[R-390] Pulling crystal frequency in R390A

[Bob Camp]

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
> 
> 
> 
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