[R-390] Pulling crystal frequency in R390A

[Bob Camp]

Hi

The crystal it’s self may have drifted. The same is (unfortunately) true of things
like mechanical filters. Lots of possibilities ….

Bob

> On Dec 2, 2015, at 10:53 AM, Nelson Gietz <ngietz at mymts.net> wrote:
> 
> 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
>> 
>> 
>> 
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