Repair of Philco 40-125 RF Coil - Printable Version
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+--- Thread: Repair of Philco 40-125 RF Coil (/showthread.php?tid=3657)
Repair of Philco 40-125 RF Coil - Ron Ramirez - 08-25-2011
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/40-125/40-125a.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/40-125/40-125b.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.
Re: Repair of Philco 40-125 RF Coil - Carl Travis - 08-25-2011
Ron,
How did you determine the values for the resistor and cap?
Re: Repair of Philco 40-125 RF Coil - Ron Ramirez - 08-25-2011
An internet search turned up this website:
http://www.vcomp.co.uk/tech_tips/transformers/bypass_transformers.htm
Notice they suggest 22K to replace a primary winding of an audio interstage or IF primary, and 200 pF as a coupling cap in an IF transformer.
I felt this information could apply to my bad RF coil.
I used 15K instead of 22K to keep from significantly reducing the plate voltage of the 7C7 since the 40-125 is an AC/DC set and the B+ isn't very high. As it turned out, the repair worked so well that I didn't bother to measure the plate voltage of the 7C7 post-repair. I probably could have used 10K or even 5K. Maybe I'll pull it back out of the cabinet and measure the 7C7 plate voltage later on.
And since I had some 250 pF caps on hand but no 200 pF caps, 250 pF is what I used.
Re: Repair of Philco 40-125 RF Coil - Ron Ramirez - 08-25-2011
I just pulled it back apart and made some measurements.
The plate voltage on the 7AH7 RF amp is 97.7 volts at full line voltage (122 the last time I measured it). On the other side of the 10K resistor, the B+ is 102.3 volts. So the resistor drops 4.6 volts @ under 0.31 mA (that's roughly 0.0014 watts), certainly acceptable here.
I forgot to mention earlier that the 7C7 in the radio was weak, and I did not have any 7C7 tubes here, so I substituted a 7AH7 for the 7C7 after conferring with the NJ7P tube database.
http://www.nj7p.info/Tube4.php?tube=7C7
Now, for your amusement, some photos.
A quick photo of the modified RF coil (10), showing the added 15K resistor and 250 pF capacitor:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125c.jpg]
How the underside of the rebuilt chassis looks:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125d.jpg]
What looks like brand new rubber insulation on the wires is actually heat shrink tubing in various colors, which I installed on the original wires (after I removed the old, rotten rubber) to replicate the look of the old rubber insulation.
Front view of chassis:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125e.jpg]
It's hard to tell that this 40-120 chassis originally had no pushbutton assembly, isn't it?
Top view of chassis:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125f.jpg]
Back view of chassis:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125g.jpg]
In this close-up of the back, you can see "MODEL 40-120" ink stamped on the chassis:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125h.jpg]
Finally, a picture of the assembled 40-125:
[Image: http://www.philcoradio.com/images/phorum/40-125/40-125i.jpg]
I still need to sand it down, fill the grain, and refinish. It should be a pretty nice set when it's complete.