08-25-2011, 08:05 AM
Recently, I rebuilt a Philco 40-125 which I acquired in April 2010. This set had numerous problems; it had been recapped by a previous owner but none of the old rubber-covered wiring had been touched; the rubber insulation was crumbling and was in dangerous condition. In addition, someone had actually removed the original 40-125 chassis and installed a 40-120 chassis (which has no pushbuttons); the pushbuttons were then glued to a piece of wood which, in turn, was glued inside the cabinet to give the set the appearance of still having pushbuttons.
I acquired a 40-125 pushbutton assembly, and my original intention was simply to add this assembly to the 40-120 chassis. That was easy to do. But when I found the poor rubber insulation under the chassis, I took the time to completely rebuild the chassis.
After it was complete, it worked but had very poor sensitivity. After going over the chassis, the trouble was found to be an open primary in the interstage RF coil (10) in the partial schematic below.
[Image: http://www.philcoradio.com/images/phorum...0-125a.jpg]
I found the broken wires after some digging. However, the resistance was showing in the megohms, not 17.5 ohms as shown in the partial schematic above.
I then had a decision to make: Either try to find another 40-120 or 40-125 chassis to obtain another RF coil...or to jury-rig a solution.
I chose the latter.
The old, open primary of (10) was completely disconnected. I then installed a 15K, 1/2 watt resistor across terminals 1 and 2 of the RF coil, and a 250 pF mica capacitor across terminals 1 and 3 of this coil. You can see the modification in the partial schematic below:
[Image: http://www.philcoradio.com/images/phorum...0-125b.jpg]
The 250 pF cap feeds the RF signal to the control grid of the 7A8 mixer, while the 15K resistor allows B+ to reach the plate of the 7C7 RF amp so that it will function normally.
The repair worked so well, that I am going to leave it as is. Since this RF stage is untuned, this circuit is well suited to this type of workaround.
I wanted to post this here in case any of you ever run into a similar problem.
I acquired a 40-125 pushbutton assembly, and my original intention was simply to add this assembly to the 40-120 chassis. That was easy to do. But when I found the poor rubber insulation under the chassis, I took the time to completely rebuild the chassis.
After it was complete, it worked but had very poor sensitivity. After going over the chassis, the trouble was found to be an open primary in the interstage RF coil (10) in the partial schematic below.
[Image: http://www.philcoradio.com/images/phorum...0-125a.jpg]
I found the broken wires after some digging. However, the resistance was showing in the megohms, not 17.5 ohms as shown in the partial schematic above.
I then had a decision to make: Either try to find another 40-120 or 40-125 chassis to obtain another RF coil...or to jury-rig a solution.
I chose the latter.
The old, open primary of (10) was completely disconnected. I then installed a 15K, 1/2 watt resistor across terminals 1 and 2 of the RF coil, and a 250 pF mica capacitor across terminals 1 and 3 of this coil. You can see the modification in the partial schematic below:
[Image: http://www.philcoradio.com/images/phorum...0-125b.jpg]
The 250 pF cap feeds the RF signal to the control grid of the 7A8 mixer, while the 15K resistor allows B+ to reach the plate of the 7C7 RF amp so that it will function normally.
The repair worked so well, that I am going to leave it as is. Since this RF stage is untuned, this circuit is well suited to this type of workaround.
I wanted to post this here in case any of you ever run into a similar problem.
--
Ron Ramirez
Ferdinand IN
