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Model 90 1st RF question - Printable Version

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Model 90 1st RF question - mgutierres - 03-18-2011

I noticed that the sensitivity of my 90 was not that great--big, local stations came in okay, but not the regional ones, and at night, only the big blow-torch stations came through, albeit very weakly. I tested my coils, and they all seemed okay, but I was still suspicious of the 1st RF, so I took it out and looked it over:[attachment=1]
Seemed okay, but it looks different than what most people describe as a 1st RF coil: the primary is inside the tube on its own spool. Here's what it looks like underneath: [attachment=0]

If I bypass this coil and hook the antenna directly to the 2nd RF, reception is much, much better, more sensitive and more clear. So there has to be something wrong with the 1st RF, right? But they all test continuous on my ohmmeter, and none of the coils shows more than 6.6 ohms resistance.

Question: how would I go about rewinding the primary on this coil? How do you get the center spool out of the form? Is this even the correct RF coil for this radio? Could someone have substituted it from something different? If the coil is okay, what could be causing the weak reception through it?

Lots of questions. Hope someone has some insight!

Mike

Re: Model 90 1st RF question - Ron Ramirez - 03-18-2011

That is very odd that your antenna coil would be, in essence, blocking reception if the continuity of both windings are good. Moisture absorption is the first thing that comes to my mind.

To answer your questions:

1. Once the primary is removed, you will be able to see the direction the turns need to be wound. I don't have my notes next to me, but it's either 200 or 250 turns of 38 gauge wire.

2. You remove the center spool by unsoldering the leads - first making notes on how the leads connect to the coil lug terminals, of course - then, using an instrument such as a socket wrench, gently pushing up on the spool. It is held in place by two tabs on top. One of the tabs may break; if it does, it can be glued back together.

3. Yes, it is most likely the correct coil, as the original antenna coil was made just as shown in your pictures.

4. While it is possible that someone could have substituted an incorrect coil, I sort of doubt it.

5. As I said, it could be moisture absorption. You might try baking the coil in your oven for 30 minutes at 180 degrees. This baking process is usually mandatory for Model 89 and 19 oscillator coils, and would certainly benefit other coils as well. Usually, a lightning hit will completely blacken that winding, and will normally make it go open as well.

Re: Model 90 1st RF question - Radioroslyn - 03-18-2011

I had a similar problem with model 370 (model 70 chassis) It has the same style coil. Mine had a broken wire and I was able to resolder it. In your case If the continuity check ok than the problem isn't the coil. Would check the plate, screen grid, and cathode voltage of the rf amp tube. While you have the coil out check the resistance of the rf section of the tuning condenser (should measure open with the condenser all the way closed).Rf amp tube ok?
If you really think it is the primary of the coil, take a cardboard tube small enough to fit inside you rf coil and wind 50turns of small gauge magnet wire. With the set all back together take the coil you just made and place it inside the rf coil in the set. Take one of the wires from that coil and hook it to your antenna. It the reception is much better then it is the antenna coil.
Terry

Re: Model 90 1st RF question - Steve Davis - 03-19-2011

If you need to rewind this coil, a plastic sewing machine bobbin makes a good coil form.
Steve

Re: Model 90 1st RF question - mgutierres - 03-23-2011

Thanks for the feedback. I've learned through recent experience that a coil that shows continuity on an ohmmeter does not necessarily mean it's good. I had a similar problem with my recently completed 48-482: OK reception on the AM band, but not nearly as good as a radio with 3 IF amplifiers should have, so I tested everything again, and every one came up solid. My only clue was that when adjusting the IF padders during alignment, the 2nd primary made no difference no matter how tight or loose I adjusted it. Luckily, John Kendall had just the replacement can I needed, so when it arrived, I swapped it in immediately and voila! Sweet, strong reception all up and down the band, even though the original tested fine.

So I'm very anxious to test the improvised RF primary, as well as running through all the voltage tests on that stage, this weekend. Results to follow...

Re: Model 90 1st RF question - Radioroslyn - 03-24-2011

I was going mention that that primary coil isn't very critical as it is used just to couple the rf energy to the secondary. The secondary is part of the tuned circuit, primary isn't. You could if you wanted to rewind it like the other coils with the primary on top of the secondary. Or you probably could use an ant coil from a model 20 or 70. The only thing that matters is at the tuning cap is the same value in mmfd's. Then it will tune the proper range.
Terry

RE: Model 90 1st RF question - brett2752 - 01-28-2024

Did you ever find resolve for this issue? I have been looking for a 1st R.F. Transformer. For Philco Model 90 and 90A. Part # 03013. 2nd version chassis that uses one 47 as a single ended output. Mine is definitely bad and the inside coil is burnt, the outside coil is fine but I am unable to rewind it.

Can you bypass this coil? Heck I would even pay to ship this to someone who could rewind! If anyone has a spare, I’d gladly take it off your hands.

Thanks,

Brett

RE: Model 90 1st RF question - Chas - 01-28-2024

You could try a 2.5 mh choke inserted into the open end as a primary. Secure with hot melt so it is an easy removal if need be.