A rainy day here and thought I'd dig into the SX-117 again and investigate the AVC pumping situation that's been discussed in the past.? For those not familiar with the problem, the symptom is that AVC "pumping" and associated desensitization occurs with strong nearby signals outside the receiver passband.? I have a late model 117 corresponding to the "D" level schematic.? I've moved the tap for the AVC amplifier to much later in the IF chain, to pin 3 of T8.? That's the source of the IF signal for the product detector and my thinking was that the AVC amp shouldn't be seeing any signal that I'm not hearing.? Turns out that isn't true.
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Testing was done using the calibrator signal.? In SSB mode with the selectivity in the 5 kHz position, I don't see the problem.? Same is true in the .5 kHz position.? But in the 2.5 KHz position it is there in spades.? When I tune across the calibrator signal, I see the expected rise and fall of the S-meter and when I'm tuned about 3 kHz above the carrier frequency, the S-meter reads about nothing as it should.? But, as I continue to tune below the calibrator frequency (in LSB mode), as I approach 4 kHz above the calibrator signal, the S-meter rises again and reaches a peak very slightly greater than the peak seen when tuned to the signal.? When this occurs, nothing is heard from the speaker or phones of course, but the S9+ signal desensitizes the receiver.??
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It appears that the unwanted signal is reaching T8 via some other path than the desired signal.? Any ideas on what could be happening?? With the AVC source tap where I have it, the notch control works with the AVC and if I turn the notch control all the way counterclockwise, the S-meter reading of the phantom signal is reduced slightly.? Does your 117 exhibit this same behavior?
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73, Floyd - K8AC
Floyd - K8AC