[AMRadio] RE: Yet another receiver update!

[Bob Bruhns]

Hi Brett,

Hey, you got that thing going... I've never built a receiver up from
nothing.

If it drifts down as it warms up, you can compensate with negative
coefficient caps.  These are the most common type, I think.  N750,
N1500, N2200, etc; the higher the number, the stronger the effect.

  Bacon


----- Original Message -----
From: "Brett Gazdzinski" <brett.gazdzinski at wcom.com>
To: <amradio at mailman.qth.net>
Sent: Thursday, March 14, 2002 4:16 PM
Subject: RE: [AMRadio] RE: Yet another receiver update!


> Bacon,
> It drifts down.
>
> Its not the coil warming up, as that is in a shield can, and
> does not seem to get warm.
>
> Part of the problem may be the dropping resistors in the
> power supply.
> I did not have a 150 volt transformer, and used a 260 volt one,
> and drop lots in the resistors between the VR tube (vr105).
>
> This adds more heat than is needed under the chassis.
> Still, I think its mostly the oscillator tube.
> It was sort of a poor choice to use octal tubes, especially
> for the oscillator!
>
> Smaller tubes likely mean lower drift.
>
> I made some mistakes since it is my first home brew superhet...
>
> I should have used 7 and 9 pin tubes,
>
> I should have placed things MUCH closer together, especially
> the coil, cap, osc tube and mixer.
>
> I should have found the correct transformer before cutting
> holes in the chassis.
> (I originally was going to include an audio amp (pair of 6L6,s)
>
>
> Thanks for the tips,
> Brett
> N2DTS
>
> >
> > Hi Brett,
> >
> > Does the oscillator drift up, or down, as it warms up?  You
should
> > use positive or negative coefficient temperature compensating
> > capacitors to reduce the drift.  Some of the comp caps can be
next
> > to the coil, other comp caps can be closer to the 6J5.  Nothing
is
> > perfect, but optimum compensation should help a great deal.
> >
> > Also, you could build a frequency lock loop, using your
frequency
> > counter reading and a small amount of "set" memory.  When you
have
> > the oscillator on a desired frequency, you save the frequency
> > reading in the "set" memory.  The FLL would then compare your
"set"
> > frequency with the readout, and apply a slow correction to the
> > oscillator to keep it on frequency.  The FLL must be disabled
when
> > you tune; when you get where you want to be, you enter the
reading
> > in the "set" memory, activate the FLL, and there it stays.
That's
> > how some old and very useful signal generators work.
> >
> >   Bacon
> >
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