[Hallicrafters] Fw: sx101a crystals

[Rodger]

Hi Jack,

The easiest way to check the crystal frequency is to use an external 
general coverage receiver with an accurate readout that covers the range 
of the two crystals.  If you have such a receiver, set the receiver to 
exactly 5 or 10 megahertz to receive WWV, choose SSB reception, and if 
the BFO pitch is adjustable set it for exact zero beat.  This step 
calibrates your external receiver accurately.  Now without changing the 
BFO pitch setting tune the receiver to pick up either the 1600 or 1700 
Khz xtal and the carefully tune the external receiver for zero beat and 
the dial readout is your crystal frequency.  Although this is not as 
accurate as using a well calibrated frequency counter it is much easier 
and should get you within a few 100 hertz of the actual frequency which 
is plenty close for what you are doing.

If your counter is sufficiently sensitive you can pick off the signal 
from the 12AT7 oscillator but many (most?) counters are not going to be 
sensitive enough to pick up enough signal for counting without 
excessively loading the circuit you are trying to measure.  If you have 
a scope that supplies a vertical signal output (most Tektronix and HP 
scopes will provide this) you can use the vertical output to drive your 
counter and use the scope to sample the signal off of the 12AT7 oscillator.

By the way, all is not lost if one or the other oscillators is a little 
off as you can compensate during calibration.  For example, it the 1600 
KC oscillator is on frequency but the 1700 is actually at 1702 Khz. then 
the difference between the two is 102 Khz instead of 100 and half of 
this would be 51 instead of 50 so during alignment your BFO adjustment 
would be calibrated for 51 Khz. at zero beat and the low IF would now be 
aligned for 51.75 instead of 50.75.  This will retain the same passband 
characteristic and allow carriers to remain at zero beat when switching 
between upper and lower sidebands.  If one xtal is too far out then this 
will not work because you will exceed the adjustment range of the low IF 
transformers but for a small deviation it is far easier and cheaper to 
do this alignment than trying to acquire a new xtal.

73, Rodger WQ9E

Jack Schrader wrote:
> Sent: Tuesday, November 20, 2007 8:51 PM
> Subject: sx101a crystals
>
>
> Another stupid question from w1jhs.
>
> I am trying to restore my sx101a per the ER articles and one of the cautions is to make sure the two crystals are on freq, (1600 & 1700).
>
> My question is, how do I do that ? I have a freq counter.
>
> 73   Jack
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