03-18-2023, 11:01 PM
+1 on Morzh's description. The schematic is a little confusing, as the negative end of the supply is usually toward the bottom of the schematic. From your description, I believe that the supply is miswired.
This supply is built in a manner common to some of the early and mid thirties radios but was not done this way in radios from the 40s and later. The + terminals of both caps go to the filament connection of the 5Y3 that is on pin 2 of the 5Y3's socket. Another wire from that same pin distributes B+ to the rest of the radio. Pin 6 of the 6K7 is blank but may be being used as a tie point. I think that it would be better to connect the + terminals of both caps to pin 2 of the 5Y3 socket. The B+ terminal of the audio output transformer, the 6K6 screen grid and B+ to other circuits should also connect here. Where the negative terminals of the caps is connected is very important. The negative terminal of Cap 29B, the 12uF cap should connect to the center tap of the high voltage secondary of the power transformer, Pin 1 of the speaker plug (Leg 1 of the field coil) and to one side of R54, an 800K resistor. The negative terminal of Cap 29A, the 8uF cap connects to Pin 2 of the speaker plug (Leg 2 of the field coil), and to one terminal each of R53, R51 and C60. In other words, the negative terminals of C29 A and B are not tied together, but coinnect to either side of teh field coil. The purpose of the confusing negative terminal connections is to consider the chassis to be B- / C+ and the Transformer secondary center tap to be C-, the bias for the 1st and 2nd AF tubes.
I generally like to up these caps to about 20 uF. Others may disagree, but the tolerance of electrolytics back in the day was +80 / -20% New caps nowadays are usually +/- 10 or 20%, so one cn go up a little. However, i think that your hum is due to the fact that the negative terminals of the 2 caps don't straddle the filed coil, and the + side of one of the caps may be connected to the wrong place, depending on what is connected to that terminal strip.
This supply is built in a manner common to some of the early and mid thirties radios but was not done this way in radios from the 40s and later. The + terminals of both caps go to the filament connection of the 5Y3 that is on pin 2 of the 5Y3's socket. Another wire from that same pin distributes B+ to the rest of the radio. Pin 6 of the 6K7 is blank but may be being used as a tie point. I think that it would be better to connect the + terminals of both caps to pin 2 of the 5Y3 socket. The B+ terminal of the audio output transformer, the 6K6 screen grid and B+ to other circuits should also connect here. Where the negative terminals of the caps is connected is very important. The negative terminal of Cap 29B, the 12uF cap should connect to the center tap of the high voltage secondary of the power transformer, Pin 1 of the speaker plug (Leg 1 of the field coil) and to one side of R54, an 800K resistor. The negative terminal of Cap 29A, the 8uF cap connects to Pin 2 of the speaker plug (Leg 2 of the field coil), and to one terminal each of R53, R51 and C60. In other words, the negative terminals of C29 A and B are not tied together, but coinnect to either side of teh field coil. The purpose of the confusing negative terminal connections is to consider the chassis to be B- / C+ and the Transformer secondary center tap to be C-, the bias for the 1st and 2nd AF tubes.
I generally like to up these caps to about 20 uF. Others may disagree, but the tolerance of electrolytics back in the day was +80 / -20% New caps nowadays are usually +/- 10 or 20%, so one cn go up a little. However, i think that your hum is due to the fact that the negative terminals of the 2 caps don't straddle the filed coil, and the + side of one of the caps may be connected to the wrong place, depending on what is connected to that terminal strip.
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
