The PHILCO Phorum
Model 41-296 - Printable Version

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Model 41-296 - w5ami - 04-11-2013

Wonder if anyone can tell me what the value of the fixed cap inside the 1st IF transformer is supposed to be? The schematic, nor the parts list gives it. The schematic does show it, but no value. I have pretty much determined it must be a 3000pf, but still not sure. Using a 3000pf gives me some change with the slug, but not as much as I would expect.

Thanks!
Brian / w5ami

RE: Model 41-296 - BrendaAnnD - 04-11-2013

Can you please post a pic of that section of the schematic? I can't find that model in either Riders nor Beitman's.

RE: Model 41-296 - w5ami - 04-12-2013

Here is the schematic. The cap in question is marked 32A across the primary in the 1st IF xfmr, 32.

RE: Model 41-296 - BrendaAnnD - 04-12-2013

I would think that something closer to 100-200pF would be closer. Though the DC resistance (and therefor the nominal inductance) of the primary is only about 10% of that of the secondary, it also has the slug tuning, which raises the inductance and sharpens the "Q". These are interesting IF's, can't say I've ever seen combination variable inductance AND variable capacitance in one can before.

RE: Model 41-296 - Raleigh - 04-12-2013

First, do you have the original cap that was inside the IF can? The dot code should tell you its value. They are sometimes hard to read, but maybe we could help you out.

Second, I looked up other 1941 radios and found that the 41-295 listed the cap value inside the 1st IF can as 3000 pf (uuf). That radio has the same XXL mixer and 7B7 1st IF amp tubes shown in your 41-296, so it might use the same IF can assembly. Unfortunately, the crummy schematic I have for the 41-295 doesn't have a parts list, so I can't give you the part number of the IF can assembly.

Maybe someone else has better documentation than I have, so you could compare the part number on your set with the one on the 41-295. If they're the same, then 3000 pf is your value. If not, 3000 pf still might be a good guess.

For reference, the 42-355 also lists the 1st IF can cap value at 4000 pf. That IF can part number is 32-3794. The 42-355 uses a 7V7 for the 1st IF amp instead of the 7B7 in the 1941 radios.

RE: Model 41-296 - Mondial - 04-12-2013

That first IF transformer is indeed unusual, but it might be explained by the fact that it is driven by a triode mixer. In the usual case of a pentode or pentagrid converter, the impedance level seen by the transformer primary is on the order of 500K ohms or so. A triode however has much lower plate resistance, typically around 10K, so the primary sees this lower effective impedance and must be designed to match. This might explain the much lower DC resistance and the need for permeability tuning to permit the use of a larger value resonating cap than normally available as a trimmer.

I would guess that a higher value cap is necessary than usual to tune the primary to resonance. You could experiment with various values of capacitance until you obtain a sharp resonance peak at the center of the tuning slug range.

Did the cap you removed have any marking as to its value?

RE: Model 41-296 - w5ami - 04-12-2013

Brenda and John,

First, thanks for the ideas, etc. The old cap has lost dot one, however by the looks of it, it's orange. I have attached an image of it, however I'm not certain which way to read this since I really can't determine what is top or bottom. I can only assume the NY USA imprint reading upright would be correct, but not certain. This cap measures about 2500pf, or a little more. Based on the inductance range of about 30 to 58 uH I've measured, 30 being with the slug in all the way and 58 out all the way, 3000pf would tune 455kcs. The old cap did however seem to produce a sharper peak than the 3000pf I have in there now. Maybe because it was off, or possibly leaky. When adjusting the slug now, it's hard to tell where the peak is. It does have effect, but very broad. Maybe it's designed that way.

Thanks again for the thoughts. I think I will leave the 3000pf in there until I get further into this rig, or hear it's the wrong value.

Brian / w5ami

RE: Model 41-296 - w5ami - 04-12-2013

Mondial,

A sharp peak (tuning the slug) is not what I'm getting with a new 3000pf, and that's why I'm wondering if I've got the wrong value. On the other hand, the trimmers do give a nice sharp peak as expected. If you assume the blank dot are black, and the photo shows it upright, I am thinking that should be a 3000pf cap. I hate all the variations they had on these micas!

By the way, I am getting a good strong signal at 455kc when I have everything where I think it should be. Like I said however, it's hard to tell using my scope where the peak is tuning the slug.

Thanks for your input!

Brian

RE: Model 41-296 - Raleigh - 04-12-2013

Orange-Black-Red looks like 3000 pf to me. I think the red dot on the bottom means 2% tolerance.

Someone correct me if I'm wrong reading these dots.

RE: Model 41-296 - Mondial - 04-12-2013

Because of the loading of the triode mixer tube, you may not get a very sharp peak. The normally low triode plate resistance may not allow very high Q in the primary tuned circuit.

You mentioned that the original cap seems to tune more sharply than the replacement. I am curious as to why you are thinking of replacing it?

Typically mica caps are pretty reliable and not prone to the same degradation as paper/foil types. Silvered micas do sometimes become unstable, but from the photo, your cap seems to be the more reliable stacked mica/foil construction.

What type of cap are you using as a replacement?

RE: Model 41-296 - w5ami - 04-12-2013

Mondial, you are absolutely right about the micas. In this case, it's just a "feel good" thing since I am doing this for a friend who wants to pass this radio down to family. Everything was original, including the micas and of course over 70 years old, so I dediced to replace everything, even resistors that checked good. This cap was a no brainer since it was located inside the can, and in order to get to it, the xfmr wires and the can must be removed, so I decided it needed a good new silver mica to hopefully last another 70 years.

John, that is the way I am reading the cap, but just wasn't sure. Some of these six dot caps get me, besides I'm color blind, but not to orange and red. It's usually 1 meg and 10 meg resistors that get me! The blue and green...

Brian