Yesterday, 10:11 AM
Hi T Murphy,
First off, welcome to the Phorum. Lots of Phine Phriendly Pholks Phull of Philco Pfacts. (Notice a pattern here?)
It is kind of hard to do these from drawings. Your writing is like mine, therefore you are a genius.
Below is a copy of the schematic of the power supply for the most important connections.
The power supplies are a little more complex than the ones of the late 1940s to 1960s. This is because of how they obtained grid bias. Note the center tap of the HV (High voltage) secondary winding that supplies the rectifier. This is the most negative leg of the power supply. On newer sets, it is grounded, but on older sets such as this one, it is the most negative point and is not B-, it is more like C-. B- on this set is Chassis ground, and is dev eloped through R77, a multi-tapped resistor. This resistor develops the various grid bias (C-) voltages needed for the output tubes, 1st AF tube, etc.. Therefore, cap 21B connects between the junction of the rectifier cathode and input to the field coil (the most positive part of this power supply) and the junction of the HV Secondary center tap and R77 pin 4 (the most negative point in the power supply. This is the most important connection. (R77 has 5 pins. The way I am counting this, Pin 1 is chassis ground.) The schematic shows that the wire for the positive terminal of C21B is blue. The color of the negative is not shown, so you must verify the connection. If you only cut the wires at the cap, the positive connection may be correct. Trace it to the rectifier socket.
The next most important filter cap is 21A. The positive connection (red) is on the other side of the field coil, the junction between the field coil, output transformer center tap, etc. (the main B+ line for the radio), and pin 2 of R77. I question this connection; I think it should be chassis ground as it would be in many other radios. However, both the Code 121 and Code 122 versions of the 41-68 chassis show this negative connected to Pin 2 of R77. Consider moving that connection to chassis ground only if all else fails.
The positive terminal for the final cap, C21 (green) connects between R20, R26 and R36. The negative terminal is chassis ground. However, I don't think that this is your issue, as having the volume control full negative minimizes the input from the circuits that are fed from the voltage across this cap.
One other thing is to verify the wiring and condition of R77. With tubes and speaker connection out, measuring between ground and pin 4 of R77 should yield about 322 Ohms +/- about 10-20%. With everything in and running, CAEFULLY measure the voltage across those points. The voltage across these 2 terminals should be about 18-22 V or something close to it.
Hope this helps!!
Best Regards,
John, MrFixr55
First off, welcome to the Phorum. Lots of Phine Phriendly Pholks Phull of Philco Pfacts. (Notice a pattern here?)
It is kind of hard to do these from drawings. Your writing is like mine, therefore you are a genius.
Below is a copy of the schematic of the power supply for the most important connections.
The power supplies are a little more complex than the ones of the late 1940s to 1960s. This is because of how they obtained grid bias. Note the center tap of the HV (High voltage) secondary winding that supplies the rectifier. This is the most negative leg of the power supply. On newer sets, it is grounded, but on older sets such as this one, it is the most negative point and is not B-, it is more like C-. B- on this set is Chassis ground, and is dev eloped through R77, a multi-tapped resistor. This resistor develops the various grid bias (C-) voltages needed for the output tubes, 1st AF tube, etc.. Therefore, cap 21B connects between the junction of the rectifier cathode and input to the field coil (the most positive part of this power supply) and the junction of the HV Secondary center tap and R77 pin 4 (the most negative point in the power supply. This is the most important connection. (R77 has 5 pins. The way I am counting this, Pin 1 is chassis ground.) The schematic shows that the wire for the positive terminal of C21B is blue. The color of the negative is not shown, so you must verify the connection. If you only cut the wires at the cap, the positive connection may be correct. Trace it to the rectifier socket.
The next most important filter cap is 21A. The positive connection (red) is on the other side of the field coil, the junction between the field coil, output transformer center tap, etc. (the main B+ line for the radio), and pin 2 of R77. I question this connection; I think it should be chassis ground as it would be in many other radios. However, both the Code 121 and Code 122 versions of the 41-68 chassis show this negative connected to Pin 2 of R77. Consider moving that connection to chassis ground only if all else fails.
The positive terminal for the final cap, C21 (green) connects between R20, R26 and R36. The negative terminal is chassis ground. However, I don't think that this is your issue, as having the volume control full negative minimizes the input from the circuits that are fed from the voltage across this cap.
One other thing is to verify the wiring and condition of R77. With tubes and speaker connection out, measuring between ground and pin 4 of R77 should yield about 322 Ohms +/- about 10-20%. With everything in and running, CAEFULLY measure the voltage across those points. The voltage across these 2 terminals should be about 18-22 V or something close to it.
Hope this helps!!
Best Regards,
John, MrFixr55
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
MrFixr55
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