Telefunken V376 racked with some noise problems (with pics)

[Apache5]

Hi all! I just finished it. It took me long time by the lack of money for it... But now its "finished".
I put it with JLM kits, DI, psu and the mic between one. I put too a buffer circuit to feed a VU meter. The vu meter is not nesesary because I usually must see the levels on DAW to see if they clip or not. Anyway I put the vu because I have it, there are a lot of free space and want to try it
It sounds nice to me, but I cant compare it with other pres, I only have the ones on my mci-JH618 desk at the moment.

The problem is that it seems to run ok but every certain time the pre gives me some pops and noise, the audio signal disapears during these short noises (3 or 4 seconds) and next, audio raises again to normal level in 2 or 3 seconds. It seems like static discharges, but I dont know if the problem is the v376 pre or something wrong outside it or in wiring.
Another question is that I joined the 22 and 23 pins (output trafo center connections). I put them both to XLR output pin 1. Is that correct? I haventt connected it to ground, to leave it floating, but Im not sure about if that its ok or not or if I need to put a resistor or something else there...


On the JLM PSU there are separated locations to put GND and to put 0V, separed with a low value resistor and a capacitor in paralel. I only used GND connection on PSU connecting it to chasis, IEC gnd, and pin 30 on V376 (chasis pin). Is that ok?

Well... I'm happy with that pre but I cant use it with non confidence people, only for my projects... until I can repair noise issues...

Well, thanks all you guys for your continuous work here, in that forum
:thumb:

Arnau

 

[Apache5]

Here is the V376 schem. Sorry... maybe its a little too big

 

[chriss]

Hi Arnau,

connecting 22 and 23 together is right, connecting them to anywhere else (like XLR pin 1) is not! Undo this connection! For this floating idea I think you can leave pin 1 unconnected. Or you could do a ground-lift switch here. However, do not connect it to the middle of the transformer!
Now you can try again, but I wonder if this is the reason for the effect you described. Maybe the module needs some of the good old recapping?!..
Else I would recommend that you fix the regulators to the housing, since it looks like these are 'floating' too :wink: Could become a problem in the future...

Chris

 

[Apache5]

Thanks Chris! I'll try to unconnect center leads from pin 1 this afternoon. Would be nice if the problem is only that :grin:
Regulators are attached on a sink, but yes, maybe would be better if I attach them to chassis... I'm thinking in recaping too, maybe thats the problem... but If I have taken a lot of time to put all this together I imagine that I'll take a lot more to recap it :sad: ... firsly I'll try your simpler advices :wink:

Arnau

 

[Apache5]

If I put a switch to lift out pin 1 to ground, where I must connect it... to Ground chasis start point (PC board chasis conn), to 0V point or to chasis on the out connector?
Thanks

Arnau

 

[Apache5]

I disconnected the output pin 1 and tried to connect it with chasis and without it, nothing. :sad: The noises still appear from time to time...
Would recaping electrolithics on V376 pre solve that noise problem?
Thanks

Arnau

 

[chriss]

Hi Arnau,

did the output level change after the disconnection of the transformer-mid? Just curious...
And for the recapping: I think this is quite easy to do with the 376 since it has these plug-in-pcb's. So if you rip it apart all parts should be very easy accessible. O.K. except for the main pcb, since you have to take out the whole module. But give it a try.

Chris

 

[Apache5]

Hi Chris. I think that the level is the same now that the center transformer pins are disconnected, but I can't be sure because I didn't do a serious test before and after the change.
Yes, recaping seems to be not too hard. Yesterday I taked some photos and a list about lithics caps, to recap it. Now Im looking for nice caps for that pre. All are axial and its dificult to find desired ones in axial... maybe I take them radial and bend the legs to fit (tubing them with termal tube).
You know what brand of caps would be ok to maintain original "colour"? There are some wima lithic axial caps in v376 but I dont see them anywhere. Im loking to Elnas, they are axial, but seems nice caps ( I dont know if good for that circuit).

Anybody knows what caps would be ok for telefunken V376? I only plan to change electrolithics at the moment, only 17 I think, maybe I must change tanthalum ones, there are only two I think, but There have strip colour codes and must see what they mean exactly...

Arnau

 

[Apache5]

Chris, I can test your question these afternoon if you want.... its easy, I think. I can put a 1KHz through DI input and see if output changes with center connected or unconnected... I can put white or pink noise too if its better for that test :wink:

Arnau

 

[jensenmann]

regarding the voltage regulators you´d better isolate them
regarding caps: you won´t find any new caps that are as bad as the ones used in the old days. I´d go with BC caps, they usually fit in whatever space old caps have been in.
And definitely replace the tantalums

 

[Terby]

Arnau,
I would change all of the caps including the tants, I have had all kind of issues with older ones in these modules including burnt output transistors due to a shortend output lytic in a V672 and exploding tants in a V676a.

Andreas.

 

[Apache5]

Thanks for info guys! Jensenmann, what you mean about better isolate voltage regulators? the ones of JLM power suply? How I can isolate them better? using mica and not the foam that there is now (these foams are the ones that comes with psu kit from JLM)? Connect the sink to chasis? or fixing the sink to the box too? :roll:

I'll definitively replace the tants too, its thrue that they usually are more problematic when fail... If there is a burnt transistor on the circuit these may be the source of my problematic noises? or normally if one transistor is destroyed the pre out wouldn't give a "normal" sounding signal (comparable with the one with an ok pre)? I supose that the last question its dificult to know without high knowledge about that circuit in particular... :sad:

Thanks

Arnau

 

[Apache5]

Chris I tried what you asked me about level diferences when transformer mid is connected or not. I pass white noise through pre for DI input, with and without transformer mid connected. The level and frequency spectrum is exactly the same on both samples :wink:

Arnau

 

[3phase]

[quote author="jensenmann"]regarding the voltage regulators you´d better isolate them
regarding caps: you won´t find any new caps that are as bad as the ones used in the old days. I´d go with BC caps, they usually fit in whatever space old caps have been in.
And definitely replace the tantalums[/quote]


I would like to discuss that question regarding the old caps...
I ve run lately in the situation where after a recaping with panasonic BC´s the sound seriously suffered... regarding telefunken V672... but the situation should be similar here...

After re caping and back caping i started over again with listeniong tests after each changed capaciator.....In general the new ones brought improovement and more tightness exept with the output caps before the x-former and one cap in the feedback path of the preamp...

I needed to leave the old caps in that position ...otherwise the sound went to harsh... precise but not nice...

theese 3 caps had something in comon..they all was labeld GPF

Does this indicates a special build of a electrolytic cap?
One guy i asked stated that there was no special caps in any telefunken circuit for service purposes... And that any modern cap should be better than the ones used back than...

But...why do the old ones sound so much better?

I ve don another test yesterday... i started to check with big and expensiv elna/silmic caps and what shall i say... they sounded like the old ones..but new..with tighter bass and crisper highs... but sonical the same warm temperature than the old ones...

What does this say? Is silmic trying to copy the sonic characteristics of an old worn out cap? or does the GPF labeling indicates a special quality cap that has something in comon with a modern highend audio cap?

Or are the panasonics BC just dont sound good and should be avoided in crucial soundstages?

Has anybody any ideas? problem with the silmics is that they dont really fit in the tight space...


Am i tricked with a listening experiance that can be only achived with old worn out caps? but why the highly regarded Elna/silmics sound so damn close to the 30 years old ones in the telefunken amp?

I am slightly irritated... However.. i am close to believe that there are some special selections inside telefunken amps, opposite to the statements of a former telefunken tech...
Or maybe all old german caps are better for audio than modern standard types?...
might be another explanation that while increasing certain technical data, during the evolution of the electrolytic cap, some sound relevant things got lost that are now only availible with special audio electrolytics?

Maybe some of you guys here has more infos on the subjekt...
Anybody has experianced similar phenomens?

 

[jensenmann]

I´ve been talking about specs of course, not sound - specs doesn´t translate necessarily in sound.
New caps need to (kind of) burn in in the new circuit to reform the dielectricum. A lot of people experienced that new caps sounded worse than old ones in the circuit after replacement. But this may change after a day or so (for whatever reason - maybe only getting used to the new sound)

....ok now I see. I´ve overlooked the pictures. The regulators are already isolated - my fault. I only read the posts which sounded like not to isolate the regs.

 

[3phase]

I had the panasonic BC´s as output coupling for some weeks..they didnt sounded bad...
Was a tough decission to put the old ones back in... booth sounded good but total different..
The minus on the old ones was actually that they sounded old..so with this midrangy wobbely kind of sound ... dont know how to describe.. ab bit worn out... But gentle, warm and very transparent... The BC´s was precise, more lowend, sparkeling...
but rough and grainy in relation to the old ones...

Grainy to a degree where we still talk about a good sound... But i didnt liked it so much... Not the sound i was used to get from using the telefunkens...

However... The Elna Silmic was very close to the old ones...
Not a big difference sonically.
the same smooth transparent sound..but without beeing worn out..tight and crisp... but its still the sound i was used to...
Thats what a recap should provide... new caps..fresher sound..but same sonic signature...

Ok... I dont know how such an amp sounded in 74... ;-)
maybe i remodeled an sound of an aged amp by using theese hifi nerd caps...

I dont know yet how it sounds to use crappy standard caps... maybe the panasonic just dont fit in with theire optimized technical data?
The circuit was designed with the caps availible at that time...

Its just that i would like to know if the GPF labeling of the 3 caps with the highest impact on the sound has something to say?

in the 50´s there was specialised german audio electrolytics labeled "GLATT"..what reffers to an even surface of the used aluminum foil which is considered to sound better...

by the way... It is allways funny to get such hugh differences in the sound of a unit without seeing this reflected in the freq-response at all...

On cap has fat bass and silvery highs and another sounds midrangy and dull but you get the exact same freq response when measuring with static signals...