Audio howl in AE three-slot problem.

[mentalstampede]

I partially restored an AE LPC-72-55 DTMF three-slot awhile back. The phone had had the usual hack-job conversion done involving removal of the relay and fitting of a ringer in its place. I restored the phone cosmetically, and have been using it as it is without any problems.

Recently, I decided I would like to restore the coin-control functions as well. I have acquired a relay and all the other missing parts, and assembled things according to the wiring diagram. However, now there is a very loud whistling or howling noise audible in the circuit. When the phone is wired for 5 cent service, the noise is only audible if the coin trigger is not tripped. If I connect the leads to the microswitch in the coin chute to enable 10 cent service, the noise is audible at all times.

I'm assuming there is something in the DTMF oscillator circuit that is causing this, but I'm at a loss as to what it might be. Has anyone else encountered this problem before?