TAB funkenwerk V78m advise

Hi,
I have a V78m that hasn't been working correctly in the last couple of years. Actually it's been at my techs house for about two years now. He's been working on it on and off but it's hard without schematics. I've tried getting with TAB about this for a couple of years but no result. Understandably so in retrospect..

I'm hoping to get some clue to what might be wrong with it.
It's been giving me a lot of head ache as it's been in and out of service since i bought it new nearly 10 years ago. First a cold solder on a tube socket that took the swedish agent Golden Age Music nearly a year to fix. Then working for a while, sounding GREAT!!!. Then a bad switch. I got a new from TAB and replaced it. then working a few months, only to break up again.
It passes audio but has a really ugly, midrangey sound. My tech showed me a spectrogram he made of it and it shows a lack of low and hi frq.
My tech has been working on it but kind of hit the wall. He has checked all the components, switches and transformers and they are all working. However there are 2 points on the circuit board where a resistor has been cut of. Its still on the board but one leg cut and lifted. As we have no schematics it's hard to tell what's supposed to be there or not... Right now it's just an expensive paper weight.

I would be thrilled to have some input on this from fellow V78m owners.
/
Emil

Studio Mollan said:

As we have no schematics it's hard to tell what's supposed to be there or not... Right now it's just an expensive paper weight.

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The V77 schematic should help. The input pad and the gain control are combined in one switch which is the  W77. Schematics here;
http://groupdiy.com/index.php?topic=52362.msg668631#msg668631

As I know, the schematic of V78M is pretty much the same as V72 with added input attenuator and gain switch in the feedback loop. It should not be so big problem to find what's wrong.  Did you try replacing the tubes? Any pictures of the preamp interior?

Yes, it is based on V72, not V77 or V78. I'm not sure about gain control either, seems like variable NFB and swiched pad at the input. Probably it doesn't control it by changing cathode resistors at V1. Some good pics are here, i would rebuild it if it was mine...
http://www.m221b.com/AMI/V78/V78HI.html

Thanks for this guys! I'm forwarding this to my tech. Promising!
/
Emil

My3gger said:

seems like variable NFB and swiched pad at the input.
http://www.m221b.com/AMI/V78/V78HI.html

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That describes a V77 to me.

MatthisD said:

My3gger said:

seems like variable NFB and swiched pad at the input.
http://www.m221b.com/AMI/V78/V78HI.html

Click to expand...

That describes a V77 to me.

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It seems like almost every reviews describe it a bit differently, except for input transformer which is supposed to be based on V72S and doesn't have 1:40 ratio. Tab Funkewerk says V78m is V72S with input pad and gain control. In some posts Oliver wrote that V78m retains V72S circuit at sensitivity of -60dB and 40db of gain.
Pics aren't clear enough for me to even read resistor's color codes, it looks like it has Rl across input trafos sec. like V72S, etc.
What makes you think this is V77? It would be nice to figure out how to properly change gain in V72S, one option PRR mentioned is reducing cathode resistor and adding output attenuator. I like it better than Drip's method where NFB is changed (frequency response too), while PFB stays the same.

Best to start with the V78 schematic, which can be downloaded from Kubi's site:-
http://audio.kubarth.com/rundfunk/index.cgi

Presuming you have renewed all the electrolytics, there is a variable 1k pot on the cathode of the first tube which controls the AC feedback, that might be playing up.

There is also DC from cathode to cathode via a 50k resistor.

The screen grids are supplied via voltage dividers, so check they are in the right ballpark as are the other voltages.

A midrange sound, sounds to me like no feedback and that's the first thing I'd check.

No excuse for keeping a customers unit for years, or not obtaining the schematic, IMHO.

best
DaveP

My3gger said:

What makes you think this is V77?

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I'm not saying it is. With V77/W77 there is no DC on the gain switch.

Beware about the reference to the v78 schematic - the more recent v78M by tab-funkenwerk is not the same as the historic v78.

Are there really EF83 s inside - as shown on the pictures? Just curious, I would have expected EF86. For schematics I'd start with the V77 one and see how close it gets. V72, 77 and 78 are kinda based on the same circuit with some variations. There are really not that many parts in there that it should be possible to even trace the schematic if needed or to verify on which type the unit is based on. That thing should be fixable, let us know how it turns out 

Michael