Scheops CMC3--U Problems

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zayance

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Hi There,


I've just found a Schoeps CMC3--U with a MK41 capsule that seem to have been left aside for many years at my work, and i just grabbed it to check things out and see what is it with it..., folks at my job told me it didn't work well etc... but no other clue.

It seem to be one that will work for +48V, the R8 Resistor on the PCB is a 6.8K, i found a Schematic online...

Tested the diodes and Transistors on DMM just as simple check etc.... all seem to be fine on DMM

So plugged it in and monitored, and it's having a lot of white noise in the back of the sound... BUT it's picking up more handling noise than sound itself from the capsule... i can hear my voice from the capsule but it's not loud at all, the noise is definitely louder.

Does this sound familiar to one of you guys?
Needs better adjustment? (Old Trimmers and better change them?)
Capsule Failure?
Dead Tantalum?
Noisy Transistors?
1x BC264A FET
2x BC416 matched pairs
1x BC414


Schematic found online:

http://www.sdiy.org/oid/mics/Schoeps.gif





Thanks a lot for any tips on this one.

Best,


T.



 

zayance

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Thanks RuudNL,


Still some noise even with the Capacitor in place, lower than when the Capsule is, but still the same on the background...
My be amp and Cap issue, but capsule seem to capture sound pressure from pops as well as my voice in a clear manner, just the noise that is 5x louder on the output....
Handling noise is loud and clear...


Thanks for the help.


T
 

Gus

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after a visual solder, broken parts and smell test
I would check voltages at transistor nodes
C10 then D5. Zeners can be noise sources.
then C9
Then the semiconductors and other passive parts

Then the more tricky part if the PNP pairs are noisy they need to be matched and the bias resistors need to be selected for the beta/hfe of the transistors
 

Khron

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Is there any particular bias point one might need to aim for?

Gus said:
Then the more tricky part if the PNP pairs are noisy they need to be matched and the bias resistors need to be selected for the beta/hfe of the transistors
 

zayance

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Thanks for shimming in Gus, I was at it checking voltages when you responded.

I'm a little rusted on Mic and Equipment repairs and all, that's why i got back here, not much into DIY and Electronics these days unfortunatly; but always nice to have something interesting to get back to when we get the time, and it's a pitty to leave a nice mic like this aside for so many years....

As far as Visual the circuit looks clean:

- no smoke smell, or burned parts
-Solder joints looks good as well, redid them all with clean solder just for safety. Cleaned the solder joints with Isopropyl etc...
- No Signs of Leaking Caps, but if so i would have more buzzing sound i guess than White noise right?
- DMM tested Diodes, and they all seem to be just fine, showing a 0.7Vdc Forward voltage.
- DMM tested Transistors, no short there, but that doesn't mean they are not dead in use iirc.

Here are the values i get, the jumper on C is OUT so Voltages are around +30vdc as compared to CMC3 (CMC5 mod i think), and with a 47pf capacitor fitted instead of the capsule.

C10 = 5.87 Vdc
D5= 5.87 Vdc
D4= 1Vdc
D3= 1Vdc
C9= 31.5 Vdc


T2 and T3 have 31.5Vdc on all pins CBE

T4=
C: -0Vdc
B: 0.6Vdc
E: 4.2 Vdc

T1=
D: 4.3Vdc
S: 1.56Vdc


Thanks for the help.

 

Gus

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From your measurements
Guesses
dirt around the JFET gate and capsule input
Noisy JFET

Did you check the capsule charge voltage?
 

zayance

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Gus said:
From your measurements
Guesses
dirt around the JFET gate and capsule input
Noisy JFET

Did you check the capsule charge voltage?

No i didn't check....

I find it odd as well having 31.5Vdc on all matched pair transistor pins, like RuudNL is saying, but i can't tell, for me a transistor should have little voltage at base at least, and not full voltage...

While checking the internet I found this blog page that has some tips and a simple Matching rig... if necessary i'll see if i have the time for all this, and i could substitue and see if it makes things better or worse...

Blog had 2n5087 for substitute, any other refs that could also be good at this from your experience etc..?

https://billthompson.us/bt/schoeps-mic-conversion/


It's an old mic that seem to have +15 years at least, and might be best to have it serviced by Scheops, just for if the amp is doing better but the capsule is not etc... etc....
Not sure if they'll repair tough, i've sended them an email just for having an idea of all this, and talking about the problems encountered , they have pricing on their website, but it's not very clear for vintage mics etc...., they can reject the repair and send you the mic dismantled from what i understand haha, but i guess it's for really bad conditioned mics and not the one i have in front of me, wich is esthetically doing just fine..


Thanks for the help guys, much appreciated, i'll keep you posted and will see how i proceed...


T.
 

RuudNL

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If you are going to match transistors, keep in mind that the temperature can change the reading you get.
So if you just touched a transistor with your fingers, give that transistor some time to cool down again.
Usually I managed to match transistor pairs within one or two millivolts.
(I often used NXP BC557B, they are as low noise as any 'special' low noise transistor.)
 

zayance

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Yes i read that, thanks a lot for the advice and substitute ref.

I'll keep posted on the route i take etc..., and if i'm stuck in some odd things etc...


Thank you very much.


T.
 

zayance

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Well Schoeps takes a flat rate of 250euros for a complete refurbish, the owner of it in my job know seem to want to just send it to Schoeps know that i've investigated haha... , well anyway, at least this one might have a "longer" life. Will see.


Thanks for the help, and will see what Schoeps is able to do with this one.


T.
 

Gus

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RuudNL said:
That doesn't seem right to me...

I read that but the diodes D4 and D5 measured 1VDC (C to E?)drop if I understood reply 5 correctly
 

analogguru

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RuudNL said:
(.... NXP BC557B, they are as low noise as any 'special' low noise transistor.)
This is not surprising because they are made on the same wafer with the same chip architecture and are only selected later:

Vce > 65V ?:                  => BC556 (A, B, C)
Vce > 45V & F < 2dB ?: => BC560 (A, B, C)
Vce > 45V & F > 2dB ?: => BC557 (A, B, C)
Vce > 30V & F < 2dB ?: => BC559 (A, B, C)
The rest:                        => BC558 (A, B, C)

 

zayance

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Hi guys,


Just to give some news... the mic was in bad shape, both amp and Capsule, a little steep repair charges (250euros by elements), but at the end i would have maybe corrected one issue to highlight another etc...

Anyways...

Best,


T.
 

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