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

[Tom Frobase]

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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