[Guest]

Hi All- I am restoring a Philco model 366 (U.S. 66) cathedral set, and I am having trouble finding the value for the winding on the filter choke. I have scoured through all the Canadian and U.S. schematics and no drawings or charts list that. Would anyone have a measurment from one?

The problem I am having is that both before and after replacing all the filters, capacitors, and out-of-tolerance resistors, I still have a loud 60 cycle hum all the time. If I disconnect the filter from either side of the choke, and subsitute in a much larger value cap, the hum gets a lot less. If I put in a 150uf cap, the hum disappears altogether. The resistance across the choke seems low- less than 20 ohms. Does this sound like the problem? :

Brad

[Ron Ramirez]

Part no. 32-7018

65 henries, 2250 ohms, 10 mA DC (according to the 1941 Philco parts catalog). As the resistance across your choke is less than 20 ohms, it is pretty well shorted out. And you won't find a 65 henry choke these days!

You can substitute a 2200 ohm, 2 watt resistor for the choke, and increase the value of electrolytic (49) from 6 uF to 22 uF. This may solve your problem. Many 1937 Philco radios used a similar voltage divider network to feed B+ to the IF screen grid and the 1st det-osc oscillator "plate", but these used a resistor, not a choke, to drop the B+ somewhat and act as part of a "pi" filter which included input and output electrolytics, just as in the 66. I have not tried this in a 66 or a 45 (same voltage divider circuit), so YMMV...but theoretically at least, it should work.

[codefox1]

I agree with Ron on all points. There is a chance if you remove and open up the shorted choke, you may find the short if it is in the first few layers of the winding. The short was was most likely caused by the failure of one of the ancient electrolytics, which at the time were very expensive, more so than a delicate choke. If you can fix it, and get at least 50-75% of the winding back in service, you will be good to go. I once used the primary winding of an output transformer from a junk portable using a 3V4 or 3S4 which had a really large impedance to substitute for a smoothing choke (do not connect the secondary winding to anything,) and upped the values of the filter capacitors, and the d**n thing worked like a charm. Physically about the same size, and it'd take a real nit picker to find out that is what I did.

Don't try any really large value capacitors as they would burden the rectifier, and possible cause it to fail, or worse yet, cause a problem with the power transformer. Ron's suggestion of a 22 mfd is appropriate for the first part of the pi filter, you could do a 33 or 4o mfd after the 2200 ohm resistor and still be OK.

You can use the tool from the following web site to approximate what will happen. Remember the 80 morphed into a 5Y3, and so on. It's free, and really pretty good.

http://www.duncanamps.com/psud2/index.htm

Best of luck!

[Guest]

Thanks very much! I will try substituting in the resistor and see if that works. Also, now that I know the value of the choke, I have lots of junker chassis. I'll see if I can find a replacement. I also may rewind the choke if I find I have too much time on my hands. I have done that in the past. Lots of work, but putting a radio back as close to original as I can is worth the time and effort.

Brad

[exray]

Thought I'd butt in and mention that there are TWO chokes in the 366. One is the speaker field coil and the other is a separate discrete choke. The drill is pretty much the same.
I had a 366 and the separate choke in it was also fried. I did the resistor sub for it with no problems.

[Guest]

I was able to find a choke of almost the same value (2000 ohms) and of the same physical size. It's out of a 1940's 6 volt GM car radio chassis. I haven't had time to install it yet, but I'm sure it's going to work fine!

[Ron Ramirez]

Excellent! Let us know how it goes.

[davemc]

Do you think I could get away with using a currently available Hammond choke rated 60hy at 8ma, 2750 ohms? An original on another model 66 I have measures 2263 ohms. Since it only feeds the oscillator plate (grid) and the converter and IF amp screens, looks like the draw would be less than 8ma. And the Hammond mounting centers are close to the original.

I'd prefer to keep the radio as original as possible. And before I invest in a choke (if you guys think it will work) I'll do some surgery - sometimes you get lucky!

Dave

[codefox1]

Sure that sub should work. I also like the suggestion of using an output xformer primary from a junker portable. Either way if you eliminated whatever caused the original to fry, you'll be good to go. Just make sure you replace all the other coupling capacitors too.

[davemc]

I lucked out and was able to repair my choke! There was no sign of burning or heat, and no "green disease" near the lead wire connections to the windings, as is often the point of failure. In this case, there was a simple break about 80% into the outer layer of the windings. The only trick was getting it apart without damage. The bottom of the case surrounding the core was riveted to the top, and had to be ground off. The core construction was all separate stacks of E and I laminations, with a thin piece of paper between to form the gap. Then there was a metal wedge retaining the coil to the core. I was able to drive it out without damage using a thin metal ruler.

The gap insulator was damaged during disassembly, so I used a piece of paper removed during disassembly of the choke (probably not exactly the same thickness as the original). I normally use black friction tape when reassembling repaired chokes and transformers.

The winding now measures 2271 ohms - no leakage to ground. Not sure what caused the failure - perhaps just a weak place in the winding, or corrosion. The filter cap in this case was a wet type, which do not tend to short out.

[codefox1]

I have found such flaws in the past, especially on field coils, where the spool of wire ran out, and a new one was spliced in during manufacturing. I jave a Rola that was made of all ends of spools, and is now totally useless. Often these shortcuts were done badly with just a small bit of wax paper to cover the splice. If you got 80% of the winding left, it should be good for the next century. You win.

[davemc]

Actually, I lost only 80% of the first layer (outer layer) of the winding. I will normally remove up to 2 layers of a transformer or choke looking for an open. After that, I usually give up assuming the open is on the INSIDE, or else I would have to remove too much of the coil to find it.