I.F. coil bypass - Sort of like a heart bypass
Posts: 240
Threads: 27
Joined: Jun 2013
City: Boulder, Colorado
Hi All;
I think I have a Bad Primary in my 1st IF coil.. I figure I have about three choices.. In no particular order..
1. Try my luck at unwinding it and hope it has a break near the end I am unwinding, and not at the other end.. I don't have a winding jig for these type of coils..
2. Put a cap of (to me) unknown value between the Primary and Secondary, Plate Primary to Grid secondary.. Like the Audio by-pass..
3. take out that If Transformer and associated tube, so that my Radio has a two Stage If Section, instead of a three stage If Section.. Untill I can find another IF can..
Can Anyone suggest the best way for me to proceed ??
THANK YOU Marty
Posts: 1,562
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Joined: Nov 2008
City: Sedona, AZ/Placentia/CA
Marty, I would say #1 is a good start. After that if it doesn't work, post a WTB in the classified for the transformer. You did not give much information as to what the radio model is, that would really help. A lot of people out here that have parts that can help you. I would not proceed to #2 or #3 until such time as I would either see if I can fix it or then find another transformer.
Jerry
A friend in need is a pest! Bill Slee ca 1970.
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City: Irvington, NY
As Jerry said, probably the best first step would be to remove the transformer and examine it. If you are lucky, the coil wire may have broken or corroded where it is soldered to the mica compression trimmer. If this is where the break is, then cleaning the coil wire and resoldering would be the simple solution. If the break is internal to the coil, then complete replacement would be a better choice.
If the primary is indeed open but the secondary is good, then the bypass idea might work. You would have to completely disconnect the primary and replace it with a resistor or RF choke. Then connect a cap between the plate of the IF tube to the grid of the next stage. As a first try, I would use a 47K plate resistor and a 100 pF coupling cap, but you might have to experiment. The secondary trimmer would have to be adjusted to compensate for the changed loading and shunt capacitance of the bypass. Might well be worth a try if the original IF transformer can't be repaired or a replacement is unavailable.
Posts: 240
Threads: 27
Joined: Jun 2013
City: Boulder, Colorado
Hi All;
Sorry, Jerry.. I thought everyone knew that I have a Philco 34.. The part number is 32-1341.. - - 460 KHZ IF frequency
THANK YOU Marty
(This post was last modified: 07-25-2013, 07:31 PM by marty.)
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City: Mesa, AZ
Don't give up the ghost, Marty! I mangled the fourth IF can in my 42-400 with the obsolete fm circuit (which pulls in fm   ), and advertised on this forum for one and received a NOS transformer (in the original box, no less!) in about a weeks time! Unbelievable! If anyone can help you, it's these guys!  You just have to be specific about part #'s and all, is all. Should be listed in the schematics. 
Posts: 240
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Joined: Jun 2013
City: Boulder, Colorado
Hi All;
Thank You All, Mondial, I have opened it up, and as carefully as I could under a magnifying glass, looked at and moved the wires, a little.. The wires to the caps are fine..
In a way I would prefer number 1, but I am afraid of unwinding it all and not having that funny way of winding it back and would then be totally out of Luck with this coil.. As winding it back straight round and round would not be the same..
THANK YOU Marty
Posts: 1,562
Threads: 56
Joined: Nov 2008
City: Sedona, AZ/Placentia/CA
Marty, if you can't see anything you can fix, post a WTB or look at the resource page for vendors of parts, you can certainly find one there.
Also, I lived in Boulder back in the early 50's and it was a lovely little city. I would like to go back but like so many things, 60 years or so they have changed a lot. It was a little sleepy neat city back then and my brother and I would walk down to city center to go to movies at night. Perhaps a quarter of a mile. We were about 5 years old.
Jerry
A friend in need is a pest! Bill Slee ca 1970.
Posts: 240
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Joined: Jun 2013
City: Boulder, Colorado
Hi All;
Jerry, unfortunately, Boulder is not the same city it was back then.. I wish it was, It would be alot nicer.. Thank You for Your input and History..
THANK YOU Marty
Posts: 240
Threads: 27
Joined: Jun 2013
City: Boulder, Colorado
Hi All;
Well I have definately found out that the 1st IF Transformer is Bad, I wired up the rest of the front end.. I applied Power (no smoke, so not all is bad), and after much probing and adjusting..
I thought to check voltages on the 1C6 and I found out why the mixer was not working.. The plate voltage was 2.xx volts,, I rechecked that I had the wires were hooked up correctly and I also had it connected to +135 volts, which I did..
So the coil is bad, and in my checking, I found the secondary is now bad as well. so, while I look for another, I will try a couple of things, One is the two resistor cap hookup.. Would a 22K on the primary (Plate side) and 100K secondary (Grid side) with a 200 pf Cap in between work OK..
Another is I have a couple of If transformers from my AA5 parts bin and I will see IF I can make one of them work with de-tuning it, from 455Khz to 460Khz..
I figure it can't hurt to try..
THANK YOU Marty
(This post was last modified: 07-27-2013, 06:07 PM by marty.)
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