Gefell MV692 P48 (UM70) - no polarization voltage (dead "Tr 2" DC/DC converter?)

[zzzzz]

Mic repair isn’t my forte and maybe someone who understands this circuit more than I do can share some insight.

Huge post TL;DR: have a dead Gefell MV692 body with no polarization voltage. Have tried just about everything to get it to work, and have compared & measured voltages with an identical second functional MV692 and I *think* the source of our issue is the "Tr 2" "BV-137" component which is part of the DC/DC converter scheme.

We have two MV692 bodies and multiple capsules — capsules are not the problem because all of them work fine on the functional mic. One of the mics randomly died a few years ago, plugged it in and no sound with any capsule. Sat on a shelf for a while and finally got to fixing it up today only to find that there's no polarization voltage at all! I guess that's a good reason why a mic wouldn't work...

Both mics were refurbished by Gefell in the late 90s / early 2000s. They were shipped to a shop Nashville for modification/upgrades (I think The Mic Shop?) who definitely just mailed them to Gefell. They came back “re-skinned” with new sleeves, matching PCBs, components, the P48 conversion using the little SMD "regulator" daughterboard from Gefell, and even new serial numbers two apart lol.

I have read through every single post about the MV692 and related Gefell bodies on the forum here and any other information I can find about repairing them. Every potential issue is discussed except for the DC/DC converter scheme. I'm assuming "Tr 2" is dead, but I can't find any info on that piece, how to replace it, or if a replacement component exists. The pins/leads are clearly extremely fragile.

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diagnostics:

Attached schematic of our exact mics. Jan's post here was helpful in diagnosing. Pretty much combed through anything I could think of.

• There is a light audible click/thump in the signal path when engaging the pad switch and/or hpf switch so the signal path seems to be working.
  
  
• The SMD "regulator" adaptor PCB from MTG works fine and puts out 12v just as it should.
  
  
• Voltages measure the same on both MV692s just about everywhere, other than after the DC/DC converter scheme of course.
  
  
• The junction of VD2 and VD3 on the good mic measures around 59vdc and seeing +/-40vdc at L1 and L2 where the capsule connects. On the bad mic, there just isn't any voltage at all after the DC/DC converter scheme
  
  
• Voltages around VT2 are roughly the same on both mics.
  
  
• Voltages on every single pin of the "IS" 29-25 integrated circuit thing are the same on both mics. They can go bad but regardless, I don't think this would affect polarization voltage.
  
  
• Deep cleaned all flux from PCB carefully with 99.9% IPA
  
  
• Temporarily swapped VT2 with a similar transistor based on datasheets (2N3903 and/or 3904) just for the hell of it with no changes other than roughly 0.5v lower voltages measured around it.
  
  
• Took C6, C7, R4, R5, and R6 out of circuit and all tested fine. Replaced C6, C7, and R4 just because they were already out. No changes.
  
  
• Removed all components in the polarization scheme (C1, C2, C3, C4, R1, R2, VD2, VD3) and measured them to make sure they were correct / functional. No problems.
  
  
• Replaced all electrolytic caps. No changes. Old electrolytic caps did measure much worse than the new Panasonic FR's I replaced them with, though.
  
  
• FET on both is the soviet KP303W (КП303В) transistor. Tested out of circuit on little LCR component meter and it seemed to measure fine? Hard to know without datasheets. Also temporarily swapped the FET with a PF5102 just to see if it would make any difference anywhere in the mic.

An aside: a bunch of people all over several forums mention something along the lines of “noisy old FET” in these mics. Moby had a thread asking for alternatives, but none were actually mentioned.

additional notes:

"Tr 2" is obviously labeled like a transformer so before I got into all of this, I removed all surrounding components so I could measure between all of the pins "out of circuit" to check for continuity or shorts.

• Pin 1 to Pin 2: 0.3 ohms
  
  
• Pin 1 to Pin 4: 9.3 ohms
• Pin 2 to Pin 4: 9.1 ohms
  
  
• Pin 1 to Pin 3: 1.5 ohms
• Pin 2 to Pin 3: 1.4 ohms
  
  
• Pin 3 to Pin 4: 7.8 ohms

 

[Khron]

"Tr 2" is obviously labeled like a transformer so before I got into all of this, I removed all surrounding components so I could measure between all of the pins "out of circuit" to check for continuity or shorts.

Too much trouble to do that on a working mic too? Should be getting sane readings even without removing stuff around it - maybe just measure once then reverse the probes, to double-check?

 

[Evertide]

Did you swap TR2 between the mic's ?

 

[Murdock]

Try to tighten the screw of Tr2. Sometimes it gets loose and this influences the inductance and thus messes with the dc dc circuit.
It’s on the solder side of the PCB.
But don’t over tighten it or else you gonna crack the plastic.

 

[zzzzz]

Too much trouble to do that on a working mic too? Should be getting sane readings even without removing stuff around it - maybe just measure once then reverse the probes, to double-check?

Did you swap TR2 between the mic's ?

Didn’t have time / concerned about fragile wires breaking if swapping / need it working for a session so full disassembly isn’t an option today.

Easier to measure between pins on the bottom of the PCB for “Tr 2” as well, for future reference.

 

[Evertide]

Did you measure ac and dc voltage's around VT2 ( oscillator ) and TR2 ( auto-transformator ) ?

 

[Khron]

Did you measure ac and dc voltage's around VT2 ( oscillator ) and TR2 ( auto-transformator ) ?

Well...

Voltages around VT2 are roughly the same on both mics.

Temporarily swapped VT2 with a similar transistor based on datasheets (2N3903 and/or 3904) just for the hell of it with no changes other than roughly 0.5v lower voltages measured around it.

 

[zzzzz]

Did you measure ac and dc voltage's around VT2 ( oscillator ) and TR2 ( auto-transformator ) ?

yes, pretty much same between them up until "Tr2" as described.

Tried tightening "Tr 2" screw as Murdock suggested -- no change.

I ended up getting a good deal on a complete MV692 circuit board, just so we could get the mic up and running fast. It doesn't have the little 48v to 12v "regulator" but I can just transplant the one from my dead board over to it easy since that is working just fine. At least now we will have a spare Bv 130 transformer and weird IC from the "dead" unit.

That being said, I wonder if there is any solution to this issue because our mic just randomly stopped working one day. No drops (to my knowledge), no weird power surge, etc. and there doesn't seem to be a source for parts in the 12v to 48v DC-DC scheme? My friend said "should just lay out a new PCB with a different polarizing scheme" but I don't have time for that (or need anymore... for now)

Related but unrelated because this might help people in the future: because at one point in time I had considered buying a replacement MV692 body (no capsule) but without the P48 conversion, I had laid out a clone of the little SMD board Gefell uses since no one is currently offering the 48v / XLR conversion kits anymore. Open-source free gerbers (optimized for JLCPCB) to download here. I am not manufacturing these or trying to make a profit off of them.