[NJARC] Philco 37-650 RF Question

[Al Klase]

Hi Scott,

We may have to take this a step at a time.  If I had to guess, I'd say 
there is a problem, possibly an open connection , with trimmer cap "16A" 
the BC-band oscillator padder, a.k.a  "compensator", capacitor.   If 
this cap is not in the circuit the oscillator will not be able to tune 
the low end of the band.

What test equipment do you have?  An oscilloscope or frequency counter 
would allow you to observe the behavior of the oscillator.  
Alternatively, if you have a communication receiver with a BFO you can 
listen for the oscillator as if it were a CW signal.  When working 
properly, the oscillator will be 470KHz above the tuned frequency.

Regarding AVC: See RECOGNIZING AND DEALING WITH AVC PROBLEMS
in the "Communications Receivers" -> "Maintenance" page of my website
 http://www.skywaves.ar88.net/commrx/Maintenance/agc.htm

Regards,
Al


Scott Roberts wrote:

>Visit our web site - See http://www.njarc.org
>_______________________________________________
>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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-- 
Al Klase - N3FRQ
Flemington, NJ
http://www.skywaves.ar88.net/