Most common faults with the older detectors such as SD 2000 2100 2200 is the input N channel Fet going noisy , short circuit or open circuit. This component has around 185 volts of clamped back emf from the coil across the drain to source connections, mind you that this part is rated at 200v or some are 240 volts it can still fail by a number of causes. Removing the coil when the detector is operating can generate high voltage that can blow it up. A bad coil connector pin and socket on the coil connector, or shorting coil wires can cause it to pop. The device is very old, it have a ohmic loss of 10 ohms and that is bad, newer replacements are only 1 or 2 ohms loss, it makes a big improvement when driving the LM394 differential preamplifier. It is easy to find, follow the yellow coil wire to the circuit board, using a multimeter find the connection to the black transistor with 3 legs, unsolder and replace with the same part VN10 or get a new input get with instructions from me. The pins are in different positions and needs explaining on how to do this.
