[R-390] To swap or not to swap those aging "reddish" rectangularmica caps

[Larry Haney]

Hi John, One of the known problems with some silver micas is 'silver
migration' that can cause changing values and shorts.  One of the circuit
conditions that exacerbates this problem is when high voltage is applied to
one, such as C249 - C254, C281 and  C289.

It seems like I have replaced 1 or 2 of these in most of the RF decks I've
worked on.

Regards, Larry

On Thu, Jul 4, 2024 at 2:12 PM jkharvie via R-390 <r-390 at mailman.qth.net>
wrote:

>  Jordan,Thank you for these excellent suggestions. I now have these on my
> check list!
> I will let you know how it goes.  Looking forward to checking the check
> list off as I make progress.
>
> cheers
>
> John N3JKE
>
>     On Thursday, July 4, 2024, 3:45:49 PM EDT, Jordan Arndt <
> outposter30 at shaw.ca> wrote:
>
>  From experience, before you start on the transformer cans, you might be
> wise
> to replace the 51pF, 68pF and 51pF caps across the 1st IF cans. C318, C329
> and C334.
> One other cap you might look at replacing is C227, a 100V .047 canned
> paper
> capacitor. It sits between the 6DC6 and the 6C4. You can cut the lead at
> the
> base of the cap, and then fish it through to where you can mount a film
> cap
> to replace it.
>
> I once managed to connect a C227 replacement cap to the tube base pin
> successfully, but I wouldn't go through that process ever again.
>
> It's not necessary to replace all the caps in the topside cans, but what
> you
> can and should do is inspect them internally, make sure the variable caps
> are not frozen (they can be repaired), clean the pins and spray some
> contact
> cleaner on the pivot point of the variable while turning it.
>
> The real test of the components inside the RF cans will be in alignment
> and
> testing, where you might see poor tracking, cores that won't tune or tune
> only at the extremes of insertion depth, etc...
>
> I hope this info helps...Good luck..!
>
> 73...Jordan VE6ZT
>
>