[NJARC] Philco 37-650 RF Question

[Robert Bennett]

 Scott -
                I had a similar problem with a restoration of a 40's Zenith
console. I couldn't get the BC band to work under 1000KC. What I learned was
that I had replaced some "out of tolerance" resistors in the RF section that
were "carbon film" instead of carbon composition (like the originals). A
fellow enthusiast informed me that the "carbon film" or "metal film"
resistors will not pass current as well. I replaced the resistors again and
Voila! I had the whole BC band again! Since then, I only use carbon
composition resistors in the RF and IF sections. Hope you find it!

Bob Bennett
----- Original Message ----- 
From: "Scott Roberts" <ng19delta at yahoo.com>
To: "New Jersey Antique Radio Club" <njarc at mailman.qth.net>
Sent: Thursday, February 23, 2006 5:24 PM
Subject: [NJARC] Philco 37-650 RF Question


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> _______________________________________________
> Greetings! The story so far: I have a Philco 37-650
> which I have been rebuilding for some time now. I've
> gotten most of the kinks out, but am still having a
> bit of an issue with the BC band. The two SW bands are
> operating nicely at this point, and the BC does work,
> but only to a particular point.
>
> In rebuilding this radio, all the caps- electrolytic,
> paper and mica, as well as bakelite, have been
> replaced, or rebuilt with newer replacement parts.
> These parts were within the 10% rule of thumb when
> original values were not obtainable. Also, the vast
> majority of the wiring was replaced, and there are
> only a couple sections which were not, for the simple
> fact they seemed to be ok. However, in doing so, I
> replaced I replaced sections of 20 or 22 gauge solid
> wire with 20 gauge stranded.
>
> My problem is this- The BC band works- between 900 and
> 1700. Below 900, it is all squeals and sqwauks and
> burrs. I have aligned the IF coils as specified in the
> tech sheets, and also the antenna, RF and Osc.
> sections, for both SW's and BC band. But it seems like
> the BC band is "squished"- If I adjust to specs,
> nothing in BC is quite where it should be. If I
> realign by ear 1210 to the proper dial indication,
> then turn to 1060, 1060 is high. Readjusting 1060
> moves 1210, and 950. Everything stays above 900, and I
> can't get anything below 900. I have fiddled, twisted,
> and contemplated to no avail. I have checked and
> rechecked the components in the RF subchassis:
> switches are clean as I can get them, coils are fine-
> even discovered a resistance value issue with leads
> I'd replaced, and restored it to correct value.
> Everything checks out OK.
>
> It has been suggested that the RF current moves along
> the outside of the conductor: and that in replacing
> the solid wiring with stranded, I may have disrupted
> this flow's proper characteristics. Is this possible,
> and would it cause the symptoms I am experiencing?
>
> Any replies appreciated- I'm almost pulling my hair
> out with this one. As my first major rebuild of a
> radio, it is my learning experience: And it almost
> works, except for this issue.
>
> Also, can anybody explain "AVC" to me? In this radio,
> the cpncept does not seem to match reality. Louder
> stations are LOUD, and quieter ones not, so I get neat
> differences when changing, rather than reduced volume
> of loud stations, when set for the quiet ones to be
> heard.
>
> This radio has a whole group of topics occupied on the
> Philco Phorum, and I can send along the link if
> desired. Long story short, I haven't gotten a working
> solution to the BC problem yet, so I thought I'd bring
> it here...
>
> Thanks for any help,
> Scott
>
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