Telefunken M60 pair

[Gus]

When you power down the microphone do you measure a short from source to drain?

Do you have a scope?
If so check if there is any high frequency oscillation at the drain.

 

[mikeyB]

When you power down the microphone do you measure a short from source to drain?

Do you have a scope?
If so check if there is any high frequency oscillation at the drain.

Hi Gus, when you say check for oscillation at the drain, is this with the capsule disconnected? Should I solder a 50pF in place of the capsule as suggested by Khron? I'll measure Drain to Source unpowered, I think I measured this early days and recall it being approx 50ohms, but I check again.

Thanks In Advance.

 

[Gus]

I would leave the capsule connected. Oscillations are caused by positive feedback and changing things changes the distance or orientation of traces, wires etc.

This is a puzzle it reads like the JFET is shorted but if you turn it off and change a cap it fixed itself.

Have you turned it off waited a 10min or so and turn it on again does it work?


The reason I asked about the scope is sometimes high frequency oscillation can give odd readings on a DMM

Have you google etc this issue to find out if others have this issue with the microphone?

 

[mikeyB]

Hi Gus, very tricky to keep the capsule connected as the capsule screws to the body, so nothing to hold it in place with the body off! Could I mimic the capsule by soldering a cap in it's place as suggested by Khron? - something like 47p or 100p?
The other question about switching on 10 mins later, answer is yes it does come back working but then eventually fails. Then I replace the 10n cap and the process repeats itself???!!! I'm very wary of X7R caps I have to hand, they are very old stock and have no voltage rating on them. I've ordered some new 100V 10n caps and also some film caps. I had a few experiences of bad multilayer ceramics in decoupling situations!! I'll keep you posted, thanks for the quick reply.
ps. I have a scope, (that indeed pointed me toward the ceramic cap problem. The scope smoked and it was a shorted cap, just like the one in the mic, hence my suspicions!!) I will check for oscillations.

 

[Gus]

Are you changing the cap between the 100k and 1 gig?(or what ever high value it is) resistor that goes to the capsule, if so I don't understand how this could be causing the issue

What I would try next is lifting the 470pf lead at the JFET gate node.

This removes anything before the input cap. Power it up and measure voltages, if it fails there is an issue in the gain stage, if it does not fail the issue is most likely in the charge circuit and maybe the input cap.
When you have an odd fail like this if is often good to break the circuit up in parts to test each sections function.

THERE IS AN ISSUE it is very easy to melt a polystyrene cap so you need to work fast with a good soldering iron and a good pair of needle noise pliers to heatsink and remove the cap lead from the circuit