Gefell MV 691 - Low Output

[smilan]

Hi,
I have a Gefell MV 691 with a M71 capsule.
The output of the 691 is significantly lower compared to a MV692 with the same capsule.
I'm suspecting that the DC to DC booster is not supplying enough voltage to the capsule, I measured the voltage between the junction of R10 with C9 and R9 with C8 and it is 41V
The voltage across D1 and D2 is +\- 37V.
So it seems to be too low, isn't it?
I measured the inductance between the coil leads to the ground bus, so BV137 seems to be good.
What else I should test to find the problem with this mic?

 

[Murdock]

I think sometimes the screw wich holds the core halves of the coil together comes a bit loose and therefore reduces inductance. Worth a try to tighten it a bit maybe. But don't overdo it.

 

[smilan]

I think sometimes the screw wich holds the core halves of the coil together comes a bit loose and therefore reduces inductance. Worth a try to tighten it a bit maybe. But don't overdo it.

Yes, after spending hours trying to find what's wrong with the 691I found the solution for the low output, I was needed to screw the capsule harder than I usually do and the output came back to normal

 

[jpg.jpeg]

Hi folks, I am running into a similar issue with my mv691. I have a 691 and a 692, and when testing with the same M70 capsule (which sounds great on the 692), the 691 has a high noise floor and virtually no output. I'm using a 48 to 12v converter cable for the 691. I've measured the voltage on the connection where pin 4 comes into the PCB, and it measures a steady 12v, so it seems to be getting the correct power.

What's weird is that when I tap on the metal body or rails on the side of the PCB, it sounds exactly like tapping on the side of a live mic - a full frequency response (as far as the sound of tapping goes). But there's basically no signal off the capsule. Only the occasional thump (and a steady noise floor) with 50-60dB of gain applied.

My first thought was that some of the capacitors may be bad/underperforming. So far I have replaced C1, C2, C4 and C5 to no improvement. I have parts to replace all other capacitors in the circuit, as well as T1 and T2, but am trying to be methodical before I just gut and replace all the components.

Can anyone offer advice on how to approach this? Ideas for test points, insight on what might be going on, anything is welcome!