Sony C38B microphone - strangely behaved hiss.

Hi!

I'm troubleshooting a Sony C38b mic which has a suspected bad capsule with very little gold left.

Yesterday I tried a different capsule in there for testing purposes (chinese '747' type).
With the test capsule I get a "normal" amount of signal but also a lot of hiss.

The High Cut switch (S2) makes no audible difference to the hiss level. The pad switch reduces the hiss by 8dB, so I suspect that the hiss is produced in the first stage of the head amp.

Here's the strange thing -  the hiss gets progressively louder when the mic is switched from M to M1 to V1 to V2. This switch (S3-1 on the schem below) just changes the output capacitors - the hiss gets louder as the capacitor gets smaller.

Does anyone have any thoughts on why that should that happen?


Here's the schematic



Some other observations

1. With phantom power I get about 10.2V in the positions marked '9.2V' on the diagram.
2. The LED does not light up when switched on. (That may be a red herring, as I hazily recall that they only light briefly when first switched.)
3. These are not the easiest mics to play with - quite a bit of delicate dismantling needs to be done to test voltages etc.

Thanks loads!

Stewart

Hi Stewart!

I don't see no led in the schematic... Could it be a bulb? PL for instance? (Power Light maybe?) With C8, R13, R14, & Q3 being the one blink gizmo...

Axel

You can substitute a cap for the capsule 50pf to 100pf to ground this removes the DC to DC converter maybe the DC to DC converter has noise on its output.  OR disconnect a capsule lead and switch in the pad for 100pf to ground this removes the capsule and DC to DC.

Do you have a scope to look at the power supply nodes?  Could be noise in the DC power

Check C9 it is there as a cap multiplier and also helps with zener noise(R11, C9 RC filter)
Voltage at the Zener D2 cathode?  Emitter of Q4 should be about .6VDC more than the zener voltage.  There is a NOTE at the bottom right "readings are taken with a 20,000 ohms per volt VOM"

Check C7 filter for amp and C10 filter for DC to DC converter

Lamp is wired for a brief pulse note C8, R13 and R14 and the use of an NPN transistor with the lamp in the collector leg

I would check S3-2 contacts

Is C3 an electrolytic?  If so check it for leakage.

Thanks for the help so far guys.

Axel, yes, I did mean PL. On the bench it looks very much like an LED.

Gus, yes I do have a couple of scopes, and will try the capacitor-for-capsule trick when I get back to work. I popped in the new capsule because listening can tell you a lot. In this case wiring in a capacitor would be good for bench testing as the mic will need a lot of disassembly.

I had wondered about leakage and was suspecting C6 because it is always in series, but of course C3/4/5 are in the circuit all the time too via R6/7/8. C3 is indeed an electro.

Cheers!

Stewart