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

[dmartin]

Regarding an inability to get two peaks on some of my 1st and 2nd IF can trimmers, in 
some cases even after replacing the trimmers and fixed caps bridging the coils, advice 
from the list was again right on!

In almost every case I've tried so far, the "fix" was to turn the coil slug in or out 
a turn or two and attempt to re-peak the trimmer. Doing this very often yielded a "two-
peak" trimmer. I would then go back to the low end of the alignment process, i.e., 
1.200 for the 1st IF, and then re-peak all the coil slugs. Doing this would invariably 
cause me to back out the slug just a little, back slightly in the direction of where 
it was, but when next redoing the caps at the higher frequency, i.e., 7.600 for the 
1st IF, I found I >still< had two peaks on the trimmer.

Point is, there >is< a lot of interaction between the L and the C in LC circuits. Such 
a fact is Electronics 101, to be sure, but I did not grasp the significance of it and 
found I was tending to strictly align the slugs and trimmers individually, in 
isolation. Thought I'd share it with the group in case others, too, and trying to 
align their slugs at one frequency and caps at another without consideration for how 
each drives the other. Seemingly perfect peaks on the slugs may or may not yield two 
peaks on the corresponding trimmer. Often, there is a very narrow, mutual "sweet spot" 
of slug and trimmer position and I've learned to align both together.

Thanks again for all the on and off list replies!

Dan
 

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---------- Original Message -----------
From: "dmartin" <dmartin at visuallink.com>
To: <r-390 at mailman.qth.net>
Sent: Mon, 11 Sep 2006 13:25:52 -0400
Subject: Re: [R-390] 0.5-8.0 Low Sensitivity (Long)

> To Cecil's point, it >does< seem to me that if I wanted to go nuts over not 
> getting two peaks on every trimmer the only recourse might be to start 
> empirically adding a little fixed capacitance, above what is factory original. 
> The 1st IF uses 51 (I replaced with a 50 pF)and 68 pF and the can in question 
> is the one with 50 pF. CDE makes a 56 pF. I could try that, or parallel other 
> combinations. Experimenting like this wouldn't be too bad in the 2nd IF and RF,
>  where the fixed caps are all pretty much in the easily accessible cans. Bit 
> of a drag to do that in the 1st IF though, where the fixed caps are under the 
> deck. Be good to hit that one right the first time! I've had rf decks out a 
> bunch of times over the years. It's kind of like mowing your yard in August -
>  not hard, just takes awhile and you don't look forward to it!
> 
> Dan
> WB4GRA
> 
> > > Have you tried screwing in the slugs a few more turns? What happens when 
> > > you
> > > screw in the slugs a few more turns? Does the signal continue to peak or 
> > > does
> > > it start to fall off? If it starts to fall off then the single peak would
> > > appear to be near correct even though you only get one peak when the slug 
> > > is
> > > maxxed out. I wouldn't worry too much about a single peak if the 
> > > sensitivity is
> > > back to normal. 73 Todd WD4NGG
> > > _____________________________________________________________