Well, I am certainly obliged to deliver! From what I have read, there is a mounting scarcity of IF transformers. For such a simple, passive component, I have no idea why this seems to be the pinch point when searching for parts. One would think that some niche company would still be producing these.
I've just ordered the new inductors and capacitors. Will reach back out with an update after they arrive! We will come up with a solution. This is the perfect forum for peer review, and I feel that we are quickly refining the design.
The new components arrived this evening. I built a new prototype per my last description. I ordered a 3.3 μH coupled inductor and two individual 3.3 μH inductors. This time I went with axial leads; I found that they have much tighter tolerances.
I noticed that I get a better range of adjustment if I use an 82 pF fixed capacitor in parallel with a 20 pF trimmer. I still also find that the individual inductors perform best. However, my oscilloscope shows that I am obtaining a resonant frequency of 7.0 MHz.
I decided to compare against a working IF can. I’ve been trying to create a replacement for the 1st IF can, but the 2nd IF can is original. I found that its resonant frequency was also close to 7.0 MHz. I am wondering if something about the way I have my test set up is introducing capacitance.
I’ve decided that the only way I will know if this is working is if I insert the device into the actual radio for a test under real conditions. Tomorrow I will attempt to solder it into the circuit.
One last note — it’s not seen in my photo, but I did use an experimental resistance on the primary side.
We have lift-off! I soldered on some lead wires, and inserted the proto-board circuit into the radio. I tried inserting a video of it playing for a few seconds, but apparently I cannot upload videos. Anyway, I captured a video of the radio tuned into 100.9 KTSO (Tulsa’s Soft Oldies) for just a few seconds.
So far as I am concerned, the range of adjustment is great with the 82 pF fixed capacitors and the 20 pF trimmers. My next step will be to start on a PCB design to tidy it up.
Indeed it is! I feel relieved that it is working, and am looking forward to producing a PCB design now. I’m curious to also reattempt the coupled inductors; perhaps they will work just fine under actual load. But, if not, we do have a working solution with the axial lead inductors.
I have gone ahead and designed a PCB this evening. I will likely sit over it for a couple of days before sending the design in for production, to make sure I am satisfied with it. The old IF can measures 0.75" x 0.75", so I designed this to be 0.725" x 0.725" to be inserted into the gutted can. Have a look at the finalized schematic and PCB 3D model images.
For grins, I did also try connecting the prototype the design with the coupled inductors to the radio. It wasn't tunable at all, unlike the success I had with the individual inductors. So I will be moving forward with the individual inductor design.
Just curious as to where you will have this PCB manufactured? Will you have access to the trimmers for final alignment once installed back into the can? Your project is coming along nicely!
I was thinking late last night about those trimmers! This final design has changed quite a bit from my original concept, and so it would seem that they will not be accessible with the can installed. One possible solution came to mind this morning—I could very slightly enlarge this PCB, so that it will not fit inside the can. I would use it as a base for the can, with cutouts for the brass clip which secures the can from underneath to fit through. For service, the can would be removed. Once adjusted, it would be reinstated. I will draft up an idea today for better clarity.
I have used OSH Park (https://oshpark.com/) in the past for PCB production. In fact, I used them to produce a PCB for the replacement multi section capacitor (see photo).
Hi Joseph, thanks for the link. With a relatively high frequency such as your 9.1Mhz, alignment might be affected once you reinstall the can so not sure how much if any. That’s usually why all tube shields should be in place if there are any before doing an alignment. Just something to keep in mind.
That is an excellent point. I did not notice much change when placing the shield over my prototype board; then again, I did not have the prototype installed in the permanent location and did not have it connected to the chassis ground. I encounter different results if I don’t proactively address the issue now.
I may go back to the original design of two PCBs. I’ll need to keep the two inductors on the same PCB, but I can put the fixed capacitors and trimmer caps on opposite boards. I’ll have one at the top (trimmer cap facing up) of the IF can and the other at the bottom (trimmer cap facing down).
I mistyped the last sentence from the first paragraph of my last post. I meant to write, "I may encounter different results if I don’t proactively address the issue now."
In any event, I have worked on a redesign in order for the two trimmer caps to be accessible while the can is in place. I have designed two PCBs, an upper and a lower. The lower board will comprise the majority of the system. It has connections 1, 2, 3, and 4 (one in each corner) for the four incoming/outgoing connections to the rest of the radio. It also has a space for the primary trimmer cap (to be mounted on the underside for access from the bottom of the chassis), the primary fixed capacitor, and both inductors. I have confirmed from experimentation that these two radial lead inductors need to stay side by side for best results. There are also four through-hole cut-outs for jumpers to the upper PCB. For both structural and electrical connection, I plan to use these vertical header pins which are used for connecting two stacked PCBs. I will only need two sets of single-row, double-pin connectors.
Here are top and bottom views of the lower PCB.
The upper PCB will consist of the secondary fixed capacitor and trimmer cap (centered and facing up for access from the top of the can). It features the same four through-hole cut-outs for soldering to the vertical header pins. Here are top and bottom views of the upper PCB.
For clarity, I have attached the two schematics for the upper and lower board.
You are exactly what this hobby needs! I haven't done a calculation regarding inductance, charge current etc. since Physics II in college, some 48 years ago. Brain turned to mush since then not from drugs or alcohol but just from 48 years of living. At least I learned how to express a complex equation just by typing, (or did i forget that I did so merely 2 years ago when I volunteered to support my company's Lab informatics product when my right hand went up before my left hand could pull it down and smack it). Definite hair on fire from brain meltdown.
My education was in Life Sciences and Clinical Laboratory Technology (BS, 1978) but I taught myself mechanics, electricity, electronics, plumbing and refrigeration from books and discarded items. Got pretty good at it, then morphed the hobby and education together for a 44 year career as a Field Engineer in the bio-medical and clinical laboratory field.
What us old heads and new engineers like you have in common is the ends that we will go to preserve these pieces of history and make them work again. BTW, excellent workmanship!
"Do Justly, love Mercy and walk humbly with your God"- Micah 6:8
Best Regards,
Looks like you have all bases covered now. Looking forward to seeing the completed unit. You put a lot of time and effort into this. Like the saying goes, where there’s a will there’s a way!
Thank you both! Yes, I agree that necessity is the mother of invention. I had researched this idea for about a week prior to presenting it here on the Phorum, and all I kept reading was that a broken or missing IF transformer (for anything other than the ubiquitous 455kHz AM intermediate frequency) was the end of the road. Many folks suggested purchasing another identical radio to use for parts, but I just couldn’t accept that. I’d rather leave as many radios available to other restorers as possible.
I do not take credit for this concept. I took the idea from another video I saw online which inspired me. Another guy had come up with something similar for a AM IF transformer. This design has ended up being fairly different, but that’s because we’ve had a lot more to contend with at higher frequencies. This design would not have been possible without the feedback from everyone who has taken the time to respond. I’m more than happy to share our findings here for all to benefit.
Though this radio is just about done, this won’t be the last of my time on the Phorum. You all have welcomed me in, and I love it! I have a 48-482 in my garage which needs a restoration, but that one is going to be a lot of work. EVERYTHING in it is coated in a thick layer of dust and cigarette tar. It’s pretty yucky. But it was sold here in Tulsa according to a stamp inside of it, and so my heart is committed to restoring it and keeping it local. It is also missing the FM1000 tube, which I understand to be another scarce commodity these days. I may go the route of testing substitute tubes and altering the circuitry to accommodate.
I have also been seriously considering a 46-480 that I’ve seen listed lately. It’s a beautiful console, and I have always wanted a console. It’s a bit rough looking, but I enjoy both the woodworking and the electrical restoration.
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