[R-390] Tracking down low sensitivity 0 to 8 mhz problem
Jordan Arndt
Outposter30 at shaw.ca
Sun Mar 31 12:55:35 EDT 2024
I'm re-sending this msg because the original response seems to have drifted
off into the ether somewhere...
Hi Larry and the group...
I normally use a DVM with a 10Mohm input impedance for DC measurements and a
50Mohm VTVM for RF/AC voltages. I've used both on all types of tube gear.
Normally, I measure over 7Vdc at E209 on my other 390As, but this one won't
peak higher than 2.02Vdc at E209.
I'd use the VTVM to measure that, but the probe won't fit into the space
around E209. There is a tube in the probe head so it's a little too large to
get in there.
I'll do some more checking and testing today and see what I can find out.
Is there any concern using disk ceramics in place of C327 and C268, rather
than the original mica caps...? The ceramic caps I removed both test fine
for value and insulation resistance, and I was advised that replacing those
mica caps with ceramics was fine and a normal procedure back 25 or so years
ago.
73...Jordan VE6ZT
----- Original Message -----
From: "Jordan Arndt" <Outposter30 at shaw.ca>
To: "Collins R-390 HF Receiver List" <r-390 at mailman.qth.net>
Sent: Saturday, March 30, 2024 12:24 PM
Subject: Tracking down low sensitivity 0 to 8 mhz problem
> 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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