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

[hamfish at comcast.net]

Craig checking in:

Just a thought on 3MHz, I believe this is the 2-4 MHz octave within the RF racks. Have you gone though an alignment of all the RF transformers? If so, is there a slug on the 2-4MHz rack that is adjusted so low it is about to fall out of the rack? This could be a sign/clue one of the silver mica caps in that transformer has seen better days.

WD8KDG:Craig 

-----Original Message-----
From: r-390-bounces at mailman.qth.net <r-390-bounces at mailman.qth.net> On Behalf Of Jordan Arndt
Sent: Sunday, April 14, 2024 7:28 AM
To: Bob Camp <kb8tq at n1k.org>
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

Hi Bob...

Yes, I have an Ailtech 460 sig-gen, and yes, none of the bands are totally dead.

To check the sensitivity, I use the sig-gen at 50uV and again at 1uV, in AM mode with 400hz modulated at 30%. I also use the Carrier Level meter and/or the Line Level meter accordingly.
I also check to see if I can peak the internal tube noise by adjusting the Ant Trim and checking for a peak in the noise as indicated on the Line Level meter with the receiver input terminated in a 50 ohm load.

Wth a 50uV input signal, the Carrier Meter shows a drop off below 8Mhz of ~
4 to 6dB per band, with the 3 Mhz band being the outlier showing a ~ 12dB drop which I believe is due to a weak crystal which produces only 2.4Vdc at E-402. The rest of the lower band crystals all meet the -4 to -11Vdc spec as measured at E-402.

Aside from a minor tracking problem in one of the 1st variable IF transformers, the rest of the RF xfrmrs appear to be working normally and track properly. I do note some scratching when the Ant Trim is rotated quickly, but that only appears with no antenna connected. The Ant Trim peaks normally on all bands.

I hope this answers some of your questions...

73...Jordan VE6ZT

----- Original Message -----
From: "Bob Camp" <kb8tq at n1k.org>
To: "Jordan Arndt" <Outposter30 at shaw.ca>
Cc: "Collins R-390 HF Receiver List" <r-390 at mailman.qth.net>
Sent: Saturday, April 13, 2024 12:47 PM
Subject: Re: [R-390] Tracking down low sensitivity 0 to 8 mhz problem



HI

This seems to be running on for a while.

Backing up a bit:

How do you know the sensitivity is low at < 8 MHz?

I’ve been assuming that you have a signal generator and have checked each of 
the bands. I’ve also assumed we’re talking about “a bit deaf, but not dead”. 
If any of that is not correct, then shame on me :).

If this is the case, just how low are you seeing relative to (say) 9 MHz? 
(are we chasing 6 db or 26 db?)

Is it just as dead at (say) 600 KHz as at 1.5, 2.5 and 5 MHz? (is one of the 
front end sections getting into the act as well?)

The point being, some of us (ok, maybe just me ….:) )  may be making 
assumptions that aren’t correct. If we are, our advice isn’t going to be 
quite as helpful as it could be.

Bob 

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