10-27-2013, 09:56 PM
Yes, that would be very helpful if you could post the voltage readings of each of the taps on the voltage divider resistor and the pins of the AVC tube. Make all measurements referenced to chassis ground.
This is a difficult and confusing set to troubleshoot because of the floating voltage divider supplies. RF and IF tube cathodes are floating at about +90V, and are actually directly connected to the converter tube's screen grid whose cathode is referenced to ground.
So if the RF and IF cathodes are at +90V, the control grids will have to be at about +87 to give an effective negative bias of -3 V for full gain. On a strong signal, the control grids will be maybe at +75 V for a effective AVC bias of -15 V.
The AVC tube supplies the variable +87 v to + 75 V grid bias from its plate circuit, controlled by its grid voltage obtained from the detector cathode. The localizer control should set the plate voltage to +87 (3V less than RF and IF cathodes) with no signal so the AVC feedback can then reduce the plate voltage further as the received signal increases.
This is actually DC amplified AVC and probably keeps the audio level very stable with varying signals, but its very complicated and finicky as to adjustment. RCA used a similar circuit in its first AVC equipped radios, but you can see why it never became popular.
This is a difficult and confusing set to troubleshoot because of the floating voltage divider supplies. RF and IF tube cathodes are floating at about +90V, and are actually directly connected to the converter tube's screen grid whose cathode is referenced to ground.
So if the RF and IF cathodes are at +90V, the control grids will have to be at about +87 to give an effective negative bias of -3 V for full gain. On a strong signal, the control grids will be maybe at +75 V for a effective AVC bias of -15 V.
The AVC tube supplies the variable +87 v to + 75 V grid bias from its plate circuit, controlled by its grid voltage obtained from the detector cathode. The localizer control should set the plate voltage to +87 (3V less than RF and IF cathodes) with no signal so the AVC feedback can then reduce the plate voltage further as the received signal increases.
This is actually DC amplified AVC and probably keeps the audio level very stable with varying signals, but its very complicated and finicky as to adjustment. RCA used a similar circuit in its first AVC equipped radios, but you can see why it never became popular.
