[ARC5] 400784741321

[AKLDGUY .]

> I'm not going to, but I WOULD be extremely interested in how the Brits did
> that!

I pointed out the proper way to do it many months ago. I don't know whether
the Brits did it that way or not, but will repeat here how it should be
done.

Do NOT remove plates from the tuning capacitor to raise the frequency !!!
Instead, solder a small value capacitor in series with each gang of the
tuning
capacitor. The value must be determined by experiment until 30 MHz is
reached with the tuning capacitor plates nearly fully open.

Now check for performance at 28 MHz to see where the plates are. You may
need to juggle the value of the small capacitor and settle for a reduction
in
coverage (less than 28-30 MHz) or over coverage (28-30 MHz appearing at
less than full swing of the dial) - see below.

The addition of the small capacitor causes an immediate rise in
the frequency
since it is small (high reactance) in comparison to the gangs, and therefore
has a dominating effect on the total circuit reactance (gangs plus stray
capacitance).

The swing in gang capacitance from fully open to fully closed now has a
smaller effect on the total circuit capacitance at 28-30 MHz than it
would if
only the gangs were present.

The best way to do this is probably to start with the ARC-5 oscillator.
Solder in a small capacitor in series with its gang.
Allow the capacitor at least an hour to cool down.
Set the ARC-5 tuning dial to maximum (9.1 Mc/s, presuming the 6-9.1 Mc/s
model).
Check the oscillator frequency using a nearby general coverage receiver.
You're aiming for 27.170 MHz (30 MHz minus 2.830 MHz IF).
Change the capacitor as needed to get oscillator swing of 25.170 - 27.170),
remembering to allow for cool down at every change.
The trimmer capacitors will help in frequency setting.
The resulting capacitor value now gives you a ball park figure for the
capacitors
that must be added to the RF and mixer gangs. The capacitors for them will
be
slightly higher in value.

My guess is that the Brits did it that way, but also took great care to
remove
exactly the right number of plates in order to get exactly the correct
frequency
swing from one end of the dial to the other.

If the result is under coverage (28-30 MHz not fully covered) and you are
not
satisfied, you could remove one plate at a time, increase the series
capacitor
to compensate, refit the tuning cap cover, and recheck.

If over-coverage (28-30 cramped on the dial) and not satisfied, add
**parallel**
capacitance to each gang, **reduce** the series capacitor to compensate,
refit
the tuning cap cover, and recheck.
These latter changes may be rather small in value. Remember to allow the
capacitors and anything else you solder plenty of time to cool down before
rechecking.

73 de Neil ZL1ANM