08-23-2023, 05:16 PM
Yes, this is kind of interesting but IMHO, here is what happens:
Usually, AVC is developed from either tthe detector anode or AVC Anode. this will be a negative voltage. It is usually impressed on the secondary of the ist IF transformer and on the "grid" of the convertor, either through the antenna coupling transformer or the loop antenna. In this set (The 42PT94 Transitore and other sets are similar), the AVC is impressed on the RF section of the convertor like most other AA5s, but instead of impressing on the secondary of the 1st IF transformer, Philco impressed the AVC Voltage on the Suppressor Grid of the 7B7 IF Tube. Impressing a negative AVC Voltage on the IF Suppressor will have the same effect as impressing it on the other lead of the IF input transformer secondary as RCA and most other manufacturers did. Putting a 3V battery across the 0.1 uF Cap Item # 5 on the schematic, with the+ terminal of the battery to the B- side of the Cap and the - side to the antenna coil side of the Cap will force max gain.
Where your real problem may lie is with the radio-phono switch, which is quite unique. If you look at # 36 on the schematic, it is an SPDT Switch that switches common B- / neutral (we hope) between the phono motor (for Phono) and the cathodes of the 7A8 Convertor and 7B7 IF. In other words, with the pickup raised, the phono motor is turned on and the cathodes of the Convertor and IF tubes are disconnected from B-, therefore rendering these staves of the radio inoperative. It seems that the volume control may be incorrectly drawn. The instructions in the 1941 yearbook describe a "New type volume control". I assume that there are parallel pots. If you recapped the radio and still have hum, disconnect the Audio output of the tonearm from the radio. The crystal pickup is likely open.
I hope this helps. (I hope I'm right).
Usually, AVC is developed from either tthe detector anode or AVC Anode. this will be a negative voltage. It is usually impressed on the secondary of the ist IF transformer and on the "grid" of the convertor, either through the antenna coupling transformer or the loop antenna. In this set (The 42PT94 Transitore and other sets are similar), the AVC is impressed on the RF section of the convertor like most other AA5s, but instead of impressing on the secondary of the 1st IF transformer, Philco impressed the AVC Voltage on the Suppressor Grid of the 7B7 IF Tube. Impressing a negative AVC Voltage on the IF Suppressor will have the same effect as impressing it on the other lead of the IF input transformer secondary as RCA and most other manufacturers did. Putting a 3V battery across the 0.1 uF Cap Item # 5 on the schematic, with the+ terminal of the battery to the B- side of the Cap and the - side to the antenna coil side of the Cap will force max gain.
Where your real problem may lie is with the radio-phono switch, which is quite unique. If you look at # 36 on the schematic, it is an SPDT Switch that switches common B- / neutral (we hope) between the phono motor (for Phono) and the cathodes of the 7A8 Convertor and 7B7 IF. In other words, with the pickup raised, the phono motor is turned on and the cathodes of the Convertor and IF tubes are disconnected from B-, therefore rendering these staves of the radio inoperative. It seems that the volume control may be incorrectly drawn. The instructions in the 1941 yearbook describe a "New type volume control". I assume that there are parallel pots. If you recapped the radio and still have hum, disconnect the Audio output of the tonearm from the radio. The crystal pickup is likely open.
I hope this helps. (I hope I'm right).
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
