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What is this doofungus? - Printable Version

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What is this doofungus? - rocketeer - 08-16-2011

I have a Transitone table radio model 46-200 that someone has been in before and messed up. There was an oversize electrolytic tied on top of the chassis and connected underneath via two long wires that ran around the end of the chassis. When I flipped it over there was an oversize 2 section electrolytic can covered in masking tape tied underneath with just 1 section also connected via very long wires. The original speaker is gone and was replaced with a tiny 3" one that is held on by a metal strap fastened to it with a screw and soldered to the chassis. So, having seen all that mess I am somewhat apprehensive about the thing in the photo. Is it original and if it is, what is it?
[attachment=0]

Thanks,
Larry

Re: What is this doofungus? - Ron Ramirez - 08-17-2011

It is sort of original - sort of in that I see it has the Sprague name on it, so it is a replacement for the original Philco part.

This cap and coil combination was intended to act as a wave trap at the set's IF frequency, 455 kc, and to shunt stray IF currents to ground.

You can read more about it on Chuck's site.
http://www.philcorepairbench.com/tips/svctip37.htm

In replacing this part, you can either replace it with a 0.22 uF cap and not worry about the wires wrapped around it, or you can wrap a new wire around the new cap if you wish. You will see how it should be wound when you undo the original.

Re: What is this doofungus? - Dan Walker - 08-17-2011

On one of the radios I worked on [can't remember the name] I had the same thing ,a wire wound cap.. When I repalced the cap I did not wind it with wire and the set did not work right ,so I wound the new cap and then it worked ok. That is not to say you should wrap your new cap with wire but thats my story..Dan

Re: What is this doofungus? - rocketeer - 08-17-2011

Thanks guys.

Larry

Re: What is this doofungus? - rocketeer - 08-20-2011

I replaced it with a .22 cap and rewound it with the wire. Other than the squeeky little speaker she plays really well. Awaiting a new dial cover from Palmquist before I put it back together.

Larry

Re: What is this doofungus? - TexMac - 08-27-2011

This explanation was found when reworking one of my sets. I carefully detach the leads to the actual cap inside, remove the core and restuff the cardboard tube with the appropriate axial lead yellow cap, reattaching the coil leads as before. I know just enough theory to be dangerous, and I feel certain the smaller amount of metal in the newer caps must shift the response some, but I suspect such an arrangement was sloppy and a little broad in frequency response to begin with. So far, I have not found any feedback problems in sets I have done this way.
[Image: http://i75.photobucket.com/albums/i305/mcaliron/Coilsoncondensers.jpg]

Re: What is this doofungus? - Arran - 08-29-2011

Actually it may have been an original part even if it did not have the Philco name on it, the model 76 (Canadian 46-1201) had two Sprague caps it in and they were original as far as I could tell, one was inside the cardboard sleeve covering the wave trap. That set still had the original electrolytics installed under the chassis and it did not look like anyone had ever removed the chassis from that cabinet, to remove the chassis from a 46-1201 is a bit of an affair. I think that in the post war years Philco may have been buying capacitors from outside sources to keep up with demand, that or Sprague manufactured Philco labeled capacitors for Philco in those years, I know that prior to 1938 they made everything in house except the transformers and tubes.
Regards
Arran

Re: What is this doofungus? - exray - 08-30-2011

When I was doing a restuff on a Philco once, forget which model, there was a completely intact Sprague capacitor inside the Philco covering. I have no doubt that it was original - I guess they did some restuffing of their own!

Re: What is this doofungus? - codefox1 - 08-30-2011

This comesd up once a month or so here and elsewhere. Onc upon a time there were navigational beacons close to 455 KC which would interfere with radio reception. No longer an issue.

Re: What is this doofungus? - exray - 08-31-2011

codefox1 Wrote:This comesd up once a month or so here and elsewhere. Onc upon a time there were navigational beacons close to 455 KC which would interfere with radio reception. No longer an issue.

But this component has nothing to do with that. It uses a series-resonant 'wavetrap' configuration for improved bypassing at the IF frequency. Wavetraps to minimize beacon interference are located at the antenna input. Philco should have used a better term to avoid confusion.
Aerovox (and others) also made them without the need for the external winding and sold them as "resonant capacitors".