[ARC5] power supply hum

[Scott Robinson]

...and another way to get rid of whiskers is this:

DISCONNECT ALL CIRCUITRY FROM THE CAP FIRST, except the frame ground!!!

Take a 300V power supply, put a 10K resistor in series with it, and 
apply that from stator to ground. Now rotate the shaft back and forth 
and watch the nice light show. When the light show stops, the whiskers 
are really gone. Watch where you put your fingers...

So long as you disconnect ALL wiring to the capacitor except the frame 
ground, this will do no harm. I learned it from a 92 year sold radio 
repairman here in San Francisco and have used it to good effect.

/scott

On 1/22/15 5:47 PM, Tim wrote:
> 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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