U87 trouble shooting

[pucho812]

1. Mic powers on
2. Mic passes signal for a second
3. Loud pop then mic is silent not passing signal.

Capsule?

 

[Whoops]

It could be the capsule yes.

Do you have a spare LDC capsule that could replace the original one for testing purposes?

 

[pucho812]

It could be the capsule yes.

Do you have a spare LDC capsule that replace the original one for testing purposes?

Hmmmm. I have two single capsules.. I suppose for test purpose a u87 in cardioid only be OK.
good idea.
thanks

 

[ubxf]

If you have access to another one the head basket is removable so you can easily swap the capsule

 

[pucho812]

If you have access to another one the head basket is removable so you can easily swap the capsule

that I know but I do not have access to a second unit at the moment.

 

[mrclunk]

Swap the capsule with a couple of 47pf caps and check voltages?

 

[pucho812]

Swap the capsule with a couple of 47pf caps and check voltages?

One for each side?

 

[mrclunk]

One for each side?

yeah, might be best to make a dummy head with the 300k resistor in it too. Then you don't have to unsolder the capsule wires.
Wrap in tin foil to keep out interference.
(I'm no mic expert, just thinking out loud!)

 

[pucho812]

yeah, might be best to make a dummy head with the 300k resistor in it too. Then you don't have to unsolder the capsule wires.
Wrap in tin foil to keep out interference.
(I'm no mic expert, just thinking out loud!)

I’m no mic expert either. But I do fix from time to time.

 

[pucho812]

Ah hah, I am not getting any voltage at T1 on the 1972 schematic. I am measuring less then 1vdc at any of the g, s, d connections.

I got a board 930 21

 

[MagnetoSound]

So you might have a shorted FET, or any of C7, C8 or C10 might be shorted or leaky …

You should have 24v on the supply side of R14 (the zener side) … if not there are a few components there to check.