[gprutchick]

I ended up making the insulators using a 3M transpearency, which worked just fine. Remember these things were used on overhead projecters in the past.

Thanks for all the comments.

[radiorich]

Hello gprutchick,
Great Idea I never thought of the 3M transpearancy !
Sincerely Richard

[MrFixr55]

That'll work!!

[RodB]

Let's see if Mike can come up with a dielectric constant for 3M transparency. Will it be close to mica?

[gprutchick]

RodB

I was thinking of trying to use the 3M PET in lieu of mica for padders and trimmers.

I thought that I would ask ChatGPT for the dialectric constant of 3M PET transparency film,

For PET film, the dielectric constant (relative permittivity, εr) is typically:
εr ≈ 3.0 to 3.3 (at 1 kHz to 1 MHz)

For most RF and capacitor-type calculations, using:
εr = 3.1
is perfectly reasonable.

The dielectric constant varies slightly with: Frequency, Temperature
Exact formulation (some 3M films have coatings)

If you're working in the AM broadcast band (~500–1700 kHz), using 3.0–3.2 is absolutely fine for design calculations.

Dielectric strength for PET film is typically:
~200–300 V/mil
(very respectable for thin capacitor experiments)

For the mica used in trimmer and padder capacitors, the dielectric constant (relative permittivity, εr) is typically:
εr ≈ 5 to 7. Most commonly in radio service work, you’ll see: εr ≈ 6

I guess we can conclude that 2 layers of the 3M transparency would be equivalent to a single layer of mica if used in a padder or trimmer capacitor. However, the dielectric constant of the 3M material varies slightly with: Frequency, Temperature.

So to copare the temperature stability of mica with the 3M PET:

Mica (natural muscovite)
Dielectric constant (εr): ~5–7
Typical temp coefficient (TCC):

~ ±50 ppm/°C (often better in good capacitor-grade mica)

That’s very stable. Across a 30°C room change: 50 ppm/°C×30°?=1500 ppm=0.15%

PET (Polyethylene Terephthalate – 3M transparency film)

Dielectric constant (εr): ~3.0–3.3
Typical temp coefficient: ~ +400 to +700 ppm/°C

Let’s use 500 ppm/°C as a reasonable average.
Across 30°C: 500 × 30 = 15000 ppm = 1.5%

That’s 10× worse than mica.
In oscillator circuits, that’s noticeable.

Final Practical Verdict

For: RF padders, Oscillator trimmers, IF fixed caps, Anything affecting frequency accuracy Mica is vastly superior.

PET is fine for: Coupling caps, Bypass caps, Non-frequency-determining uses

But for frequency-setting components in your Philco-style circuits you want mica.

Learning something new every day!
gprutchick

[Arran]

Good thing I have a stash of mica, but then again I have a roll of flat nichrome heating element wire too, like the kind they have in toasters, the mica I will need in some quantity at least, the heater wire, I don't know, unless I end up with a vintage appliance I need to repair. By the way, you can find mica in a discarded hair dryer, though it is likely too thick, another possibility is an insulator from under a power transistor.
Regards
Arran

[gprutchick]

Good to know. I did not think about using power transistor insulators.

[gprutchick]

I am looking for instructions on how to rewind the ANT Coil for the broadcast band on my Philco 38-10.

I have rewound the secondary but the primary looks to be a much bigger task.

The radio works fairly well. The IF alignment is good at 470KHz and the oscilator tracks across the entire band at 470KHz above the dial setting. Stations are received at the correct position on the dial, however weak at best. Touching the cap of the 6A8 greatly improves reception and reduces noise considerably. I suspect the primary of the ANT coil or someting else???

Thanks,
gprutchick

[RodB]

If the primary is one of those eighth inch thick windings, try wrapping masking tape around the form till you get the thickness you need. Then make a space for the winding and wrap another multiple layer of tape. In the gap, wind the primary using an adhesive to hold the wire in place. You don't need to remove the tape when done.

[gprutchick]

Thanks RodB

So that I understand, the two masking tape wrappings create a "channel" for the new winding to rest in. Is that correct?
Do you know how many turns? I suppose I can try to count the turns as I remove the old secondary.

One other problem, the clip used to mount the coil broke. Any suggestions? I guess I could hot glue the coil in place?

[RodB]

That's correct. You build a channel for the winding with the tape. Being it's an antenna transformer it's not critical to have the same number of turns. You can measure the resistance, get the physical size and a close number of turns and it should be sufficient to come close enough to do the trick. Maybe even add a few turns for good measure.

Hot glue is good.

[Arran]

Another option, though I have not tried it, is to use rubber washers that will fit onto the form.
Regards
Arran

[gprutchick]

I have removed the secondary winding of the BB ANT Coil. There was approximately 15' of Litz wire. I did some research and think I need 50 strand / 44AWG wire. Does anyone know where I can pruchase a small quantity of Litz wire. For this coil I only need about 15 to 18 feet.

[RodB]

Here's a link to a recent thread, page 7, discussing litz wire:
https://philcoradio.com/phorum/showthrea...light=Litz

[gprutchick]

I have rewound the secondary of the Range 1 ANT coil using Litz wire. The AWG of the Litz wire was a bit different in that the final coil resistance was about 4.3 ohms, the inductance was 185uH. I did the calcs and this inductance is good given the range of the tuning capacitor which is 15pf to 420pf. The IF is aligned and crisp. The RF alignment is good. Known local stations are received at the correct dial setting. The 6A8 osc is strong, the osc grid ranges from -14.2V at the low end of the dial and -11.3V at the upper end.

Unfortunately the sensitivity of the radio has not changed. So, I rewound the primary of the Range 1 coil. It did not change the sensitivity. I substituted 8 different 6A8 osc tubes with no change. Since the Range 1 ANT signal passes through the secondary of the Range 2 ANT coil I tried to short the Range 2 secondary thinking it was the culprit, but no change.

There doesn't seem to be much coupling between the pri and sec of the Range 1 ANT coil. Touch the cap of the 6A8 and the radio works fine.

I checked the AVC voltage using an RF signal generator as the antenna signal source. With no signal the AVC is around -2.2V. With a fairly strong signal, it gets up to around -11V with the dial at 1500 and -14V with dial at 550. I think I can conclude the AVC is working ?

NOW WHAT? I still think the Range 1 coil is the problem, but I just don't know how to solve it. How do you improve the coupling of an ANT coil, which is wound on an air core. I don't have much experience winding coils.

I just discovered something that I think is strange. I was monitoring the antenna input signal with my scope when I did the AVC check. I disconnected the RF generator, the scope was still connected to the ANT terminal. When I switched to the SW band, there was a significant amplitude of a sine wave, close to the frequency selected by the dial. Is this normal? I don't see how this can be unless I made an error during the electrical restoration. I double checked everything, but ?

I have put many hours into this one. Maybe I need to put it aside for a while?