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

[Jordan Arndt]

Salut Jacques...

R210 is 56K and measures well within spec.
I had removed the original C327 Mica cap years and years ago for that very 
reason. I replaced it with a 1kV 100pF ceramic disk cap. I had also replaced 
C268 with the same type of cap. I'll be replacing them with SM caps anyway 
while I'm in there.

I'll have to check the RF level at J221, what should it be in either p-p or 
RMS...?

73...Jordan VE6ZT

----- Original Message ----- 
From: "Jacques Fortin" <jacques.f at videotron.ca>
To: "'Jordan Arndt'" <Outposter30 at shaw.ca>; "'Collins R-390 HF Receiver 
List'" <r-390 at mailman.qth.net>
Sent: Saturday, March 30, 2024 2:19 PM
Subject: RE: [R-390] Tracking down low sensitivity 0 to 8 mhz problem


Hello Jordan,

Some questions:
1_ What is the value fitted for R210 ?? 56K or 220K ?? Have to be a 56K for
proper 17MHz output to V202.
2_ Did you replaced C327 ? That cap was flagged as failing in some R-390A.
Just put a new SM there, re-peak T207 and retry.
3_ What is the pp voltage measured with J221 directly routed to a 'scope
input (with T207 peaked) ??

73, Jacques, VE2JFE in Montreal

-----Message d'origine-----
De : r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> De la
part de Jordan Arndt
Envoyé : 30 mars 2024 14:24
À : Collins R-390 HF Receiver List <r-390 at mailman.qth.net>
Objet : [R-390] 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