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

[dmartin]

I do a pretty "classic" 1st IF alignment - peaking the slugs back-to-front at 1.2 megs 
and then peak the trimmers back-to-front at 7.6 megs, with signal input at the 
balanced antenna connector, one pin grounded. No, I've never tried to screw the slugs 
back in (or out!) to see if doing so would enable a two-peak trim cap. I'm sure the 
diode load VDC would drop off if I moved the slugs from wherever their "natural" 1.2 
meg peak is, though. However, Todd and a couple of other private replies have 
suggested this so maybe I'll play around with it tonight. 

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!

Let's see what other posts come in. It is taking well under 4 uV for -7.0 VDC diode 
load voltage on 0.5-8.0 frequencies right now, which is TM-spec for field checking 
sensitivity. That's from my not necessarily calibrated HP-606 with no effort at 
impedance matching. Point is, sensitivity seems fine, but I do wonder what else might 
be available ???

I haven't decided whether to get paranoid about going for two peaks on all the 
trimmers (could be fun!) or just move it from the bench to my radio desk and enjoy it!

Dan
WB4GRA

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---------- Original Message -----------
From: "Cecil Acuff" <chacuff at cableone.net>
To: <ToddRoberts2001 at aol.com>, <r-390 at mailman.qth.net>
Sent: Mon, 11 Sep 2006 10:12:44 -0500
Subject: Re: [R-390] 0.5-8.0 Low Sensitivity (Long)

> Well that's another tip off that there is a resonance problem in that 
> particular tuned circuit.  The slug tuning should never be at it's threaded 
> limit to achieve the peak....(it's actually not the peak because you are at 
> a physical limit prior to reaching the peak)
> 
> I've had to add and sometimes even remove fixed capacitance to get within 
> the range of the trimmer.  It's seems to me the inductance value has changed 
> from when the can was manufactured.  Maybe there is fine particles of slug 
> material worn into the coil form.  Maybe the slug has a chunk missing off 
> the bottom of it...who knows.  The factory value of the fixed mica cap is no 
> longer correct and requires custom matching to the current circuit 
> conditions.  I've had to do it several times and sometimes the effect on 
> performance is dramatic because you don't know how far off of the real peak 
> you are until you can achieve two distinct peaks with the trimmer capacitor 
> at different positions in their rotational range.  It's worth the effort....
> 
> Cecil...
> ----- Original Message ----- 
> From: <ToddRoberts2001 at aol.com>
> To: <r-390 at mailman.qth.net>
> Sent: Monday, September 11, 2006 9:59 AM
> Subject: Re: [R-390] 0.5-8.0 Low Sensitivity (Long)
> 
> >
> > 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
> >
> > 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
> > _____________________________________________________________