[R-390] Pulling crystal frequency in R390A

[Nelson Gietz]

Hi Bob,
    Thanks for the observation.  I've pretty much concluded there's nothing 
I can or
should do regarding the crystal frequency itself.
    The issue I'm now trying to correct is to get the 1 kHz circuit to peak 
on that
frequency as called for in the filter neutralizing procedure (TM-11-5820-35, 
P.119).
It seems to be too high, out of the range of adjustment of L503.  It's 
beginning to
look like C524 may have drifted low over the past 60+ years.  My math (?) 
suggests
even a 1 pf change in it could swing the LC circuit by up to 3 kHz.
    I might try to change some lead dress...
Nelson


-----Original Message----- 
From: Bob Camp
Sent: Tuesday, December 01, 2015 6:47 AM
To: Nelson Gietz
Cc: 390 list
Subject: Re: [R-390] Pulling crystal frequency in R390A

Hi

Except for the calibrator crystal, pretty much every crystal in the R390A is
in a circuit that does not intentionally tune it. Drift of things like 455 
kHz crystals
over the anticipated military life of the radio was not considered an 
important issue.
One wonderful quote on the subject “tuned crystals? Not in the spec and the 
tuning
cost money …”.

The R390 was hardly unique in this respect. A *lot* of gear that was 
commonly fielded
in the 1960’s and 1970’s shared this trait. In some cases “crystal swapping 
parties” were
the only way to get the gear to operate properly.

Bob


> On Nov 30, 2015, at 9:39 PM, Nelson Gietz <ngietz at mymts.net> wrote:
>
> Going thru the alignment process on this R390A on the bench, I find that 
> crystal Y501 for
> the narrowest bandwidths is about 80 Hz above the required 455 kHz. In the 
> neutralizing
> procedure C520 adjusts well, but I can't get L503 to tune low enough to 
> resonate the 1 kHz
> bandwidth at the same frequency. There is no visible evidence of damage or 
> tinkering in T205.
> Could the two problems be related? (The crystal resonant frequency is 
> about 455.076 kHz
> out of circuit.)
> Also, has anyone heard of or tried "pulling" that crystal frequency to 
> compensate? How
> would that work in this circuit?
> Nelson
>
>
>
> ______________________________________________________________
> R-390 mailing list
> Home: http://mailman.qth.net/mailman/listinfo/r-390
> Help: http://mailman.qth.net/mmfaq.htm
> Post: mailto:R-390 at mailman.qth.net
>
> This list hosted by: http://www.qsl.net
> Please help support this email list: http://www.qsl.net/donate.html