KM84 Problem

The Output Level of my KM84 is very low, but there is no distortion audible. When I turn the Phantompower off at the Micpre, the Output rises to a normal Level for about 10sec. till there is no more Energy in the PH.
The MicPre is okay.

Anyone a Idea ?? 

Just write songs that are shorter than 10 seconds... ;D

It sounds like somehow pin one on the mic  is not finding ground in the mic pre. when you switch phantom off the phantom resistors are referenced to ground through a bleed resistor.  Its a long shot,  but you should definitely check to make sure the XLR cord you are using has both the pin one grounds connected. Every once in a while I will find an XLR cable that only has the shield connected to pin one on one connector.

So change the cable and try another mic pre before you tear into the mic.

Common fault is failure of the tantalum power supply bypass. Also, the polarisation voltage components may be sus. You can check the amp by putting a signal generator in instead of the capsule.

THX Guys...I will check this and tell you if it works !

Common fault on KM84 actually is - like BluBird is pointing out - that mic's pin1 misses connection to the cable XLR because of the mechanical design of the barrel (and/or the rubber gasket that is hiding inside the XLR-M?). Try with a different type xlr cable? Verify connection between pin1's? Connect pin1's to xlr housing/chassis ground?

Jakob E.

I checked the cable and grounding, but everything is okay. I have done some measurements and found out that potentials in the Polarisation-path are much too low. i draw it into the schem, maybe someone have a idea whats going on !

another try for the schem

hmm the upload wont work :-( so
i try to describe.... Between R5 and R8 : 25V; R1:R8 16V; Capsule: 0,5V
All other potentials as seen in the original schematic are okay...

Keep in mind that your voltmeter is loading the point you are measuring at!
The capsule is fed through a 1 G.ohm resistor.
The internal resistor of your voltmeter is probably low compared to the 1 G.ohm resistor, so your measurements will give inaccurate results when measuring at this point. (Especially at the capsule.)

Trucki,

There are seldom serious fault on KM84, I have  repaired/refurbished hundred of them over the years, it's mostly mechanical fault, screws as loosened and some time a shorts in a tantalum electrolyte.

But of course, there have been some exceptions, such as the failure of the capsule, bad FET transistor, interruptions in the transformer.

If you have about +10 volts on the FET transistor Drain, probably the amplifier works as it should, and it is said earlier, you can not measure the polarity voltage with a standard "DVM" meter, so if you have the opportunity to try another capsule, you should do this first of all.

However, if you suspect fault on polarity voltage, try changing the capacitors C6 and C10, but probably they are ok.

--Bo

THX for the Suggestions Guys. And now it is clear to me that i cant measure the voltage on the capsule with a DVM !
I think the FET works fine, because the Output-Signal is not distorted, only very low.
i will change the Caps and look whats happening then.
I wonder whats the function of R13 ( the 10 Ohms Res before the XLR Pin1) ..... can someone tell me ?

Trucki,

Normally when done in this way, it is with mains powered units that can be connected to other units, and a resistor like this this prevents ground loops between signal ground, chassis and mains earth.

But in this case, the idea of ​​a 10 ohm resistor in series with the signal/phantom ground, is probably supposed to suppress RF disturbances, I would guess.

--Bo