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[Zoors]

Hi all.

I have a couple of these KM84 here that are very weak in signal.
If I remove the capsule, and measure the +48 spot (where the capsule clicks in, I only got some 3-4 volts. Before the 1 gig I do have around 40 volts.

My guess is that it is difficult to get a true value at this point as the voltmeter (anyone) is grounding this voltage that goes thru a 1 gohm resistor.

Is it time to swap capsule on this one, or might the 1 gig be bad?

Thanks a million for any experienced hint!

[David Satz]

Your guess is correct; you can't measure a voltage correctly unless the impedance of your meter is an order of magnitude higher than the source resistance of the voltage you are trying to measure.

In other words, you can't measure the phantom voltage of a KM 84 where the capsule contact pin goes, unless you have a very unusual measuring instrument.

[Klaus Heyne]

I venture an opinion: neither your capsule is bad, nor are any of the high impedance resistors.

Usually, when there are complaints about a "weak" signal coming out of KM 8x, the source of frustration is either the current output strapping (set to 50 or 150 Ohms, with pad resistors engaged, rather than the 200 Ohms European standard) or a defective FET.

Considering that you mentioned more than one KM84 suffering from the same 'weak' output (you did not specify how much down, in dB, these are compared to other condenser mics), I lean towards a low-output strapping.

If that is the case (you would need to physically inspect your mics' insides), set them to 200 Ohms, and you will  be fine.

Add on, after reading your response: good 3819 FETs are not easy to find. You can always use the types Neumann used, after they discontinued the 3819 FET in their mics: K105 or SK107.

[Zoors]

Hi Klaus.

Thanks for very fast response!
I have gone deeper into it, and I lifted the 47k resistor. (R2)
I wanted to see 10V there, but it was to low, just some volt.
Then I could measure 3.5kohms between the D and S on the FET. So I disconnected the FET too, and yes...it is a bad FET.
I guess a 2N3819 will do there...instead of the S2436.
Good mike on my test setup gives -20, but this one gives -70.
Lets say it is 50dB under what it should. The other KM84 I have is dead. I will process it when I got the FETs landed.


Its on 200ohms, as I am a Swede  

Thank you very much for a valuable forum, and a good response!
I will try to share my experience all I can.