Gefell MV692 IC 29-25 schematics and internals

[bezen4uk]

Hi,
I had the broken MV692 for repair.
The problem located in the white IC named 29-25.
I saw this chip about 50 times and never had the chance to look inside.
Now it's time
There is schematic.

 

[bezen4uk]

internal view

PS: So, it was dead P-channel FET BSS84 (marked as PD).

 

[Dr_J]

That's cool!!!
That rocks!!!

ROCK-ON!

 

[Sredna]

Great, thanks! 


BTW, I might have one lying around, do you need one?

Best,

Anders

 

[bezen4uk]

Hi Anders,
Thank you for kind offer.
I'd like to try modern SMD clone.
If it works, I'll post PCB file. 

 

[berl]

Hi,
I think it is a PNP transistor and not a mosfet.
Check on a broken one of mines and it is recognized as a normal PNP.
I repaired one of my microphones with the same back-ingeneering (I found slightly different resistors values) and it does the job for more that two years. 
BTW I made a small pcb for easiest future repairs.

 

[sneak]

Hi Anders,
Thank you for kind offer.
I'd like to try modern SMD clone.
If it works, I'll post PCB file.

cc: @berl
Were you all able to make these PCBs? I have a UM70 that I think needs the same treatment

 

[sneak]

Great, thanks!


BTW, I might have one lying around, do you need one?

Best,

Anders

Heya, Anders do you still have extras laying around by chance? Any guidance on testing would be appreciated as well

 

[jp8]

I have three of them lying around, waiting for repair/upgrade/modification. One of them I converted to P48. Can you describe the problem you're facing with your mic? Maybe I can do some measurements for your reference.

Mind you: there are different versions of the module for different ranges of Idss of the JFET. So mine may measure slightly different voltages/currents/resistors than yours.

Jan

 

[sneak]

I have three of them lying around, waiting for repair/upgrade/modification. One of them I converted to P48. Can you describe the problem you're facing with your mic? Maybe I can do some measurements for your reference.

Mind you: there are different versions of the module for different ranges of Idss of the JFET. So mine may measure slightly different voltages/currents/resistors than yours.

Jan

Thanks for reaching out!
My mic is a UM70s and it has a crackly static noise, low output and drops signal.

Wondering if this could have anything to do with the T1 transistor or IS 29-25.

All I've done so far is tear the mic down and check for obvious broken connections.
Could you share your plan for debugging the three you have?

 

[jp8]

The IS 29-25 would not be my first suspect. Could be the capsule, JFET, elcaps or flux or debris on the PCB. I would start by ruling out the capsule. Proceed as follows:

• Remove the capsule from the body. Without capsule, the mic will pick up lots of mains hum, so you cannot listen to it through a preamp this way.
• To minimize the hum, take a 100nF or so film cap with long leads and fold the end of the leads as pictured, then push this leg into the center terminal as pictured.
• The other leg of the capacitor should be tied to the mic body or XLR shield (pin 1). I did it as pictured, because I had this body lying open.
• No connect to a preamp and listen to the microphone. Gently turn up the gain. You will still hear hum, but I guess the noise, if still present, should be audible.
• No noise? Then unfortunately your capsule is bad. I had two of my M70 capsules repaired by Thiersch, Germany. Takes a long while before he'll have your capsule repaired, but they are worth it.
• If the noise was still there, open the mic body. There is a guide online to guide you in doing that. If you cannot find it, I'll try to find it and share with you.
• Take out the PCBA, which is held by two small screws to the inner frame. Be carefull when loosening the screw next to the round transformer in the middle. This transformer has fragile wires, which you do not want to touch or they might be cut.
• Clean the PCB with a suitable flux cleaner for rosin flux, or IPA with a cotton swab. You may have to carefully scratch off thick crusts of flux.
• Replace all the elcaps. Axial elcaps are hard to get nowadays, but radial will fit. I replaced all of them with 10uF 25V panasonic FC series caps. See picture. I've als tested new 1uF and 33uF capscto replace the C8 and C9 decoupling caps and noticed that 10uF lowered the noise compared to the original 1uF. 33uF gave no improvement, so I put 10uF in.
• You could try again now. When odds are with you, the noise etc. will be gone. If not, the JFET is the next suspect. If you google for it, you will find suitable alternates, but you may have to select for right Idss an Vp.

Good luck!

 

[jp8]

Didn't read your post carefully: you already took the mic apart. Anyway, regarding my plans for the three MV692s: Two had broken capsules. Reskinned and repaired by Thiersch. One has broken switches and I use it for experiments. One body was OK and the third has a motorboating sound. I'm not going to repair that one as two out of the three will get new preamp electronics I will design myself. I'll leave one original, except that I will give it an XLR and make it P48 compatible. All long term plans, btw.

Jan