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

[jkharvie]

 Dave and Larry,
thank you for your thoughts and recomendations regarding capacitors that enable highly resonant tuning circuits, including those within the intermediate frequency plug-in ("cans") assemblies located on the Radio Frequency subchassis.
I received and have replaced the one capacitor (C227) unit with a new "high Q" cap, and will save the old part just in case this new part does not work out so well.
I have completed detailed inspection, and cleaning of the inside of all the "cans" on the RF deck.  Quite a bit from many years of grime, dust and grit.  All cans had green caps except for 5 cans mounted with red capacitors.  Each of the multiple variable caps have been carefully cleaned, silver contacts are now clean, and the cans appear to be ready for operation.  I wrote down which cans have the red caps to remind myself when we enter electrical alignment.

making steady steps forward
best regards

John (N3JKE)


    On Friday, July 5, 2024, 11:44:50 AM EDT, David Wise <d44617665 at hotmail.com> wrote:   

 At least in my R-390A, C227 is hermetically sealed.  Those are the longest-lived paper caps, and 99% are still fine today.  I would not replace it unless it was proven bad.Not to mention that it will never see more than a few tens of volts, and usually just a few volts, period.  If an immortal monitored a radio over the centuries, C227 would be the last to go.
Dave WiseSWL (inactive)From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Larry Haney <larry41gm2 at gmail.com>
Sent: Friday, July 5, 2024 2:37 AM
To: jkharvie <jkharvie at verizon.net>
Cc: R-390 Mailing List <r-390 at mailman.qth.net>
Subject: Re: [R-390] To swap or not to swap those aging "reddish" rectangularmica caps 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
>
>
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