GR 78

[Bill Coleman]

On 12/21/01 4:47 PM, W.A. Strong at maytag at WECNET.COM wrote:

>Need help in getting  GR 78 aligned .
>
> All my attempts to set the frequency(s) are frustrating ( a very mild term)
>
>Problem is I cant get it  in the freq range -- Adjustment ment of the
>slugs and the caps in the circuits always give the same response- The
>frequency range is shifted as a complete move.. If I move or try to change
>the top end the low end will move the same way the same amount..  Is there
>something I am missing in that I cannot make any adjust to one end or the
>other .  40 meters not obtainable  the high end is 7005,  can't get 80
>tuned  either .

I don't know what a GR-78 is, exactly, so I'm just giving some general
advice here.

If you have tuned circuit for a local oscillator that has a variable L
and variable C, there's an infinite number of combinations that will
result in it oscillating at one frequency. If you have a little less L,
you can add more C, and vice versa.

Where this becomes important is if you have a pre-printed dial and you
are trying to match up the actual frequency range of the radio with the
dial. Increasing the amount of adjustable C will cause the tuning
capacitor to be swamped by a little more capacitance, therefore it won't
tune over quite as far a range. Decreasing the C allows the radio to tune
over a little more range.

The trick here is to vary the L/C ratio to get the radio to tune
according to the dial. So, you set the frequency at the bottom of the
band with the L (coil). Then you tune the top of the band and adjust the
C. Back to the bottom of the band and re-adjust the L, then back to the
top of the band and adjust the C. Go back and forth 3-4 times should do
it.

Note that none of these dials is ever perfect, but you should be able to
get it close.


> Another question : Is there any decided difference in the capacity
> features of copper plates versus aluminum?

Nothing significant. Aluminum isn't as good a conductor and such a
capacitor will have a lower Q, but it shouldn't be a big deal.

>In this receiver  3 sections,  2 have aluminum plates and one section
>copper.  (VFO) circuit.
>  The aluminum ones are identical. The section with copper plates has
>more plates than the others .  Readings of the capacity  gives me  lower
>than the aluminum ones with less plates .

Makes sense. The LO is probably at a lower frequency than the tuned RF
stages.



Bill Coleman, AA4LR, PP-ASEL        Mail: aa4lr at arrl.net
Quote: "Not within a thousand years will man ever fly!"
            -- Wilbur Wright, 1901

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