[Central-Electronics] 20A drift: new one just like the old one

[Jim Candela]

Hi Group,

    Today while looking through my crystal box I stumbled upon a CB 
crystal, 27.025 Mhz with holder HC-13. After swapping out some lead 
thickeners from another crystal I had a HC-17 holder.

    So in my 20A on the bench, the 9 Mhz crystal was at 8998 Khz. 
Plugging in the CB crystal gave me 9005.6 Khz. The shift didn't seem to 
upset the alignment much. So a 3rd overtone crystal operating on the 
fundamental is not at exactly 1/3 the stamped frequency.




   The drift seemed to be 100 hz down in the first 3 minutes, and then 
down at a steady rate 150 hz more after 1 hour. The drift was gone after 
that. At first I had some jumping around, but that seemed to be the 
crystal contact with the socket. After fiddling with the crystal in the 
socket, the jumping completely went away.

    Looks like older CB crystals in be used in the 20A. Channels 2-6 TX 
being good candidates with channel 4 (27.005) being the best choice. I 
assume that there will be an offset from stated frequency/3.



Jim
WD5JKO


On 9/23/2011 6:59 PM, k6jek wrote:
> I upgraded my very early 20a to the newer 6U8 oscillator circuit from the old 12AU7 circuit that was only used for three months.  Alas, the new circuit isn't much better than the old one for drift but at least it doesn't have the 50 Hz pull problem.
>
> Next I suppose I'll try some temperature compensating caps although with the old circuit, just putting a small fan on the outside left blowing in to the lower vent made a big difference, enough that my enthusiasm for fooling with caps is diminished.
>
> What was the 50 Hz pull problem?  If the CARRIER knob was turned beyond 8, the oscillator was pulled 50 Hz.  I would do just that to join a QSO -- switch to a dummy load, turn the carrier up, zero beat the QSO.  Pretty soon some honest soul would say "Jon you're 50 Hz high" (or maybe it was low). I would check by using the same procedure.  I'd be dead on.  They'd say, no 50 Hz high. How could that be?  It was making me crazy. I thought the crystal was jumping.  No the oscillator was being pulled by the carrier insertion.  No wonder they junked that circuit after three months.
>
> I didn't figure out the mechanism of the oscillator pull.  Instead I just rebuilt to the 1955 schematic.  No more pull.
>
> I am disappointed this version is no better than the last for drift.  I'm not surprised since others with newer transmitters also noticed the 9 MHz drift. Still I had hope.  At the same time that CE changed the oscillator, they put 33 pF N750 caps across the RF phase shift coils which must help the stability of the phase shift. They did not put any temperature compensation in the oscillator itself, however.
>
> Some numbers.  After 5 minute warm-up measured every 10 minutes.  Regulated 115 VAC, 75˚ F controlled room temperature.  Drift in the last ten minutes
>
> Time	Old	New
> 10		-9	-29
> 20		-46	-53
> 30		-46	-45
> 40		-38	-37
> 50		-32	-25
> 60		-26	-16
> 70		-21	-13
> 80		-17	-10
> 90		-12	-8
> 100		-10	-6
> 110		-9	-6
> 120		-5	-2	
>
>
> Cheers,
>
> Jon
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