[RF7590]

Hello all, currently working on a 46-421. One of the caps ( .2-400 ) has wire wrapped around it to make a coil.
No problem in replacing the cap, but the new cap will much smaller in diameter so the coil of wire will not be tight against the cap.
Does this matter or should I re-wrap the the wire around the new cap. ( wouldn't that change the value of the coil).

Thanks

RF

[klondike98]

I've just restuffed the old cap and kept the wire wrapped around it.  Here's a 42-322 with one of those caps.  There is one of those yellow film caps inside the paper cap body; note the wire is remains wrapped around the body of the cap. (sorry, its a bit out of focus)

[Image: https://dl.dropboxusercontent.com/u/5462...22_cap.jpg]

This was my first radio restore, so I wasn't restuffing all the caps but I did that one.  I believe however, that the radio will be fine without the wire.  

[sam]

you can glue it on cap.  but I have left them off as plastic is isolation
sam

[K7Sparky]

"you can glue it on cap.  but I have left them off as plastic is isolation
sam "

Wire on a cap is a new one on me.

Is it connected in the circuit as an inductor like parasitic chokes wound on 2W resistors on plate caps?

Sams comment makes me think it is extra insulation over the outer foil in a plate circuit.  Can't tell the hookup from Bobs picture.

Just curious

[klondike98]

In the case of the Philco 42-322 its part # 21 in the schematic below and is described in the parts list as a 0.2uF condenser and RF choke...so its part of the circuit.
[Image: https://dl.dropboxusercontent.com/u/5462...Fchoke.jpg]

[sam]

so the capacitor is connected to wire coil and then to ground ?

[Ron Ramirez]

Chuck has info on these on his site:
http://www.philcorepairbench.com/tips/svctip37.htm

[K7Sparky]

Thanks for the link Ron.  I just learned something new that is old

Since paper foil caps rarely go bad (in my experience) the best thing is to leave the IF trap alone or pull it and sweep it.

The chance of 2 of the old caps having the same C and self L would be rare.  A modern one would seem remote.

Anyone built a new one?

A SWAG of procedure to get a satisfactory replacement would be to get new 0.2 MFD wrap some turns around it hook one end of the coil to the ? inside foil  ? end of the cap and the other end of the coil is the other end of the series resonate circuit.  Not sure if the new caps have a band to mark the outside foil end.    The one in klondike's picture is either hooked to the outside foil side hiding the mark or that cap does not have an outside foil stripe.

Guess to bench test / tune to IF frequency add a film or carbon resistor like 50 or 100 ohms in series.  Monitor the voltage across the resistor and sweep with a signal generator to find the initial resonant frequency (peak voltage across the resistor).  If close to IF frequency squeeze the coil to raise L or pull apart to lower L.

If the resonate frequency is too far off, change the number of turns.

Alternatively sweep the replacement cap to find self resonance.  Then grab a copy of an old "Allied Data Handbook" off the web and use the "Single-Layer Would Coil Chart" for air wound coils to get the initial coil in the ball park.  Not sure how much the Aluminum foil of the cap will effect resonate frequency.

As Ryder says in the link it would be a broad resonance.

Probably a bit of hot glue to hold the coils spacing when finished.

[codefox1]

Agree with all. Just replace the cap with a plain modern one, maintain original lead dressing, and you'll be fine. There has been much chatter around for a long time about wave trap methods used to mitigate interference from radio navigation beacons (long since obsolete in the U.S.) that operated near IF frequencies. Dunno if true or not.

[morzh]

I thought wavetraps became non-essential ever since we have no more of those stations.

[Chuck Schwark]

They did have those low freq stations when these radios were new……   

[K7Sparky]

I too had thought that there was a reception problem caused with navigation signals so went on a search of LORAN data.  Here is a GREAT read on LORAN development and deployment during WWII http://www.jproc.ca/hyperbolic/loran_a.html

Frequency range of LORAN A was 1750 to 1950 KHz no where close to IF.  The first LORAN A pair Montauk Pt, NY & Fenwick Is, Del went live June 1942

I reread the Rider document given by Ron's link in post #7 and it talks about only IF feedback caused by chassis isolation in the universal radios.  The chassis was not part of common so could propagate the IF and cause feedback.

The cap only would be Rider "How it Works" Figure 1 method A where the one used in the 46-421 is method C.  The trap only conductes close to IF frequency where just a cap conducts any AC / RF and the higher the frequency the lower Xc gets.  Maybe that's why Philco used C.

I am just starting an old Zenith 6R886 universal in a box with a 78 turntable that uses method B.

Didn't even know about this subject till the thread.  Really a great education and just in time.