[ARC5] power supply hum

[Tim]

Also, beware of the possible presence of "tin whiskers" growing on the
variable capacitor plates.  Those drove me nuts on a GRC-9 receiver until I
figured it out - one whisker shorted out the local oscillator section at
some tuning positions. The cap plates were apparently tin plated...
Practically invisible, a few microns in diameter, and were pretty resistant
to removal with an air duster can...  Some interesting physics going on
with those little linear crystals.

Tim
N6CC

On Thu, Jan 22, 2015 at 5:30 PM, Kenneth G. Gordon <kgordon2006 at frontier.com
> wrote:

> On 23 Jan 2015 at 8:12, Leslie Smith wrote:
>
> >   DS suggested I look at the tuning gang, and I intend to return to this
> >   area.
>
> It could be that the plates are not centered, or there is a bent plate,
> that is
> shorting the cap when it reaches a certain point.
>
> >   The LO works when the tuning is set to around 200kHz.
> >   The LO stops working when I tune the set above 270 .. 280 kHz.
> >   I put the CRO on the oscillator plate lead.  ( see 15 to 20 volts RMS)
> >
> > I put the CRO on the oscillator grid (triode section, pin #5).
> > I see nothing.
>
> That would not surprise me. Unless you are using a very high-impedance
> probe, or an RF probe, you will be shorting out anything that "is" on the
> grid.
>
> Ken W7EKB
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