07-10-2016, 12:16 PM
I checked the four mica capacitors that are in the RF signal path (C400, C407, C409, and C415). All were good - values were close to spec and no leakage indicated (up to 40 megs - the limit of my DVM). I then connected up the signal generator and monitored the AVC voltage. On the broadcast band with no signal, AVC was -2.7 volts. Set for a loud output, I measured -8.7 volts. The AVC then seemed to gradually increase to -10.2 volts at the maximum output of my generator.
There seems to be a quite high level of "hiss" which generally goes away on stronger local stations, but it is still there on weaker stations. I am not quite sure what to do next. Although all tubes were tested, should I try substituting tubes in the RF and IF section at this point? Another concern is the screen voltage for the converter and IF amplifiers. I measure about 87 volts. The tube manual for a type 78 (first IF amp) says the screen voltage should be 100 volts with 250 volts on the plate. All of the resistors that are part of the circuit that supplies the screen voltage are in tolerance. But my power transformer is not original. I may play around with the screen voltage and see if that makes any difference.
I pulled out my restored Philco 37-670 and compared it side by side with the 46-888, using the SAME antenna. The performance, including the hiss level, was comparable. However, the 37-670 only has ONE IF amplifier stage. I would have thought that the 46-888 would blow it away, but they appear comparable (subjective).
There seems to be a quite high level of "hiss" which generally goes away on stronger local stations, but it is still there on weaker stations. I am not quite sure what to do next. Although all tubes were tested, should I try substituting tubes in the RF and IF section at this point? Another concern is the screen voltage for the converter and IF amplifiers. I measure about 87 volts. The tube manual for a type 78 (first IF amp) says the screen voltage should be 100 volts with 250 volts on the plate. All of the resistors that are part of the circuit that supplies the screen voltage are in tolerance. But my power transformer is not original. I may play around with the screen voltage and see if that makes any difference.
I pulled out my restored Philco 37-670 and compared it side by side with the 46-888, using the SAME antenna. The performance, including the hiss level, was comparable. However, the 37-670 only has ONE IF amplifier stage. I would have thought that the 46-888 would blow it away, but they appear comparable (subjective).
