I think I figured out what the earlier guy was doing. After removing the modifications and restoring the proper connections, after alignments, repairing damaged components I think I found why the circuit was modified. I noticed that the gain in FM was really low. As it turns out the 2nd IF Transformer resonates well above 9.1 MHz and the gain is absolutely terrible at that frequency. I read a really good posting where a person rebuilt an IF transformer from scratch. I noticed with the FM alignment, adjustments of C301A and C301B do not peak at some value but causes the measured voltage across C314 to continually climb and saturate. From passive measurements, it looks like Z301 is trying to resonate way too high. I think the earlier guy tried to bypass the primary of the transformer Z301 by removing the ground off the secondary and injecting through that path. I'm not really sure if that was the best technique. I've been debating how to address this issue. This transformer mostly uses the top half adjustments for FM and the lower half secondary adjustments for AM (along with one fixed cap). There's no switch contacts in this circuit, only L & C with some small R. It's actually a pretty clever design to operate at 455 kHz or 9.1 MHz. Now I'm trying to decide how to resolve this issue. Parts like this are impossible to find. Any suggestions?
I’ve actually built one IF can from scratch. I did it by placing to axial inductors side by side. The “primary” inductor will induce a signal upon the “secondary” inductor, just as these coils work in the original IF cans. You’re lucky here that it’s the second IF can; I personally dreaded the thought of trying to reconstruct that third IF can while I was restoring my own 46-480.
You’ll need to create two paired inductor circuits, one for AM and one for FM. For each one, start with your known capacitor value. Using the formula
2*pi*f = 1 / sqrt(LC)
we find that
L = 1 / ( (2*pi*f)^2 * C )
where L is inductance (in Henries), pi=3.14, f is your resonant frequency (455kHz for AM or 9.1MHz for FM; convert to Hertz), and C is your known capacitor value, in Farads.
I read your post earlier - I thought you did a really nice job with the analysis and fabrication, very professional work - great job! The thing I'm concerned about is passing the 455 kHz and 9.1 MHz. I'm sure the Q will not be as high as it would be for a resonant 9.1 MHz transformer. The AM actually works great - the FM is the one that does not resonate at the proper frequency so I think it's related to the top portion ( or maybe a bad connection inside). I read that these AM/IF transformers have a more flat response as compared to either a 455 kHz or either a 9.1 MHz transformer. I know the dc resistance is actually very low and I am thinking that the schematic is showing the Inductive Reactance (X_L), but I don't know if it's for the higher or lower frequency. I am debating on pulling out the can but I'm really afraid I will really mess it up. I'm also thinking about doing an analysis to see if there's an 'easy' way to tweak the higher resonance by adding some capacitance to the primary side to get some gain out of it for 9.1 MHz. That requires some engineering work just because of the complexity of this transformer. I'm really debating this - Fix the FM and forget about the AM , pull out the can for repairs - I'm really afraid of damaging the 'guts' when I pull everything out of the can. Sometimes those screw down slugs are tough to pull apart and put back together. No simple solutions - I see now why the guy was trying to modify the circuit. If he picked off of pin 5 on the switch, then AM would have been untouched by his change. Never easy answers
I’ve actually had that very IF can open. Mine had an out-of tolerance mica cap and needed new fish paper lining. The good news is that these have fixed inductors with variable caps, so you don’t have to worry about that fragile core crumbling!
Disassembly of these was fairly simple. There’s a spring leaf of sorts on the underside, locking the innards into place. The entire inside comes out as one.
jrblasde: That is really good feedback! I feel a lot more comfortable about disassembly now. Yes I see where the screws are for caps, not slugs. Do you have any idea or measurements of the internal components - the span of C and possibly, dc resistance and uH for the coils?
jrblasde: I pretty new so I don't really know to navigate to your posting about the IF Transformer. I read it about two or three days ago, but I have no idea what the Topic name was or how to get back to it. Can I get a link or directions? Thanks
Hey, good afternoon! I found my fish paper on Amazon. Here's the link to what I bought: Fish Paper
Essentially, fish paper is specially made to resist current and should not burn or smolder.
For your second IF can, there's one fixed capacitor and three trimmer caps. I don't have measurements for the trimmer caps, but that fixed cap is a 300 pF mica.
By the way, I gave the forum an updated schematic for the 46-480, to publish to the Philco Library's "Service Info" documents page. Here's a link, if it may help you: Schematic
I formatted it for 11" x 17" paper, so it's much easier for most folks to get printed. I can print these at Office Depot for $0.35 a sheet.
As it turned out, the 300 pF cap was bad. I didn't know taking this can apart was that easy - Thanks. AM and SW work great now! FM is still a little weak - I'm still looking at a number of things
Hello Harry,
I bought a roll of fish paper from eBay, but Amazon carries it too. GC Electronics makes it, for example, and here’s the one I purchased. I’ve had to rebuild the IF can many times.
radiorich Wrote: I bought a roll of fish paper ... GC Electronics makes it, for example ...
I also have a roll of GC fish paper. It is 0.01" thick.
I have a lot of GC hardware in stock. Some of you will remember the GC hardware which came in transparent plastic boxes with yellow paper labels inside the boxes. I acquired mine years ago at an estate auction - it came from the stock of a radio-TV repair shop.
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