[R-390] 0.5-8.0 Low Sensitivity (Long)

[dmartin]

Some of you may recall my post of a few weeks back, from which I got a number of 
helpful replies. The good news is that replacing the three under-deck caps across the 
1st IF coils (C318, C329, and C334) and replacing all three trimmers seems to have 
solved the issue. I get a good and unvarying alignment of the 1st IF now, that holds 
over time, and I've experienced no 0.5 to 8 meg loss in sensitivity over time. The 
puzzling thing is why one of the three trimmer caps, the one for Z-213-1, still has 
only one peak and it is at max capacitance? By replacing all three under-deck mica 
caps and all three trimmers with NOS 5-25 pF I would have thought to have double peaks 
on all three trimmers. Additionally, I have one single peak at max capacitance trimmer 
in my 2nd IF, Z216-2. I replaced C292-2 with a 100 pF silver mica and also put in a 
NOS trimmer but still get only one peak, at maximum capacitance. 

In both cases, this suggests to me that the factory OEM values of the caps I replaced 
are no longer quite enough since the trimmers cannot quite trim up enough?

All else works fine now and I'm tempted to just leave well enough alone. However, I'm 
puzzled why fresh trimmers and new micas at spec values cannot get two peaks during 
alignment.

Got any ideas?

Dan
WB4GRA


---------- Original Message -----------
From: "dmartin" <dmartin at visuallink.com>
To: "r-390 at mailman.qth.net" <r-390 at mailman.qth.net>
Sent: Wed, 16 Aug 2006 06:28:04 -0400
Subject: [R-390] 0.5-8.0 Low Sensitivity (Long)

> Have a 390A here with the low sensitivity problem below 8 megs. It isn't the 
> first 390A I've dealt with having this problem. Problem on this one is 
> characterized by a 1st variable IF alignment that won't "hold", sensitivity 
> that is good at first but can go numb after several hours of operation, and 
> generally odd alignment results. Threads for slugs on Z213-2 and -3 cans may 
> be all the way out during one alignment effort and then have to be screwed all 
> the way in flush with the rack during a subsequent alignment hours later. With 
> my IF gain set for -7.0 VDC with 150 uv of 455 kHz into J- 513, "good" 
> sensitivity on lower bands is defined as a carrier level of 40-60 db with the 
> calibrator signal and -7.0 VDC at the diode load requiring but 1-2 uv out of 
> my HP sig gen. Hours later this may be only ~20 db for the calibrator signal 
> and 30-40 uv for -7.0 VDC! When the rig drops to the "low" sensitivity values, 
> realigning the 1st IF, with vastly different final placement of slug positions 
> and accompanying changes in the trimmer caps usually brings it back.