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Philco 40-120 rework/recap
#4

Hi T32,

Did you have to replace any wiring on the set also?  Recheck wiring.

We call these sets "hot chassis" but they are not quite so.  Looking at the schematic, the B- and filament string "cold" end is not to the chassis but is a series of connections that include the output tube cathode resistor, the 7C6 First Audio cathode and 1 filament leg, the power switch, one end of the volume control and other points.  This is usually referred to as the B- Bus.  The chassis connects to B- through #8, a capacitor and coil pair., with the cap in series with the coil.

With power off and set unplugged, connect one end of the meter to the power switch contact that is connected not to the power cord but to one leg of the volume control and wiring that goes to another B- connection.  Then check the following points.  All should give zero Ohms:
  • The "ground" (B-) end of the volume control.
  • The "ground" (B-) end of the output tube cathode resistor # 27 (as opposed to the 35A5 cathode end).
  • The "ground" (B-) end of resistors # 28 (Output Tube grid leak), 23 (1st AF Tube grid leak) and R5 (1st RF and Convertor cathode resistor).
  • The 1st AF Tube and IF Tube cathodes (Pin 7) and the 1st AF Tube.  
  • The "-" connections of filter capacitors # 33
  • The "ground" (B-) end of capacitor # 9 (AVC Capacitor).
  • Note that the tuning cap (and push button caps)
          connect to the chassis and not the B- Bus.  the connection between chassis and bus is through capacitor #8, a 0.25uF cap.  The coil is not                  highly critical; it is merely several turns of wire around the capacitor.

As RadioRoslyn (Terry) stated, there should be a high or infinite resistance between the B- Bus and the chassis.

Hope this helps!

"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards, 

MrFixr55


Messages In This Thread
Philco 40-120 rework/recap - by t32above - 03-12-2024, 06:30 PM
RE: Philco 40-120 rework/recap - by morzh - 03-12-2024, 08:34 PM
RE: Philco 40-120 rework/recap - by Radioroslyn - 03-13-2024, 09:56 AM
RE: Philco 40-120 rework/recap - by MrFixr55 - 03-13-2024, 06:40 PM
RE: Philco 40-120 rework/recap - by t32above - 03-14-2024, 03:23 PM
RE: Philco 40-120 rework/recap - by RodB - 03-14-2024, 08:07 PM
RE: Philco 40-120 rework/recap - by radiorich - 03-15-2024, 07:00 AM



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