Today a condenser microphone found its way to my bench for repairs. It had a nasty intermittent problem which occurred so now and then with a drop-out of signal whilst adding noise. I opened it up and found an interesting and transformerless circuitry, where the pcb is populated with through-hole and surface mount components. There’s a small daughter pcb shielded in it’s own metal case. At first I thought that it would be some delicate filtering circuitry which needed RF shielding, but after further investigation it turns out that the it contains a simple oscillator on the back. The small pcb appears to be a voltage multiplier circuit where the housing shields the sensitive audio circuitry from the alternating high voltages. A oscillating transistor creates an alternating voltage where the multiplier and filtering circuit cranks it up to a clean 90Vdc to supply the condenser capsule.
The fault appeared to be a small cable, stuck in between the metal frame and a capacitor. Because of this, the insulation of the cable got perforated over time and started leaking to the chassis. I removed the pcb, got the cable loose and rewired it different so that it would be loose. At last I applied a drop of silicon coating to the bruise so that it wouldn’t leak again if the cable would be touching something else.