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

[Jim Candela]

On 9/23/2011 6:59 PM, k6jek wrote:
> Snip
> 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.
> SNIP

Jon,

    I still have my QRO 20A on the bench. I looked more at that oscillator, and mine pulls about 8-10 hz when the carrier pot is advanced fully. I did find out what triggers the shift though. When the drive is advanced, the peak RF final grid drive eventually matches the fixed bias. When we go beyond that the RF goes from class Ab1 to Ab2 operation. In class Ab2 we draw grid current, and this changes the load impedance on the driver stage. The bias supply being high impedance also goes more negative with increasing drive. I'm not sure what mechanism is getting back to the oscillator, but I am sure the trigger is grid current on the finals.

73,
Jim
WD5JKO