  Reply With Quote

Reply


Hide this!Advertise here!
Thread Tools Search this Thread
Search this Thread:

Advanced Search

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are Off
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
QED P300 service manual ..very long shot butI'm, stripping my dads QED P300 to get my Fivetide Parts 2 8th August 2011 08:40 PM
QED robinmicheal Swap Meet 0 4th May 2011 01:30 PM
qed digit any good? anyone? honsten Digital Source 5 19th March 2006 09:28 AM
QED P300 power amp bias question DirkD Solid State 2 21st March 2005 08:13 AM
Does anyone know the brand "QED" for cables & interconnects? Bricolo Parts 7 25th June 2003 10:32 AM


New To Site? Need Help?

All times are GMT. The time now is 04:32 AM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright 1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2