KM84 Repair

[lukegalea16]

Hi! I'm attempting my first microphone circuitry repair on this KM84. This microphone has no output (loud hum only) and when I lightly touch the mic's outer shell I can hear some short noise/distortion. I'm following this schematic. So far I've changed C8 temporarily as you can see from the photos below and restored the voltage back to 24V. C5 has around 20.5V as well. Between R8 and R5 I get 22.2V and between R1 and R8 I get 15V. Thus I would like to change C6 and C10 but they seem to be sandwiched between the two PCBs as shown in the photos. Any experience working with these? I'm not sure how I can take out the red caps sandwiched between the boards in order to try and change them?




Thanks!

 

[Khron]

Between R8 and R5 I get 22.2V and between R1 and R8 I get 15V.

You DO see that R8 and R5 are 10Meg each, right? What input impedance spec does your multimeter have? And what sort of voltage dividers get formed, when you probe those voltages?

C6 and C10 are probably just fine (film caps, by the looks of them?).

Is the JFET's drain voltage ("after" R4) even close to the ~10V it should be?

If all that checks out, it's probably safe to assume the electronics are fine, and your next suspect is (unfortunately) the capsule...

 

[lukegalea16]

You DO see that R8 and R5 are 10Meg each, right? What input impedance spec does your multimeter have? And what sort of voltage dividers get formed, when you probe those voltages?

C6 and C10 are probably just fine (film caps, by the looks of them?).

Is the JFET's drain voltage ("after" R4) even close to the ~10V it should be?

If all that checks out, it's probably safe to assume the electronics are fine, and your next suspect is (unfortunately) the capsule...

Hah very fair point that I overlooked! Thanks.

After R4 it's around 5V which is odd as well. I replaced C4 and C5 but had no improvements. I'm thinking of replacing the JFET but it's a bit tricky as well. Correct me if I'm wrong but if it's the capsule I'm not sure if it's worth repairing?

 

[RuudNL]

I have repaired many KM84's. For some unknown reason often the FET is the problem.
But it is not as simple as only replacing the FET, because you also have to rebias the FET for the specific one that you used. (FETs have a very big tolerance, that's why.) This means that you have to change the source resistor to bring the FET in the optimal working condition. You can do this by temporarily replacing the source resistor with a (trimmer)potentiometer (10 or 20 K.ohm) and if you have found the correct value, measure the resistance of the potentiometer and replace it with a fixed resistor with a value as close as possible to the value you measured on the potentiometer.
Another common problem is a failed tantalum capacitor, but as long as you measure volatage across them, I wouldn't suspect them in the first place. (Most of the time a failed tantalum capacitor shows a short.)

 

[lukegalea16]

I have repaired many KM84's. For some unknown reason often the FET is the problem.
But it is not as simple as only replacing the FET, because you also have to rebias the FET for the specific one that you used. (FETs have a very big tolerance, that's why.) This means that you have to change the source resistor to bring the FET in the optimal working condition. You can do this by temporarily replacing the source resistor with a (trimmer)potentiometer (10 or 20 K.ohm) and if you have found the correct value, measure the resistance of the potentiometer and replace it with a fixed resistor with a value as close as possible to the value you measured on the potentiometer.
Another common problem is a failed tantalum capacitor, but as long as you measure volatage across them, I wouldn't suspect them in the first place. (Most of the time a failed tantalum capacitor shows a short.)

Good to know! I'll have to be careful how to replace it as it's position is a bit uncomfortable. I already replaced one tantalum which improved the voltage at that stage. Can you recommend a good JFET substitute please?

 

[RuudNL]

The 2N3819 is still available. You could also use a 2SK30A. (Same pin positions.)

 

[lukegalea16]

It was the JFET! Thanks

 

[Latie]

I have repaired many KM84's. For some unknown reason often the FET is the problem.

Too bad you're not in my neighborhood. I have a very old KM that needs a doctor