Semrau EB068 unbalanced master out section _ HELP!!

[Psychaos]

Hello all ..

i have an old German desk by Semrau .. its an amazing little modular 8 channel broadcast mixer with Haufe transformers and a zero ohm summing section with Haufe outs and a funky limiter ..

I got the desk over a year ago but have always had a balance issue at the master stage, But now it has got worse.

A brief outline of whats happening ..

When the mixer is powered from cold the left channel shows to be about -4db quieter on the master LED.
Then as it warms up (30 mins) it levels out a couple of dBs.
After a long session and the desk is definitely hot/warm the problem slowly gets worse again but its more obvious in that the LEFT channel loses bass definition (less bass) and has more treble compared to the RIGHT which seems just perfect.  IF the left channel is raised in level to meet the right channel some distortion artefacts can be heard as it gets closer to unity gain (0dB) on the master fader.

I did some tests with a measurement app .. Fuzzmeasure Pro and this confirms what im hearing.
I can upload them if need be and pictures of the mixer/master section.

I have eliminated any wiring issues and have checked all channels for consistency.
Im not bad with a soldering iron and have a multi meter to hand.

The desk was recapped and serviced when i brought it and now it is out of 'warranty'.
I feel capable of fixing it if i can track down the problem.

Any ideas what this might be?
I have full schematics and have checked the master section with a magnifying glass, there is nothing that looks like it has blown.. caps look good.

if any of you brainy bods might have an idea of what would cause this and where to start looking it would be most appreciated !!

Psychaos

 

[gyraf]

Bad electrolytic probably.

Are all electrolytics in signal path biased the right way 'round? If not, this error would be what to expect..

Schematics, or it didn't happen.....

Jakob E.

 

[Psychaos]

Bad electrolytic probably.

Are all electrolytics in signal path biased the right way 'round? If not, this error would be what to expect..

Schematics, or it didn't happen.....

Jakob E.

Hi Jacob .. thanks for chimming in .. thats something i will check .. good shout ..

what do you mean .. "schematics, or it didnt happen" ?

 

[synthiaks]

Hi,

I´m on Jakobs side here. Bad caps in the audio chain. Frequency, warm/cold all point towards it

.Semrau are as high quality as it gets, they did the smaller consoles for german ARD and such. I think the EB 068 was like 1000s of euros (then it was Mark) back in the days.

I think Jakob just wanted to say that its hard to tell what could be faulty without a schematic, then it's just guesswork and no fun . Sadly the schematics are hard to find so if you have them i'm also interested


S

 

[Psychaos]

Hi,

I´m on Jakobs side here. Bad caps in the audio chain. Frequency, warm/cold all point towards it

.Semrau are as high quality as it gets, they did the smaller consoles for german ARD and such. I think the EB 068 was like 1000s of euros (then it was Mark) back in the days.

I think Jakob just wanted to say that its hard to tell what could be faulty without a schematic, then it's just guesswork and no fun . Sadly the schematics are hard to find so if you have them i'm also interested


S

Hey there .. yes i understand now ..
the desk was 11k euros in the 90s .. and i Do have some schematics .. i can send them to you if you like .. they are really good quality ..

Ive had a good look and traced with schematics .. ive found one cap that in the schematics says its correct .. but on the PCB its wrong .. there are tiny little positive "+" signs on the back of the board and there is one that is in correctly according to the schematics but wrong according to the printing on the board .. could the schematics be wrong ?

Anyway just swapping that cap around now and ill know ..

I have schematics of a good quality if anyone wants ..

https://www.dropbox.com/s/z5tcqsvgoybo1j7/Semrau%20EB068%20Schematics.zip?dl=0

 

[synthiaks]

WOW!!!

Thank you sooo much!! I have 2 of this model but with slightly different setups. some without full AUX or Filter some just line and a couple of full ones.

Start with c101/102, 104/204, 110/210, 118/218

It must be one of them i think, if not maybe an OP-amp might be dead in the master section, Try and flip the OPs in the sum1/sum2 output stage. Does the faulty channel flip?

Just some thoughts,

S

 

[synthiaks]

looking a bit more i would say that if you want to swap ICs i would start with IC0101/201 and IC103/203. That is if the Limiter is not doing anything to the level. This console also has a VCA and i haven't looked into that part of the circuit yet but i would start with the ICs and then the Caps ( If the ICs are in a sockel )

P112/212 are Threshold of the Limiter i would say

S again

 

[synthiaks]

Sorry... it should be IC301/IC303,

IC101/201 are part of the limiter P111/112 could be limiter channel calibration and P301 for the clip LEDs

 

[bruno2000]

Thanks for the schematics!
Best,
Bruno2000

 

[Psychaos]

awesome info guys .. gonna get the sleeves up and the glasses on and get tucked in ..

speak soon..

 

[L´Andratté]

Thanks for the schematics!

Yes, crazy limiter, btw.

 

[Psychaos]

Pff .. tried  swapping out a few of the IC's mentioned .. tested quite a few caps .. its a bit worse now .. up to 10db difference on the left channel now ..  :-\

05:50am .. better get some rest and get back at it later ..

thanks to all for your input .. lets try again later 


Psychaos

 

[gyraf]

Thanks for schematics! Apex 1537 VCA, nice.

Quite a few electrolytics directly at opamp pins there. Failing opamps are likely to upset those. Maybe even a failing power line (+ or -) can make serious disturbance if left on for some time.

Try tracing a low-freq signal through the unit (routed to both L and R, comparing the voltages at same points) to find the stage where you loose lows. Then measure voltage polarity across the electrolytics in the area, and replace observing this polarity.

(Not very likely, but I've had 4066 CMOS switches behaving weirdly like this one time)

 

[Psychaos]

Thanks for schematics! Apex 1537 VCA, nice.

Quite a few electrolytics directly at opamp pins there. Failing opamps are likely to upset those. Maybe even a failing power line (+ or -) can make serious disturbance if left on for some time.

Try tracing a low-freq signal through the unit (routed to both L and R, comparing the voltages at same points) to find the stage where you loose lows. Then measure voltage polarity across the electrolytics in the area, and replace observing this polarity.

(Not very likely, but I've had 4066 CMOS switches behaving weirdly like this one time)

hmm .. thats a bit out of my scope knowledge wise ..  :-\

 

[Psychaos]

Oh .. i forgot to add the technical data sheet (in german) to the schematics ... for those interested please find below  8)

https://www.dropbox.com/s/711sv4f9qgaust2/SEMRAU%20SIE%20EB%20068%20tech%20data.PDF?dl=0

 

[Psychaos]

YAY !!

good news .. after changing all of the caps around IC301 .. mostly 47uF i now have a balanced stereo out !!! 

thanks to all for your help and ideas/support .. feeling very content and satisfied .. now i can get on with making some music !!  8)


Joti

 

[synthiaks]

well that is cool!

so i guess i can forget the PM?

And by looking at the Info PDF i can see that i was wrong its P111/211 that is Threshold for the limiter. The P112/212 is
klirr/distortion

Maybe you should think about a full recap of the Semrau, this might not be the only bad cap.


S

 

[gyraf]

Or at least measure DC polarisation voltage at the electrolytics that you changed with success.

Some of them may be better the other way around than what's in schematic (because of unpredictable DC offsets)

Jakob E.

 

[synthiaks]

Or at least measure DC polarisation voltage at the electrolytics that you changed with success.

Some of them may be better the other way around than what's in schematic (because of unpredictable DC offsets)

Hi Jakob,

Could you please explain a bit closer, or point me to somewhere where i can read up on it. I hate it when i feel lost

/Sebastian

 

[Psychaos]

Or at least measure DC polarisation voltage at the electrolytics that you changed with success.

Some of them may be better the other way around than what's in schematic (because of unpredictable DC offsets)

Jakob E.

OK .. sounds good .. ill check that out as soon as i get back in there .. i have a busy week this week music wise .. will keep updates  posted here  ..  8)