The PHILCO Phorum

Hi Boys and Gals:

Restoring a Philco 482 and discovered the 4th IF transformer resonating on 340 kHz, rather than 455 kHz. The mica trimmer has no effect on the frequency. I am sweeping it with a HP 3325B triggering a Tektronix 465M scope--e.g. Poor Man's Spectrum Analyzer. The curve is beautiful. But it won't move when I adjust the 455 kHz trimmer.

So I pulled the transformer from the chassis, opened it up and checked the mica cap inside (270 pf) which was reading 318 pf and leaking 200 uA with 600 VDC across. So I replaced it with modern silver mica (263 pfd). Swept it out of circuit and it is still resonating low--now about 315 kHz when stuffed into its can. The mica trimmer till has no effect.

Did the resistance checks on the coils with Heathkit analog IT17 and all resistances spot on. Took apart the mica trimmer partway, and pulled two of the mica insulating plates. Both have "tabs" on one end that are broken off. When I push down the plates after reinserting them, the mica trimmer now shorts. This is after I lifted one end off the coil to isolate it from the coil when making resistance measurements. So now I am trying to figure out how to "repair" the missing tabs on the mica insulators.

Just for fun, I swept the transformer with the 455 kHz trimmer disconnected from the coil, and it shows the same resonance--about 315 kHz--as before. And then also for fun I dropped a 100 pf across the 270 pf mica cap inside the transformer, and the resonance dropped in frequency slightly.

I predict after squaring away the mica insulation problem and putting the can back in the radio's circuit, the proper 270 pf mica cap installed will make the can resonte on 455 kHz and tune as per factory. Fingers crossed.

If I'm missing something, please let me know 8D

Is an isolating resistor of at least 10k used in between the generator and the inductor? I use 50K but my gear is older than the radios.
Try warming the coils with a heat gun, enough to make the wax fluid then let it cool. That should drive off any moisture, check again. If all fails time to shop for a replacement transformer.

Chas WA1JFD

Thank you, Chas, for thinking about this puzzle. Your response has pointed out to me something I did not consider. Let me tell you a little story, for I am an old man.

I thought about the resonance curve getting skewed by loading the circuit down with a low impedance prior to removing the IF can from the radio. So I swept the can before it--the 3rd IF can--with the radio off. It showed it was parked on 455 kHz and moved +/- by adjusting its 455 kHz trimmer. That told me my set up was OK. https://youtu.be/6YJew2bmjfU?t=206

I then did the same thing to the 4th IF can--the problematic one--and it showed it was parked at 328 kHz and exhibited no response to turning its 455 kHz trimmer, or any other trimmer. https://youtu.be/s78BkxtmIrE?t=79

Now, Chas, the thing you have pointed out to me through your post is the tell-tale fact that when I performed these tests I had to increase the excitation signal into the 4th IF can to get the same scope reading. I think it was 0.5 V for the bad can, versus 10 mV for the good can. That is a big clue, which your thoughts now make me realize. Something in the transformer is attenuating the excitation signal. Since the coil resistances check good, this leaves the other two trimmers. Perhaps one is shorted out...

Chas--I followed your advice and the frequency moved. So I am now using an O-scope probe to convey the signal at 10X attenuation. The trimmer still makes no difference, but it was interesting to see what I assume is decoupling the generator's internal capacitance from the IF transformer via the increase in series resistance.

In another pix of the trimmer assembly, it appears there is either staining or an accumulation of wax on the metal leaves of the compression trimmer. Both mica and the wax are not mechanically compressible, therefore, the wax "layer" is preventing a minimum compression of the trimmer, thus limiting reaching a lower point of resonance. However, that does not seem to agree with the fact that the tuned circuit is resonating so low.

I would suspect a scenario where the tuned circuit is too high from this lack of compression. I do think there is a problem in the trimmer that careful cleaning will resolve, but I don't think that will fix it...Most  "radio" wax will move when bathed with 91% alcohol.

You will need a steady hand to disassemble a compression trimmer. I did but I was a teenager 50 years ago.

I also suspect there is a problem in the coil itself. FWIR if the DC resistance is around 20 ohms or so. (OH! Use an analog ohmmeter that uses a bit of current like a Simpson 260.) I would consider it O.K. but where this resonance issue exists, I think that one or more strands of litz have either gone open internally or at the connection point, start or finish of the winding.

I have seen this on magnetically tuned RCA coils. One slug was "dead". I was unable to fix but it was a popular radio so I got a parts chassis

Another tedious chore is to de-solder the finish end of the winding, separate the litz into individual strands and measure back to the start of the winding each strand. Each strand is insulated by enamel and should be all connected at the other end. One or more strands are open they will constitute a capacitance that should effect the inductance/resonance relationship. Generally, the break is external, if not then the winding is "toast". The only hope is finding a used IF transformer from the same model. Or, muster all your skills and rewind. The most important factoids on rewind is closely replicate the wire type/size/position, the number of turns and direction most exactly. In still another pix I see the coil(s) they are universal wound. If you have the time and up to the challenge I will lend you a Morris coil winder with a cam that can throw 1/4" or wider if need be. I may have usable litz too. I have not looked at the details. I have some potting bees wax to finish the job if need be and a hot plate for melting. I also have a bankers box full of used and new IF's but it is slim I'll have the right one, FWIR, this is the detector transformer. ...

BTW I did look up your call, saw your saltmarsh experiments. I am not on the marsh but .6 miles from Apponagansett bay, S. Dartmouth.. About 25 miles East of Succotash Marsh as the Buzzard flies...

Quote:...decoupling the generator's internal capacitance

 It's the generators low output impedance, 50 Ω that is the problem.

Quote:for I am an old man.

My Elmer was W1ZE...


Life has kept the power off my rigs for many years.

Powered a 1940 National RAS this past Sunday, first time in more than 50 years. Works, trying too, tunes but very hissy (electron noise) will need caps, BFO dead, hope its not a coil, probably needs caps all around at a later date.

GL

Bill...

Chas

WA1JFD

Chas,
If you need a RAS bfo coil I've got one in a parts set.
Happy to pass it along.

Terry,

That is great news! I will let you know...

I got ambitious just a 1/2 hour ago and put the RAS back on the bench supply, upside down. Did the cursory look around the BFO area, spotted a 220k 1/2w dogbone broke off the terminal shared by the gimmick cap. I'm sure that is what has killed the BFO, I'll repair tomorrow and test with the refurbished power supply.

Spent some 5 hours on that power supply scrubbing out potting wax yesterday. All clean and saved the paint job. Today I scrubbed/polished the cover for the P.S.

I have been led to believe that this radio spent its previous life tuned to 500khz 24/7, at the New Bedford airport, then stored in a bit of a damp location... To be placed in a local Ham flea in the 70's, passed by for hours until I came along, it was tagged $5...

FWIW all the caps are contract Aerovox New Bedford. Others are Cornell suspect N.B. as well.

Chas