[R-390] Crystal Osc Deck Trimmer Failure

[Drew P.]

Hi Steve,

I would surmise that Collins never anticipated that these radios would still be in service nearly 50 years later, and hadn't considered that in that time the silver plating could slowly migrate through the trimmers' dielectric (at least I think that would be the failure mode).

On installation of a DC blocking capacitor in series with the trimmer or entire group of trimmers via the switch wiper, the DC potential will divide between the seriesed caps in inverse proportion to their capacitance, so the low capacitance trimmer would see almost the entire B+ in comparison to the relatively much higher capacitance blocking cap. To what practical effect? If the failure would otherwise be slow development of leakage in the trimmer which progresses to large leakage and then to short circuit, then such leakage would, in the case of a blocking cap being used, merely keep the trimmer discharged, keeping the trimmer's impressed DC voltage low.  And if instead the failure mode would be sudden discharge across weakened trimmer dielectric, the total energy would be limited to that stored in the trimmer and blocking cap, might not be so catastrophic.

And if the trimmers' failure mode would be greatly increased power dissipation at the plating/dielectric interface, resulting in localized overheating and failure of the dielectric, caused by DC conduction via deposits of mouse pee and t*rds, then failure prevention would be best served by thorough cleaning of trimmers, and subsequent deployment of mousetraps.

Any of these possibilities of my conjecture might be avoided by installation of a drain resistor to ground after the blocking cap and  before the trimmer(s). After the blocking cap charges on powerup, the DC voltage seen by the trimmer(s) would be essentially zero. A drain resistor of a few meg oughtta do it. 

Drew


Steve Hobensack wrote:

> I Just finished repairing my '63 Imperial. I had the 3/20
> mhz trimmer cap fail, shorting out B+ to ground. The fault
> also
> took out two 3900 ohm resistors within the deck. The 1/8
> amp B+ fuse operated. The repair was difficult, mainly
> because I had to 
> trace out the wire harness with an ohmmeter.  I made
> the repair by kludging in a 0.01 dc blocking cap in front of
> the bad 
> trimmer. Replacing the trimmer was not an option due to the
> size of the wires and number of wires that would have to be
> moved.
> The bad trimmer peaks up fine, however there is a bad place
> at a certain setting that will kill the oscillation. Why
> didn't
> Collins use a blocking cap? After I put the receiver back
> together, I realized I could have put the blocking cap at
> the 
> input to the band switch and blocked B+ from the entire
> trimmer network. If you have your xtal osc deck out for any
> reason, it might pay to install a blocking cap as you would
> the "killer" cap in the IF deck.
>  
> Steve N8YE    
>         
>           
>   
> 
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> End of R-390 Digest, Vol 76, Issue 14
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