HW-101 erratic on 80 and 40 metets

[Neil Morrison]

It is not unknown that on some variable caps (esp. older ones) the
plating will peel away from the plates and will short out the cap
intermittently.
The fix is nasty - remove from circuit, connect rotor side to ground
and the other, via a 15 W lamp, to 110VAC.
Then rotate the cap back and forth until the shorts are burnt out. BE
CAREFUL - insulated gloves and tools are highly desirable.
Make sure that the fault is not an intermittent lack of connection to
the rotor, via the wipers or whatever before doing this.

Regards,

Neil


----- Original Message -----
From: "CyberGeezer" <cybergeezer74 at HOTMAIL.COM>


> [As you'll see, this is really a question about variable
capacitors.]
>
> I've been using this HW-101 for about a year now, during which time
I knew
> that it was flaky on 80 and 40, but that didn't used to matter
because I
> didn't have antennas for 80 and 40.  Now I've put up more antennas
and I
> want to get it working on 80 and 40.
>
> I've identified the problem component to be C421A.  C421 is the
more forward
> of the two variables that are operated by the DRIVER PRESELECTOR
knob.  It
> tunes the driver grid on transmit and the rf amp input on receive.
It has
> two sections--  C421A (200 pf.) and C421B (20 pf).  C421B is always
in the
> circuit.  Bandswitch section 3R puts C421A in the circuit only for
80 and
> 40.
>
> The problem with C421A is that its rotor plates intermittently
short to its
> stator plates (verified with ohmmeter).  The shaft does not appear
to be
> bent or misaligned, the bearings do not appear to be loose, and no
plates
> appear to be bent.  (And the O-ring drive belt is not too tight.)
>
> My question --  I'm not really sure what my question is --  the
plate
> spacing is so small that there's just barely room for the rotor
plates to
> get in between the stator plates, so what should I look for that's
supposed
> to guard against their making contact?
>
> This is probably a dumb question, but do they employ some kind of
dielectric
> coating on the plates in case there's contact?
>
> All ideas and suggestions will be considered.  It probably is
impossible to
> obtain a replacement capacitor, and even if I had one I would not
relish the
> prospect of replacing it--  I'd have to take out (unsolder) all the
> bandswitch sections . . .
>
>
>
> 73 de W2GERry

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