06-18-2010, 01:02 PM
I've been out of town, so I'm just getting back to this today.
The 250 mmf AVC filter capacitor tests good, so I didn't replace it.
As 7estatdef suggested, I measured the AVC voltage from the IF side of the 10 Meg AVC resistor (#13) to ground. With the radio tuned to the strongest local station, the voltage measured about 750 mv DC. On a weak station, it was about 330 mv DC. Tuned away from any station, it was still about 330 mv.
The measurements were similar with my VTVM and with my modern digital multimeter, which is rated at 10 Meg input resistance. The VTVM measured about 30 mv higher. It is rated 11 Meg/volt (1 Meg in probe), and I'm not too confident of the calibration.
I don't know how to interpret these measurements. Does this amount of difference seem reasonable?
Edited to add: I moved the cabinet around to reorient the loop antenna with respect to various radio stations. The AVC voltage when tuned to a strong station went up to 950 mv DC, but it was still around 330 mv on weak stations or between stations. The chassis was outside the cabinet, connected to the loop and ground with test leads. My workbench is in my basement, so it is likely that I'm getting weaker reception on all stations than if I were upstairs. I imagine the AVC voltage on strong stations would increase even more if I were upstairs.
Edited again: I tried an NOS 1H5GT detector tube without any change.
The 250 mmf AVC filter capacitor tests good, so I didn't replace it.
As 7estatdef suggested, I measured the AVC voltage from the IF side of the 10 Meg AVC resistor (#13) to ground. With the radio tuned to the strongest local station, the voltage measured about 750 mv DC. On a weak station, it was about 330 mv DC. Tuned away from any station, it was still about 330 mv.
The measurements were similar with my VTVM and with my modern digital multimeter, which is rated at 10 Meg input resistance. The VTVM measured about 30 mv higher. It is rated 11 Meg/volt (1 Meg in probe), and I'm not too confident of the calibration.
I don't know how to interpret these measurements. Does this amount of difference seem reasonable?
Edited to add: I moved the cabinet around to reorient the loop antenna with respect to various radio stations. The AVC voltage when tuned to a strong station went up to 950 mv DC, but it was still around 330 mv on weak stations or between stations. The chassis was outside the cabinet, connected to the loop and ground with test leads. My workbench is in my basement, so it is likely that I'm getting weaker reception on all stations than if I were upstairs. I imagine the AVC voltage on strong stations would increase even more if I were upstairs.
Edited again: I tried an NOS 1H5GT detector tube without any change.
John Honeycutt
