Noisy AKG C1000S after some time

[coolbaci]

Hi,

We've got a AKG C1000S mic powered thru A&H GL2400 Phantom power. Part way through the service (about 1 hour in) it started producing some hiss which started getting louder over a minute or so (by which time we'd found out what it was thankfully).

We took off phantom power for about 5 seconds and turned it back on. The hiss was initally gone but came back in about 30 seconds.

Just wondering if anyone know what to look for, is it servicable or throw away, I have dabbled in soldering but only know how to follow designs not find faults.

Service manual is here

Thanks,
Joel

[sien]

Maybe you could try sending it back to AKG?
AKG Acoustics, U.S.
Phone +1 818-920-3237

[daver2]

I am impressed that AKG included the schematic diagram!

One possibility is a capacitor becoming leaky. The other possibility is a semiconductor (e.g. a transistor) becoming noisy when it heats up.

One quick check for a temperature-related effect is to try and run the microphone with the circuit board exposed (remembering that phantom power is connected and that a short circuit could damage the fuse/power supply in your mixer desk - so place things on a well-insulated surface).

Once the hiss starts - cool the board down with the wife's hair dryer set to cold. If that cures the problem - obtain a can of freezer spray and spray it on each semiconductor in turn until you find the noisy device (by the hiss disappearing when you spray it).

If using a cold blast of air doesn't cure the problem - I would use a piece of test equipment called an oscilloscope to probe around the circuit from the microphone capsule forwards until the noise manifested itself on the oscilloscope trace. You have then most likely found the faulty component (or group of components).

Dave