12-28-2016, 10:26 PM
Check the voltages on TR701 and TR702. This is the circuit that detects touching the knob.
It basically works by detecting the hum created when you touch the tuning knob. Just like when you touch the top cap of a 75 tube in a radio, an AC signal is produced. TR701 and TR702 amplify the AC hum signal which then is rectified by D702, providing a DC control voltage. The DC voltage created when the knob is touched is applied to the base of TR703 which switches the AFC off and changes the meter illumination.
It seems that TR703 and the following circuitry is working fine because pressing the Auto-magic switch turns the AFC on and off and changes the meter illumination color. This is accomplished by applying DC to the base of TR703 through R329 from the Auto-magic switch.
So the problem is either the knob is not connected to the TR701, TR702 AC amplifier or the amp circuit itself is not working. Touch the tuning knob and see if you see a change in DC voltage at the cathode of D702 (19 TP on schematic)
It basically works by detecting the hum created when you touch the tuning knob. Just like when you touch the top cap of a 75 tube in a radio, an AC signal is produced. TR701 and TR702 amplify the AC hum signal which then is rectified by D702, providing a DC control voltage. The DC voltage created when the knob is touched is applied to the base of TR703 which switches the AFC off and changes the meter illumination.
It seems that TR703 and the following circuitry is working fine because pressing the Auto-magic switch turns the AFC on and off and changes the meter illumination color. This is accomplished by applying DC to the base of TR703 through R329 from the Auto-magic switch.
So the problem is either the knob is not connected to the TR701, TR702 AC amplifier or the amp circuit itself is not working. Touch the tuning knob and see if you see a change in DC voltage at the cathode of D702 (19 TP on schematic)
