[R-390] Fwd: R: Strange variation of sensitivity AGC resistance toward the chassis

[N4BE_Jim--- via R-390]

Sent from my iPad

Begin forwarded message:

> From: "N4BE_Jim at yahoo.com" <N4BE_Jim at Yahoo.com>
> Date: May 9, 2015 at 4:40:37 PM EDT
> To: Norman Ryan <nnryann at yahoo.com>
> Subject: Re: [R-390] R: Strange variation of sensitivity AGC resistance toward the chassis
> 
> Norman,
> 
> I have used Big Bath Cleaner/Degreaser to remove oil and such from phenolic wafers and the antenna tune shaft.
> 
> http://www.gcelectronics.com/order/Catalog_PDF/2007%20Catalog%20120407%2042.pdf
> 
> Be sure it says degreaser.  Available from many electronic suppliers.  Works on old phenolic tube sockets too.  Just be sure it doesn't contain lubricant. I was restoring a Collins KWM-2A once and made the mistake of spraying the tube sockets with tuner cleaner/lubricant.  The sockets absorbed the lubricant and dropped impedance.  Same happened with switch wafers.  I think the older fibrous materials are porous and soak up liquids and lubricants.  The degreaser dissipates water and lubricants.  
> 
> Hope this helps.
> 
> Jim N4BE
> 
> Sent from my iPad
> 
>> On May 9, 2015, at 10:47 AM, Norman Ryan <nnryann at yahoo.com> wrote:
>> 
>> Hi, Jim,
>> 
>> Good advice on all points.
>> 
>> Do you have a suggestion for what solvent would remove lubricant from the antenna trim phenolic shaft?  I used Mobil 1 on mine.  Fortunately, I have conscientiously kept Deoxit away from the switch wafers, but if you have a suggestion for removing Deoxit for folks who may have inadvertently sprayed the wafers, please feel free to share it with the R-390 list.  (I've responded here offlist.)
>> 
>> Vy 73
>> Norman
>> 
>> From: N4BE_Jim--- via R-390 <r-390 at mailman.qth.net>
>> To: "r-390 at mailman.qth.net" <r-390 at mailman.qth.net> 
>> Sent: Friday, May 8, 2015 8:39 PM
>> Subject: Re: [R-390] R: Strange variation of sensitivity AGC resistance	toward the chassis
>> 
>> For what it's worth, in restoring my SW radio, I found it more expedient to just "shotgun" replace every capacitor touching the AGC, especially in the RF deck.  The .005 mfd bypass caps throughout the RF deck were completely replaced with modern caps.  Didn't bother wasting time trying to isolate specific caps.  With the age of these radios you can safely assume they all leak.
>> 
>> Also check all resistor values. They typically drift with age.
>> 
>> And I can't emphasize enough the problems induced by spraying switch wafers with any kind of cleaner.  It's best to individually clean each contact wiper with a Deoxit pen.  If you spray the phenolic wafers, they will absorb the chemical and will no longer be high impedance.  And for the same reason, never lubricate the antenna trimmer phenolic shaft.  The trimmer cap is connected to the AGC line.
>> 
>> 73 Jim N4BE
>> 
>> Sent from my iPad
>> 
>> > On May 8, 2015, at 5:57 PM, r-390-request at mailman.qth.net wrote:
>> > 
>> > R: Strange variation of sensitivity AGC resistance toward the chassis
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