Output transformer: reasons for destruction? - diyAudio
Go Back   Home > Forums > Amplifiers > Tubes / Valves

Tubes / Valves All about our sweet vacuum tubes :) Threads about Musical Instrument Amps of all kinds should be in the Instruments & Amps forum

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 2nd January 2013, 07:58 PM   #1
diyAudio Member
 
Join Date: May 2012
Location: Lisboa
Default Output transformer: reasons for destruction?

Hi!
During the repair of a 50 year old Grundig open reel tape machine (TK23L), I traced the fault to the output transformer (no anode voltage on the output valve, excessive current on g2 and trough it's resistor with HT on normal levels).
The output stage is a ECL86 triode/pentode and the output transformer is directly and permanently connected to the built-in 4 Ohm speaker.
After dismantling the transformer and unwinding it's primary, the wire was open in six different locations. The interruptions originated in black spots spread throughout the windings (different layers, different places within the transformer, that is, not aligned with each other).
Transformers being generally robust and well built (particularly these old ones) what could have caused the destruction? Manufacturing defects is obviously ruled-out as well as corrosion (the impregnation rosin was still sticky inside the windings, despite the age).
I already have a replacement transformer but am not confident about turning the machine on before I understand what could possibly have happened for I don't want to risk blowing the replacement.
Any help from anyone regarding the possible causes for the destruction of the original transformer after 50 years of faithful service?

Thank you for your help,

Regards,

Renato

ps: the end of the transformer came slowly (lasted for weeks) with intermittent sounds difficult to describe, something like the sound of tearing a paper sheet very slowly or a deep 'grinding' sound.
Attached Images
File Type: jpg TK23L schematics.jpg (1.01 MB, 394 views)
  Reply With Quote
Old 2nd January 2013, 08:48 PM   #2
DF96 is offline DF96  England
diyAudio Member
 
Join Date: May 2007
Dampness causing electrolytic corrosion where the enamel wire coating was cracked due to age? In use the primary is sitting a few hundred volts above ground and the transformer frame is probably grounded. It doesn't take much moisture to get a little current flowing. A big current will take a short time to corrode metal. A little current might take 50 years.
  Reply With Quote
Old 2nd January 2013, 10:01 PM   #3
gpapag is offline gpapag  Greece
diyAudio Member
 
gpapag's Avatar
 
Join Date: Nov 2002
Location: Athens-Greece
Hi Renato
Do not install the new transformer before you troubleshoot the output stage.

The usual suspect for failed primary of output transformer is the coupling capacitor between the anode of the driver triode and the screen of the output pentode. If this cap is leaky, DC from the triode anode makes the pentode screen very positive, increasing the anode current and burning the thin secondary coil (the pentode may survive)
As your schematic has the EL95 (pentode) as the output tube and not the ECL86, I attach the output stage with the ECL86.
I would then first check the capacitors C9 and C17 for leakage.

George
Attached Images
File Type: jpg Output Stage.JPG (122.9 KB, 345 views)
__________________
["Second Law is a bitch." - SY] ["The Road To Heaven:Specify the performance & accept the design. The Road To Hell:Specify the design & accept the performance"-Bruno Putzeys]
  Reply With Quote
Old 3rd January 2013, 01:54 PM   #4
diyAudio Member
 
Join Date: Oct 2011
Location: Tauberbischofsheim, Germany
Also maybe a defective (short to gnd) erase/bias oscillator transformer indicated BV 9281-097?

Best regards!
  Reply With Quote
Old 3rd January 2013, 02:58 PM   #5
diyAudio Member
 
Osvaldo de Banfield's Avatar
 
Join Date: Dec 2011
Location: Barrio Garay,Almirante Brown, Buenos Aires, Argentina
Check the switch that disconnects the speaker, given the possibility that it has false contacts, the trafo may get overvoltages in its primary causing arcing in it and destroying coil winding.
__________________
LW1DSE
  Reply With Quote
Old 4th January 2013, 04:16 PM   #6
diyAudio Member
 
Join Date: May 2012
Location: Lisboa
gpapag,
Thank you for your fast answer! Before disclosing what was wrong, I measured all the voltages in every tube: all was as expected except for the anode voltage on the ECC86 pentode (that lead me to the faulty transformer). The grid voltage on the same pentode was low as expected (so, the transformer didn't expire from this, I think). Nevertheless I tested C9 & C17 and they are fine. I also measured the grid resistor (10K) and the grid leak resistor (2,2M) and the values matched (by the way, thank you for the schematics: I seem to have posted the wrong one - oops!).
Ruling out the chance of excessive voltage on the transformer, I decided to connect the new one and got a huge hum on the speaker. I then replaced C23 (even though the capacitance tested ok; I donīt have a ESR meter) and started getting results: some sound from the tape but dim and humming. Then I replaced C22 and the humming disappeared at normal volume playback. However, when turning the volume up, a very high pitch sound (ear piercing, almost ultrasonic) starts and the volume pot screeches (may be dirt, though, or too much current passing through and damaging the carbon track) even with the tape on 'pause'.
The voltages on every valve check with the values on the schematics (only about 20% lower) which I donīt think is significant.
So now I'm stuck. Any ideas about where to start? A brutish way would be to start replacing the capacitors one-by-one but I would like to take a more intelligent approach!
And still, the origin of the transformer destruction remains undisclosed... Six rupture points is massive, I think, for a well behaved home amplifier!

Opinions welcome!

Thank you all,
Renato

ps: I checked the speaker switch and it is working; I also checked the oscillator coil and there was no short (and measured about 1,2 Ohm on the primary).
  Reply With Quote
Old 4th January 2013, 08:26 PM   #7
gpapag is offline gpapag  Greece
diyAudio Member
 
gpapag's Avatar
 
Join Date: Nov 2002
Location: Athens-Greece
Quote:
Originally Posted by rlourenc View Post
ps:
I also checked the oscillator coil and there was no short (and measured about 1,2 Ohm on the primary).
Correct. See attachment

Quote:
However, when turning the volume up, a very high pitch sound (ear piercing, almost ultrasonic) starts
Now we are talking! (there is work for you ).

HF Oscillation can burn the output x-former primary coil at many points which are many turns apart, due to charging of inter-winding capacitance/coil inductance hf tank circuit.

Before looking any deeper, due to the many mechanical contact points that are within feedback loops (common design issue with equipment of that era), I would apply contact cleaner (with no oil) on all the switches and exercise them many times (electrical power removed of course).

I would also remove all the tubes from the sockets, apply this contact cleaner and reinstall them (handle the tubes gently!)
Then I would apply contact cleaner (oily this time) on all the potentiometers and exercise them.

After all this, if the hf oscillation is still there, I would suggest to start the rewarding part of the job.

Do you have an oscilloscope (and a sine wave generator)?
If not an oscilloscope, is there a decent AC voltmeter available?
Do you have the full schematic diagram? (I can post it )

Reply and we can proceed if needed.

George
Attached Images
File Type: jpg Coil.JPG (30.5 KB, 183 views)
__________________
["Second Law is a bitch." - SY] ["The Road To Heaven:Specify the performance & accept the design. The Road To Hell:Specify the design & accept the performance"-Bruno Putzeys]
  Reply With Quote
Old 4th January 2013, 10:49 PM   #8
Hi_Q is offline Hi_Q  England
diyAudio Member
 
Hi_Q's Avatar
 
Join Date: Oct 2007
Older Grundigs suffered from leaks in the square Brown Toffee Wima capacitors. If there are any in your machine then change every one! They had the same bad reputation as the British Hunts mould-seal capacitors.
  Reply With Quote
Old 7th January 2013, 02:33 AM   #9
diyAudio Member
 
Join Date: May 2012
Location: Lisboa
Dear George,
I'm at a loss. After cleaning all the contacts, instead of a high frequency sound, I got a low frequency one. After replacing the pentode's cathode capacitor (C20) the HF noise was back. After replacing the triode's cathode capacitor (C8) the LF noise came back...nothing makes sense here.
Regarding the signal generator and the oscilloscope I'm afraid I don't have one (and even if I did, I wouldn't know how to use it; using the multimeter is as far as a pharmacist goes!). I'm begining to think this is way above my skills but the problem is that if I donīt do it myself, nobody around here is going to do it for me.
As for the decent voltmeter I have a decent one I think I can trust. I would also appreciate if you could post the schematics: tho one I have is a poor scan and sometimes difficult to read (and I also don't have the PCB layout which means I have to follow the tracks to find a specific component...).
Thank you again for your help and patience,

Renato

Hi-Q: I can't find any of these inside. The signal ones are ERO foil IIs the WIMAs are round yellows and the electrolytic, mostly Siemens.
  Reply With Quote
Old 7th January 2013, 07:38 PM   #10
gpapag is offline gpapag  Greece
diyAudio Member
 
gpapag's Avatar
 
Join Date: Nov 2002
Location: Athens-Greece
Quote:
Originally Posted by rlourenc View Post
I'm at a loss.
Renato
Save these feelings for more serious troubles (e.g. women ). Not for electronics.

After you cleaned all the switches with the spray did you worked all the switches many times (without electric power connected)? It is important to do this.

Do all these noises occur when you play back a tape only or they occur also when you select the other inputs (microphone, radio, gramophone). Check each one.

Do you have a microphone that you can connect to the mic input? Does it work? What do you hear from the speaker?

Please, if possible, take a picture of the pcb from the components side and post it. I am interested to see the type of the capacitors.

George
Attached Images
File Type: jpg Ext.JPG (54.8 KB, 92 views)
File Type: jpg Int.JPG (61.8 KB, 89 views)
Attached Files
File Type: pdf Complete schematic.pdf (265.1 KB, 9 views)
File Type: pdf Front Section schematic.pdf (119.4 KB, 3 views)
File Type: pdf Aft Section schematic.pdf (114.8 KB, 4 views)
File Type: pdf PCB-Components.pdf (243.9 KB, 6 views)
__________________
["Second Law is a bitch." - SY] ["The Road To Heaven:Specify the performance & accept the design. The Road To Hell:Specify the design & accept the performance"-Bruno Putzeys]
  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
TOTAL DESTRUCTION OR HAPPY FET WSJ Car Audio 1 25th February 2012 01:38 AM
Stopping Speaker Destruction DQ828 Digital Line Level 10 8th January 2011 10:03 PM
Gratuitous destruction PeteMcK The Lounge 4 4th July 2010 02:56 AM
Op-Amp Self Destruction Tim__x Parts 9 6th June 2004 09:50 PM
The Horn of Mass Destruction mgoedeke Multi-Way 51 3rd June 2004 01:00 PM


New To Site? Need Help?

All times are GMT. The time now is 03:12 PM.


vBulletin Optimisation provided by vB Optimise (Pro) - vBulletin Mods & Addons Copyright © 2014 DragonByte Technologies Ltd.
Copyright Đ1999-2014 diyAudio

Content Relevant URLs by vBSEO 3.3.2