[R-390] deaf below 8MHz (R-390A)

[atfu]

Dear All,

I am the proud owner of a recently bought R-390A (Capehart) with an
excellent history: From late 60s to early 90s in the labs of the German
Aviation and Space Agency (with regular servicing); thereafter taken
home by one of the employees. I bought the RX from the estate of that
employee, so couldn't put any questions to the former owner. Compared to
other radios I saw, this one is of an exceptionally sound and clean
appearance, in and out.

When taken to my shack it came well to life on all bands, though,
perhaps, not up to the legendary sensitivity that it is capable of in
principle. (Carrier meter was definitely reacting low: about 20 dB on
strong cal signal, even lower on strong signals from ANT.)

First steps taken: Clean all contacts and retire paper caps in AF and IF
as a preventive measure. First AF. All went well and the result was an
audible improvement. (BTW all caps I found, mostly yellow Aerovox, were
within specs on my LC-meter.)  Calibration signal and reception on all
bands. (Still low carrier meter.)

Next IF.  Replaced all but C533, C536 (both under BFO shaft), C549 (a
bit tough), and C551 (in alloy cap). Checked and re-checked and
re-checked for proper values and connections -- ok. (I did this one by
one so as to rule out connection errors.) Result was a definite
improvement on all bands above and including 8 MHz, below that -- NIL,
including no cal signal. Carrier meter appears pretty dead now, also on
the upper bands.

So troubleshooting is called for. I started with the flow chart from the
Cryotology Class and procedures according to the Y2K manual.

1. Although I didn't mess with the mechanics, I first checked the
mechanical alignment (according to Y2K). All fine. V207/Pin 6 to
J208/Pin D gave 56K: so I suppose no field change 7 was made and the
value is thus spot on for R210.

2. V202, V207, V201 warm up. I don't have equipment to check them beyond
that though.

So here is my question: How should I proceed from here? In particular,
what may impede the cal signal to be heard below 8 MHz?

My test equipment includes a working AN/URM-25(H) (hooked up to a modern
freq counter), but no tubes tester.

Hope someone may be able to help!

Vy 73! Andre, DM1AA