[R-390] Tracking down low sensitivity 0 to 8 mhz problem

[David Wise]

There's a straightforward failure mechanism in mica caps.  If the end contacts are crimped and moisture gets in, corrosion degrades the contact of one or more sheets, changing the value and causing loss.  I don't know if any models use a contact method that can't do this.  I've had both molded and dipped parts fail.  Not many, but enough that nowadays I give all of them the side-eye if something's wrong.

Dave Wise
SWL (inactive)
________________________________
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> on behalf of Tom Frobase <tfrobase at gmail.com>
Sent: Friday, April 5, 2024 2:29 PM
To: Jordan Arndt <Outposter30 at shaw.ca>
Cc: Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
Subject: Re: [R-390] Tracking down low sensitivity 0 to 8 mhz problem

I have had the 100pf mica go bad in the 17mhz drive circuit.  Interestingly
it did not check bad, but greatly limited the amplitude of the mixer drive.
… Tom, N3LLL

On Sat, Mar 30, 2024 at 1:24 PM Jordan Arndt <Outposter30 at shaw.ca> wrote:

> After a long break from working on 390As, I'm back into a 390A I bought at
> least 25 years ago.
>
> It's the most mongrel 390A I've ever seen, with a modules and assemblies
> from S-W, Teledyne, Capehart in an early single fuse chassis of unknown
> mfgr. The front panel is a re-paint and re-letter and the ID tag is a
> generic after-market type.
>
> The RF deck front plate and gear assembly is Teledyne with the deck
> chassis
> being a S-W and with a Capehart xtal oscillator module refurbed at
> Tobyhanna(sp?).
>
> I've gotten a good deal of the required rejuvenation work done, but I
> found
> an issue where the 1st xtal osc. voltage is rather low when measured as
> rectified at E-209.
> I cannot get more than 2v DC measured at E-209, while T207 peaks smoothly.
>
> What I've done so far regarding this problem:
>
> Replaced the 17Mc xtal, and then the entire oven ass'y including xtals
> from
> a working radio to no avail.
>
> I then swapped out the 6AK5 osc tube with several good 6AK5/5654 tubes,
> then
> did the same with the 6C4 1st mixer tube. Again, all to no effect.
>
> Since it has been 25+ years since I originally worked on this radio, I
> suspected component problems in the RF deck, so I pulled the deck.
>
> I pulled C275 and C268 as the originals had been replaced with disk
> ceramics
> (what I had on-hand at the time) but both test just fine at 99 and 105pF
> respectively. I'll replace them with S.M. caps.
>
> I then checked every resistor in the RF deck and aside from R203, which
> had
> drifted up to 90K from 82K, they all are within spec and show no signs of
> overheating.
>
> Aside from checking feedback caps C324 and C325 to see if one or both have
> issues, is there anything else I should look at...?
>
> Any suggestions or advice would be much appreciated...!
>
> 73...Jordan VE6ZT
>
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