Neumann TLM193 circuit help?

[Moby]

Hey guys, I have this guy on my bench. No sound at all. Looks like missing capsule polarisation voltage, but I'm not  sure because I have no schematics. I wonder does aomeone has schemo or experience with this little *******?

 

[Gus]

I have not worked on one,however looking at the picture I would

1) check pins 2 and 3 voltages using pin 1 as ground when plugged in and phantom is turned on, If the microphone uses 3mA (6.8k x 1.5mA = 10.2VDC , 48VDC -10VDC = about 38VDC at pins 2 and 3 )Then check voltages at C2, C3 and R4, R3(guessing they are the output resistors to pins 2 and 3).

2) the board with the CMOS 4093 looks to be the DC to DC board.  Check for voltages If you can get a probe through the conformal coating or test the pins going to the large board. I would check for something like 15VDC at pin 14 at the CMOS 4093.

More pictures would help

 

[Moby]

Hey Gus, thx for tips. Yes, I checked phantom, it looks.Ok inside the mic. Also, it seems that audio impedance converter works ok, but I dont measure any polarization voltage.  If nobody has this. circuit I will try to reverse draw it but wierdest thing is that backplate is terminated to GND with 510ohm. Maybe I didnt trace it correctly but it looks like that, and I dont measure any voltage there... still confused.

 

[moamps]

....but I dont measure any polarization voltage....

Most likely you have problem with DC-DC converter. Try to measure output voltage at the converter's output directly,  you can't measure polarization voltages at capsule because this voltage bias is set via very high-ohmic resistors.
Some clues how the schematic looks  like you can find here:
http://www.neumann.com/forums/view.php?bn=neumann_mictec&key=1359951125&v=f

 

[Moby]

Hey Moamps, thx for the link. Yes, it looks great to see topology  even like this. Btw, I.have hiZ tube voltmeter for measuring polarisation voltages at backplate but im totally confused with this one.  I have both voltages (-48 and 9.7v )from mh660 but i still cant figure  out how they polarized the capsule?? I see its terminated to GND via 49.9ohm!  In my version.of the  mic it's 510ohm , but anyways....

 

[Moby]

Guys, I extracted the schemo so u can see it withouth opening the pdf. I really dont understand how the one did capsule polarisation. Must be that I miss something. Do you see what i miss?

 

[Gus]

I would look for -50VDC on pin 6 and 7 of IC1
I would guess one of the  backplates is at -50VDC and the other backplate is grounded with 1gig it is a multpattern capsule used as a cardiod
The outside is grounded
check for the -50VDC and +10VDC at IC2

 

[Moby]

I would look for -50VDC on pin 6 and 7 of IC1
I would guess one of the  backplates is at -50VDC and the other backplate is grounded with 1gig it is a multpattern capsule used as a cardiod
The outside is grounded
check for the -50VDC and +10VDC at IC2

Thx. I think that I checked already but hey, double check cant do harm. Btw, I still dont understand (from the circuit) how they connected -50v to the backplate

 

[moamps]

...., I still dont understand (from the circuit) how they connected -50v to the backplate ....

Imagine a resistor inside hybrid IC1 MH101 connected from pin 6 to pin 7.  There are now two possibilities: Capsule is shorted or MH101 is dead. You can check MH101 by injecting a signal via 50pF capacitor to pin 7 and observing the output (when capsule is disconnected).

 

[Moby]

Imagine a resistor inside hybrid IC1 MH101 connected from pin 6 to pin 7.  There are now two possibilities: Capsule is shorted or MH101 is dead. You can check MH101 by injecting a signal via 50pF capacitor to pin 7 and observing the output (when capsule is disconnected).

Aha, u want to say that polarisation is achieved form the  membrane side, right?

 

[Moby]

Hey guys, all voltages seems to.be ok from MH660 , capsule is ok, so  it looks like MH102 is faulty. The only source i.found is this.http://spares.sennheiser.co.uk/neumann/tlm193/neumann-tlm193-hybrid-mh102-060274.html
Do you have some  better ideas?

 

[Moby]

In the meantime owner of the mic decided not to buy replacement mh102 because of the price. He gave me the approval to hack the module. Now, there is the issue with black conformal coating. Does anybody here has experience with this ?

 

[RuudNL]

I suppose it will be an epoxy resin that is very hard to remove without destroying what's 'inside'.
On the internet I saw people dismatling integrated circuits, to have a closer look at the chip itself.
But as far as I remember, they used pretty dangerous chemicals.  (High concentrated nitric acid etc.)

https://www.youtube.com/watch?v=mT1FStxAVz4

 

[Gus]

Have you tried another capsule?

 

[gyraf]

Epoxy is removed with a heat gun and a sharp knife....

Jakob E.

 

[Moby]

Have you tried another capsule?

Actually, I tried this one on other impedance converter and it works

 

[Moby]

Epoxy is removed with a heat gun and a sharp knife....

Jakob E.

yep, thx. Same approach recomended me my local colleague. Right now I'm on vacation and surgery will start at the end of this month of course I will share the results here

 

[Moby]

Hey guys. It took me some time to remove this nasty coating, but here it is. Some of the parts just felt with the coating but topology is visible. To be honest, It looks unknown to me, except I see that Jfet (PF5101) is bootstrapped and that has some feedback for probably distortion cancellation. Right now, I'm really short with the spare time so  I will be not able to examine any deeper.  Red mark on the PCB is probably diode, but it was already dead so I was not able to measure polarity

 

[Moby]

Hey guys. I tried to trace the circuit but some details still confuse me. Anyways, I give a try with TLM103 front end(Thx to Dale). And yes, it works great. This marriage sounds really good, and the best thing is that  one day if someone decide to bring it back to original, its 15min of job here is the pic. Cheers

 

[Sredna]

Wow!

Care to elaborate on the TLM103 front end?

I have a dead TLM193 here as well...

Thanks in advance!