02-10-2018, 07:07 PM
Hi Wilbur! 
In my experience, the 250 ohm section of those dual Philco volume controls are prone to going bad as Terry mentioned above. That section of the dual control is very important as it controls the cathode bias of the RF and IF tubes. If this control goes bad (open)...the RF and IF tubes receive no cathode voltage and your radio goes dead (silent).
The 5000 ohm section controls the amount of signal getting to the first RF coil. It actually isn't as important as the 250 ohm section.
A person could effect a repair by connecting a 5000 ohm resistor across the antenna coil (2) where the 5000 ohm section of the control was, connecting the antenna terminal direct to the top of the coil primary where the wiper arm of the control currently connects. Then, if you could find a 250 ohm 5 watt rheostat with a suitable shaft, use it in place of the original control. It will turn the cahode bias of the RF and IF tubes up and down, increasing and decreasing the amplification in those tubes, and rendering the radio usable. The trouble is, rheostats can still be purchased but they are extremely expensive these days and in a quick search of Mouser and Allied, I couldn't find any of that resistance below 12 watts which is overkill for the 70 but would certainly work.
I suggest you disconnect the wires going to the 250 ohm section, then place the leads of a multimeter from one of the ends to the wiper arm (center terminal). Then slowly turn the control, watching the meter. If it goes open at any point, reclean. If that doesn't help, it's bad.
Good luck.
In my experience, the 250 ohm section of those dual Philco volume controls are prone to going bad as Terry mentioned above. That section of the dual control is very important as it controls the cathode bias of the RF and IF tubes. If this control goes bad (open)...the RF and IF tubes receive no cathode voltage and your radio goes dead (silent).
The 5000 ohm section controls the amount of signal getting to the first RF coil. It actually isn't as important as the 250 ohm section.
A person could effect a repair by connecting a 5000 ohm resistor across the antenna coil (2) where the 5000 ohm section of the control was, connecting the antenna terminal direct to the top of the coil primary where the wiper arm of the control currently connects. Then, if you could find a 250 ohm 5 watt rheostat with a suitable shaft, use it in place of the original control. It will turn the cahode bias of the RF and IF tubes up and down, increasing and decreasing the amplification in those tubes, and rendering the radio usable. The trouble is, rheostats can still be purchased but they are extremely expensive these days and in a quick search of Mouser and Allied, I couldn't find any of that resistance below 12 watts which is overkill for the 70 but would certainly work.
I suggest you disconnect the wires going to the 250 ohm section, then place the leads of a multimeter from one of the ends to the wiper arm (center terminal). Then slowly turn the control, watching the meter. If it goes open at any point, reclean. If that doesn't help, it's bad.
Good luck.
--
Ron Ramirez
Ferdinand IN
