MicParts MPV57 PCB instructions?

[Paul W]

I bought a used Microphone Parts mic that has some issues, and I'm looking instructions for their MPV57 (rev III) PCB, particularly with respect to the wiring options (omni or pad switch on the PCB) and the HF deemphasis options. (What component do I change, and what values do what?)

I contacted MP and they wouldn't give me the instructions. They said I could send the mic in for repair (at some cost, I assume) if I don't like the way it sounds. Bah.

I believe in the right to repair (and mod) so I'm asking for anybody who's got the instructions to DM me the relevant bits. (I'm not going to distribute them or give away any secret sauce, if they have any. This is just for my personal use.)

 

[Khron]

Photos of the front & back of the board, perhaps?

 

[RuudNL]

The pad switch usually places a small capacitor (in the pF range) in parallel with the microphone capsule.
In omni both membranes of the capsule are in parallel, in cardioid the rear membrane is decoupled.
For the 'corrective EQ' (as MP calls it) a capacitor is switched in parallel with the drain resistor (and eventually one in parallel with the source resistor) of the FET.

 

[Khron]

(and eventually one in parallel with the source resistor) of the FET.

Wouldn't that boost the highs?

 

[RuudNL]

Yes, no doubt! But it seems different values are used for the 'drain' and the 'source' capacitor.
I never liked this solution for 'de-emphasis', because it also increases distortion.

 

[Dylan W]

Pretty sure it’s just a Schoeps circuit with minor modifications. If you post photos of both sides of both boards, and any component values you can see, we should be able to help you.

 

[Paul W]

"A" board front:


The blob of solder connects the near left pin of the switch to one leg of T3, one end of R9, and the whitish wire, which connects to the front diaphragm. (Nothing is connected to the rear diaphragm, whose wire has been snipped.)

"A" board back:


"B" board front:


The light blue wire from the pad marked BP goes to the wired-together backplates.

"B" board back:

 

[Khron]

At the very least, I'd expect at least one more wire between the two boards - the voltage input to the capsule bias oscillator.

 

[AUDIO FREQ]

He usually will sell the manual if you lose yours. I will have to find mine. It's around here somewhere....Im sure of it...i'll send you a PM if I find it. What pages did you need?

 

[oscarhuang]

 

[oscarhuang]

Instructions I saw on YouTube a few years ago. Hope it helps.

 

[Paul W]

Is it just me or do those curves from the manual look like it's rolling off treble over a pretty wide range (with a very gradual slope) rather than mostly just taming the ~10KHz resonant bump of a K67-type capsule a la Neumann?

My C14 says WIMA and 5600/63- on one side, so I guess the last sentence on that page applies: "The C14=5600pf line is not shown; it would be midway between the green and blue traces," so slight HF rolloff.

If I replace it with a 470pf (top green line, flat) will that eliminate the distortion issue Ruud mentioned, as well as making it flat?

Is the distortion proportional to the amount of attenuation?

 

[Voyager10]

At the very least, I'd expect at least one more wire between the two boards - the voltage input to the capsule bias oscillator.

I have an older MPV57 MicParts mic (also 2nd hand, no instructions!) and there's a wire between 'PA' on the top and 'PB' on the bottom -



There doesn't seem to be this wire on @Paul W 's mic - is it an electret capsule?

 

[Voyager10]

Regarding the switch:



The options are

- If you have a 2-sided capsule, connect the rear capsule diaphragm to the terminal arrowed. Closing the switch makes it an omni, opening makes it cardioid.
- Wire a 100pF (or thereabouts) cap between the arrowed terminal and ground, to make a pad switch. Bigger capacitor gives greater attenuation.

 

[Paul W]

At the very least, I'd expect at least one more wire between the two boards - the voltage input to the capsule bias oscillator.

There does seem to be a wire missing, between the unused octagonal pad marked "P-A" near the top (and right of center) of the A board in my first picture, and one marked "P-B" near the top (and a little to the right) on the B board (in my third picture).

It seems to work (cardioid only, rear diaphragm is not connected at all) without it, and those pads are clean, so apparently that wire was never put in at all. There's no capacitor installed in the place with a cap symbol and marked "PAD," so I guess it was never wired for a pad.

It seems that the rear diaphragm may have been connected to the switch at some point, but the pin (on corner of the switch toward the capsule, see red arrow in Voyager10's picture above) got mostly broken off, and the the wire is snipped off near the rear diaphragm.

I have no idea what's going on with that.

The capsule is an externally-polarized K67-type with white rings, which I'm guessing is the usual MicParts K67. (Although it lacks a letter A in one of the circular depressions in the ring, like the K67 picture on the MicParts web site. Maybe a slightly different version?)

 

[Paul W]

He usually will sell the manual if you lose yours.

I asked for the manual and he only offered repair services, but didn't offer to sell me the manual, so I assumed he wouldn't do that.

 

[Paul W]

I didn't find a video on the MicParts site addressing my issues, but I found this unofficial build video which helped a little. (Unfortunately the guy is trying not give away any secret sauce, so it's not as clear or useful as it might be.)



About 24 minutes in he talks about setting the polarization voltage with the blue trimpot on the B board. Apparently you measure the voltage at the leg of diode 5 that's right by the label "D5" and it's supposed to be 60v. That was definitely something I wanted to know.