[R-390] 17 MHz AGC pulling problem

[Perry Sandeen]

Yo Bubba Dudes!
A challenging problem for solving by those with cold logic and a hot soldering iron.

One of the puzzling mysteries of the a is the 17 crystal oscillator drift caused by the application of AGC to the first mixer. This can be 300 - 450 Hz from the findings of Chuck Felton.
Collins went to the trouble of putting the crystal in an oven so why not solve this problem.  Best guess.  They knew about it BUT it wasn't in the required stringent spec's so engineering, knowing the cost of solving the problem, just kept quiet.
Now for causal AM listening Chuck noted that it probably wouldn't make any difference.  However e are interested in listening to the continuous sound of the 100 watt station in flea fart from Fiji, or something similar.
But for us true believers in getting the ultimate performance from our beloved A's this intolerable technical gaffe must be remedied, if not, we'll become the laughing stock of those heathen rice box lovers.

I started to investigating methods for solving the drift.
The  major difficulty is that the oscillator is located on the RF chassis making simple access impossible.
Also finding a 17 MHz crystal or a harmonically related prove futile except for SMD. Ugh

An outboard metal box approach using a SS method led to using a 10 MHz and a 27 MHz 1/2 crystal can oscillators feeding a Minicircit passive DBM then using a tuned 17 MHz LC circuit.  This would be fed through a very good .01uF/1KV ceramic cap into to pin 5 of the 6AK5 oscillator tube V207 after removing B+ resistor feed R211.
OPS! pin 5 goes to tuned circuit T207.  Square wave into LC circuit, not a good idea.
Feed the new 17 MHz through P221? Perhaps.
Plan two was to remove the 17 MHz crystal, make a new oscillator circuit then feed it into the grid pin 1 of V207 utilizing it as a buffer.
Would this work? I don't know.
This is just a starting point for others as I will never be able to pursue this.  Good luch to any who will even try (and post there results on the list.
Regards,
Perrier






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