TELEFUNKEN V76/80 INPUT TRANSFORMER BROKEN?

Hi,
i'm trying to repair a telefunken v76/80.
I recapped entire unit with the same components and same values..but it seems the input doesn't work..

I mesaured the input Dc resistance of the input transformer primary and secondary...
My tests primary DC resistance is 6.5ohms...isn't too low?


I don't have a chart with the original values..
but i foun a clone of sowter with a chart:
www.sowter.co.uk/vintage-audio-transformers.php

And the primary is 9ohms ....

My  v76 input doesn't match any microphone..
So my input transformer is broken???

Any expert here?

With a V72 you can apply a signal to the grid of the first tube (this is the hot of the secondary of the inputtransformer). This is a much used way for using it as a DI. If this works for your pre, then the inputtransformer is broken indeed.

heroes said:

I mesaured the input Dc resistance of the input transformer primary and secondary...
My tests primary DC resistance is 6.5ohms...isn't too low?

Click to expand...

I was told that putting dc on an audio transformer can magnetize the core which leads to higher distortion. I'm not a trafo expert, so I have no idea how bad the effect might be and how it can be reversed, but I decided for myself not to do any dc measurements on valuable transformers.

But since you anyway already did it: you did short the cap between the primary windings or measured the 2 windings individually? Otherwise you couldn't measure any dc resistance. Just to make sure you are measuring the right stuff.

I'd probably use a tone generator and oscilloscope to figure out what's going on where with the transformer. I've had some broken ones over the years. It is a very complex beast (for a transformer) and on top of it the shielding inside is somewhat poisoning, some mercury involved if I remember right. I just memorized not to touch it...

CJ might have dissected one here once?

I'd also like to know how good the sowter replacement is. If it was as good as the original then dying a V76 might be possible. If they can do the input, they should easily be able to do the choke and the output? With a good pcb layout it should be possible to get it done, and it might even be easier to rebuild them than to properly refurbish an original ;-)

I don't get what you mean by 'my V76 doesn't match any microfone'?

I doubt that you can just leave out the xformer and try the circuit as DI, because it seems like the grid is dc-biased through the secondary winding. (Which is kinda contradicting my 'no dc through audio transformers unless they're specially made for it'-rule, but since it connects only to the grid the current is probably going to be extremely low). Anyone can confirm this or am I overlooking something?

You have the curcuit diagram with the dc measurements? You made sure all the dc voltages are in the right ballpark? Before that I wouldn't worry about the transformer. The chances are pretty good that you have a loose wire, broken resistor or bad tube (in that order in my experience).

Michael

>But since you anyway already did it: you did short the cap between the primary windings or measured the 2 windings individually? Otherwise you couldn't measure any dc resistance. Just to make sure you are measuring the right stuff.

The unit i own already had the primary two windings tied for bypass input filter..

I have the suspect who someone broken it with a bad 48v phantom..

The only audio that pass trough that transformer is from an active di at differents gains (1khz sine wave)..but very low gain...and bad quality..

It doesn't work in the same way with any microphone..it seems dead signal...

I have done several refurbishment 
1)New caps with exact values...also polystyrene rare ones was swapped.but nothing changed

2)NOS valves..NIB all swapped..

3)Checked all traces of pcb..nothing broken..all cables..all resistors..

I don't have a big experience..but when i got this unit..was completely dead..

I don't have many skills of tube stuff..my only sucessfull repair was only a manley mic 500eq where transformer  psu was broken...

Should i replace transformer?

Any suggestion?

Check the dc voltages and expect the unexpected - meaning: don't think 'it can't break here', but recheck it. Most of it is no vodoo, but just loose wires and broken resistors. Check the wires going to the tubes, those under plastic tubes. They can be broken below the tubes. Check the choke (if the dc volts are ok then you've already done that).

If you have one, you could try a different input transformer if you can't make sure the fault is there by systematical aproach. Any cheap one will do in order to see if it makes a difference and before you buy a replacement and afterwards realize it doesn't change a bit...

Michael

With a V72 you can apply a signal to the grid of the first tube (this is the hot of the secondary of the inputtransformer). This is a much used way for using it as a DI.

Click to expand...

really, have you tried it?
it didn't really work for me. I had a signal but it was covered in noise and yes, I did pay good attention to shielding and grounds. So I changed it back to stock and am happily using it with a transformer DI in front. Would love to know how to make the gain switchable though...

Michael Tibes said:

Check the dc voltages and expect the unexpected - meaning: don't think 'it can't break here', but recheck it. Most of it is no vodoo, but just loose wires and broken resistors. Check the wires going to the tubes, those under plastic tubes. They can be broken below the tubes. Check the choke (if the dc volts are ok then you've already done that).

If you have one, you could try a different input transformer if you can't make sure the fault is there by systematical aproach. Any cheap one will do in order to see if it makes a difference and before you buy a replacement and afterwards realize it doesn't change a bit...

Michael

Click to expand...

I checked every wire in the tube area also..resoldered many parts..
I will check the choke...
Sadly i don't have another transformer here...

All i could tell that the unit when it come here was completley dead ..i ressurected  a part of v76..

I read in many forums about v76 input trafo broken with 48v phantom...

I don't have a diagram of this transformer and  values of static resistenc of total primary windings.. and secondary i suppose primary is 9ohm like the sowter clone claim in their chart.

Sending a 1khz  signal from my terrasonde audiotools to a direct box like manley to telefunken...some signal passes trough v76...but defintively not hifi quality...and a big loss of gain...very few  decibels of gain..i done ha sweep and seems that freq response i se a big gain  loss most on the mids freq...

I checked a shure sm 57,58 nothing dead signal..no preamplifcation..
Too for all my oktava ribbon microphones..
I have also some telefunken valve mics. km54,u47.but don't want to do tests with valuable mics like that..
I don't try to use phantom on it..

Mine problem is not buy a new trafo input...the problem is not get mad ...

also clean the power fuse holder, that was a problem with a V72a I had in the studio.

martthie_08 said:

also clean the power fuse holder, that was a problem with a V72a I had in the studio.

Click to expand...

Definitely, that's the most stupid weak point with most V7x. But again, the dc voltages can't be ok when there's no mains currrent. IMO you really should check them...

It's also certainly not a mistake to check all the switch contacts. They're supposed to be silver or gold shining, anything dark or even black needs cleaning. With a lot of patience and thinking the switches can be partially disassembled and then cleaned. But this is not a 3 minute job and be prepared for little things to fall off! There were several different switch assemblies and some had somewhat loose contact plates. It is no problem if they come off and you put them back, but if you don't realize it and loose them, you need to find a replacement switch - good luck with that 

There is a lot of stupid talks and and halfbaked or intentionally mysterious or glorified 'information' on the net, especially about the V7x series. Some people find the wholy grail, others believe themselves to be it. Just because someone blew his transformer doesn't mean yours has to be the source of the problem. Could be though, but I wouldn't expect the worst at first. At that point I'd rather measure and carefully look and think than to read too much about what others think what might be in case of something they heard about... With the information given so far, noone will definitely be able to locate the problem for you.
And any audio transformer, no matter how cheap and what ratio, could serve as a rough replacement in order to rule it out. You just have to interpret the results according to what you use.

Good luck  8)

Michael

martthie_08 said:

With a V72 you can apply a signal to the grid of the first tube (this is the hot of the secondary of the inputtransformer). This is a much used way for using it as a DI.

Click to expand...

really, have you tried it?

Click to expand...

No, I didn't, but read it often. But the reason I said it,  is off course to check if it's the inputtransformer dead or not. They're indeed often ruined by 48v, that's why I used blocking caps with mine.

Oh, and as a side note regarding DI with a V72: It wasn't the sound I was looking for with a DI. I have a racked pair of DDA channels, these are my favourite for DI. Knocked out my other pre´s, (my beloved Siemens U274´s, SSL 501´s, Keith 9k clones, focusrite TF).

Michael Tibes said:

martthie_08 said:

also clean the power fuse holder, that was a problem with a V72a I had in the studio.

Click to expand...

Definitely, that's the most stupid weak point with most V7x. But again, the dc voltages can't be ok when there's no mains currrent. IMO you really should check them...

It's also certainly not a mistake to check all the switch contacts. They're supposed to be silver or gold shining, anything dark or even black needs cleaning. With a lot of patience and thinking the switches can be partially disassembled and then cleaned. But this is not a 3 minute job and be prepared for little things to fall off! There were several different switch assemblies and some had somewhat loose contact plates. It is no problem if they come off and you put them back, but if you don't realize it and loose them, you need to find a replacement switch - good luck with that 

There is a lot of stupid talks and and halfbaked or intentionally mysterious or glorified 'information' on the net, especially about the V7x series. Some people find the wholy grail, others believe themselves to be it. Just because someone blew his transformer doesn't mean yours has to be the source of the problem. Could be though, but I wouldn't expect the worst at first. At that point I'd rather measure and carefully look and think than to read too much about what others think what might be in case of something they heard about... With the information given so far, noone will definitely be able to locate the problem for you.
And any audio transformer, no matter how cheap and what ratio, could serve as a rough replacement in order to rule it out. You just have to interpret the results according to what you use.

Good luck  8)

Michael

Click to expand...

I don't pretend you locate problem for me...just asking suggestions..this v76 layout is a pain in the ass working on it...and find caps for refurbish it was not an easy task..months of searching..i know it's not an easy task..i appreciate help of all of you..and experience is very important..

When mine MANLEY MIC 500EQ broken..the tech guy of MANLEY was a fantastic person he assisted me by email every step..and when someone know perfectly every aspect of the hardware could help you to repair at long distance..

If i understand this v76 is sensibile also to differents mechanical problems of age.

I would like to hear stories of successful v76 repairs..it's always interesting  every detail is important..

And most important thing this is not junk..when it works is one the best musical valve preamp i ever heard...

Im trying to understand how 48V can heat up a winding in the transformer to make it snap?
i thought you might need at least 480V for that to happen.

mich said:

Im trying to understand how 48V can heat up a winding in the transformer to make it snap?
i thought you might need at least 480V for that to happen.

Click to expand...

Consider if the current is not limited by the usual phantom power 6K8 resistors. Say you have a strong 1 amp supply...

it´s about the inductive "kick" while rapidly turning on/off/shorting the 48V. this kick gets transformed at a 1:20 (v72) or 1:30(v76) ratio resulting in a peak voltage >1000v  - the peak is very short but it can create little sparks on the fine secondary winding. if you repeat this process continuingly, the secondary will be welded together turn after turn.

-max

Somewhat related topic:

http://www.groupdiy.com/index.php?topic=37588.0

I don't see mention of a secondary measurement in your posts; the info in the tread link says you should check it.  Is it open?

These V series thingies seem to be the weakest link genetically, I recall because of the insulating materials used, and their deterioration with age. 

I advocate turning on phantom with the pad engaged, so any thump is 20 dB down.  Though, I suppose you can then say that switching the pad is also a risk with some pieces.  I haven't done the math for comparison; one of you smart guys can bang out the numbers in your head faster than I can determine the method. 

Anyone have the diagram and specifics of the transformer BV NR 511 1:30 MIC INPUT TRAFO?

heroes said:

I don't pretend you locate problem for me...just asking suggestions..this v76 layout is a pain in the ass working on it...and find caps for refurbish it was not an easy task..months of searching..i know it's not an easy task..i appreciate help of all of you..and experience is very important..

When mine MANLEY MIC 500EQ broken..the tech guy of MANLEY was a fantastic person he assisted me by email every step..and when someone know perfectly every aspect of the hardware could help you to repair at long distance..

If i understand this v76 is sensibile also to differents mechanical problems of age.

I would like to hear stories of successful v76 repairs..it's always interesting  every detail is important..

And most important thing this is not junk..when it works is one the best musical valve preamp i ever heard...

Click to expand...

Well, if you were providing other information than your belief that the transformer should be broken, than people could try to help you. And I believe I made some suggestions... So what exactly did you check??

Even doing a basic check on the xformer is not rocket science, you can get around with a soundcard and some thinking if you have no other tools at hand. But what tools do you have?

Taking you on a V76 refurbishment tour might be a bit tough though, that might end up being a very long movie.

Michael

For start i checked all connections were clean and working..wires..tube sockets etc tc..
I checked all fu%%% resistors that were ok...
The traces of pcb..there were some ruined by age..i rebuilt with a special product..
I replaced the electrolytics caps..of psu and board..
until this step the gain was not working...

I replace the polystyrene caps ones and i noticed an improvement in audio quality of course..an the gain returned to work but very low gain..

Then i replaced the tubes with new ones..slight improvent of otuput with a new  E83F..

I checked the the rotary switches...but nothing new..

I checked all connections..many times..looking the circuit i noticed the audio inductor used as filter for high freq was missing..

And the input cap+ restistor(5,6,7,8,9,54) were bypassed as i read is a trick for remove low freq filter of this broadcast version..

I checked every joint..desoldered and soldered again every fu$%$& part..also psu ..


I sent several freq trough V76...alway a low gain..always with a DI box as bridge..

I noticed an inaudibile results with any microphone i connected on it..

Last check was the dc res total of primary and secondary windings of input and out trafos..

I don't have a table for compare originale specifics of this TRAFOS..i only have some numbers from sowter clones...http://www.sowter.co.uk/vintage-audio-transformers.php


Now i'm writing every  suggestion..maybe usefull...

The idea of transformer is my supposition..i'm not sure 100%

Because if understand this unit didn't come exactly pure and intact..some one modified it for filters...and the suspect  in this modern age of phantom 48v...

As i read from an expert like Oliver Archut phantom was a common killer of  v76 input trafos..unitl he discovered how to stop this problem..



The tools i have is a normal digital tester...An Audio Terrasonde Toolbox that is a pro box that do freq analysys,scope, signal generator ,SPL..etc...
Maybe not enough...

That's all..