07-26-2022, 05:18 PM
Hi Hamilton,
My $0.02 worth-
The 2 capacitors in the upper left of your pic appear to be installed correctly, and matches Terry's drawing. It appears that the Cap with the green line indicating a positive terminal connected to chassis ground is 77 and the cap above it with the 2 positive terminals tied together is 78. If these are the originals, Philco likely tied these together at the plant because the correct single capacitor was not in stock when the radio was built. Otherwise they are replacements and the best that the repair shop had available.
I agree with Terry that you should remove that Aerovox capacitor. It appears to be paralleled with 78 and was likely added to cut down on hum. (I sure hope that the can on that Aerovox was insulated from the capacitor inside!)
This power supply can be confusing to anyone who worked on radios from the 1940s and later but have not worked on many radios from the late 1920s to mid 1930s. The common negative terminals for C77 and C78 do not go to ground. They connect to the center tap of the Power Transformer Secondary per your schematic and Terry's drawing. The Schematic is further confusing because they don't use the usual "curved line" indicating the negative pole of an electrolytic cap (capacitor).
So why does a Positive terminal of a cap go to ground? This is because the Power Supply is actually set up to supply not only a "B+", but to also supply a "C-" (Think of battery sets)! The cathodes of the '42 Output tubes are at chassis ground potential (B- / C+), but the 42s need a grid bias of somewhere around -18V if Class A Pentodes or -35V if Class AB Triodes, like your radio is. This bias is obtained because the center tap of the secondary of the driver transformer (Output Tube grids) is connected to the most negative section of the Power Supply, the HV Secondary Center Tap. Resistor Pack 66 (250 Ohm total) acts as a voltage divider between chassis ground (B- / C+) and the center tap of the HV Secondary of the Power Transformer (C-). If you look at many non-Philco radios from the early to mid 1930s (particularly RCA, Crosley, etc.), as well as Philco radios of the era, you will find a similar setup. Radios from the 1940s and later usually have the HV Secondary Center Tap and the negative of all of the electrolytics at chassis potential and have cathode resistors between this point and the cathodes of those tubes that require bias (usually the Audio Output). These resistors are usually bypassed by a capacitor, and in the case of the Audio Output, an electrolytic capacitor.
If that Audio Output circuit is in fact wired for Class AB Triodes with 350V B+, then this circuit is good for about 15W! If there is no Phono Input, add one! At high volume, this radio will ROCK any external input (until the speaker comes apart)! I fixed a Crosley 02CP for a friend and added an external input and he actually plays a guitar through it on occasion for that great Tube sound! (But insufficient sensitivity to overdrive it.
Good Luck,
John, MrFixr55
My $0.02 worth-
The 2 capacitors in the upper left of your pic appear to be installed correctly, and matches Terry's drawing. It appears that the Cap with the green line indicating a positive terminal connected to chassis ground is 77 and the cap above it with the 2 positive terminals tied together is 78. If these are the originals, Philco likely tied these together at the plant because the correct single capacitor was not in stock when the radio was built. Otherwise they are replacements and the best that the repair shop had available.
I agree with Terry that you should remove that Aerovox capacitor. It appears to be paralleled with 78 and was likely added to cut down on hum. (I sure hope that the can on that Aerovox was insulated from the capacitor inside!)
This power supply can be confusing to anyone who worked on radios from the 1940s and later but have not worked on many radios from the late 1920s to mid 1930s. The common negative terminals for C77 and C78 do not go to ground. They connect to the center tap of the Power Transformer Secondary per your schematic and Terry's drawing. The Schematic is further confusing because they don't use the usual "curved line" indicating the negative pole of an electrolytic cap (capacitor).
So why does a Positive terminal of a cap go to ground? This is because the Power Supply is actually set up to supply not only a "B+", but to also supply a "C-" (Think of battery sets)! The cathodes of the '42 Output tubes are at chassis ground potential (B- / C+), but the 42s need a grid bias of somewhere around -18V if Class A Pentodes or -35V if Class AB Triodes, like your radio is. This bias is obtained because the center tap of the secondary of the driver transformer (Output Tube grids) is connected to the most negative section of the Power Supply, the HV Secondary Center Tap. Resistor Pack 66 (250 Ohm total) acts as a voltage divider between chassis ground (B- / C+) and the center tap of the HV Secondary of the Power Transformer (C-). If you look at many non-Philco radios from the early to mid 1930s (particularly RCA, Crosley, etc.), as well as Philco radios of the era, you will find a similar setup. Radios from the 1940s and later usually have the HV Secondary Center Tap and the negative of all of the electrolytics at chassis potential and have cathode resistors between this point and the cathodes of those tubes that require bias (usually the Audio Output). These resistors are usually bypassed by a capacitor, and in the case of the Audio Output, an electrolytic capacitor.
If that Audio Output circuit is in fact wired for Class AB Triodes with 350V B+, then this circuit is good for about 15W! If there is no Phono Input, add one! At high volume, this radio will ROCK any external input (until the speaker comes apart)! I fixed a Crosley 02CP for a friend and added an external input and he actually plays a guitar through it on occasion for that great Tube sound! (But insufficient sensitivity to overdrive it.
Good Luck,
John, MrFixr55
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
