SX-117 AVC pumping
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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 |
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Finally resolved the AVC pumping problem.? As I mentioned, AVC pumping by off-frequency signals occurred only with the selectivity switch in the 2.5 position.? So, looked for what's different with the selectivity switch in that position.? That switch, S4, has four sections that switch resistors into the ground leads of T6 and T8 (50 KHz IF transformers) and switch capacitors between the coupling networks of T5/T6 and T7/T8 and ground.? Those actions change the bandwidth of the 50 KHz IF.? I checked the value of the resistors and capacitors and none of them were close to the values shown on the schematic.? After replacing the 220 ohm resistors and the? .01 uF capacitors, I realigned the IF at 50.75 kHz.? The result is that the AVC pumping has been eliminated - strong signals outside the passband no longer desensitize the receiver.? ?
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73, Floyd - K8AC |
¿ªÔÆÌåÓýFloyd, ? I¡¯m curious as to the type of capacitor that was replaced.? Hallicrafters often used the Cornell Dubilier ¡°Tiny Chief¡± type in many of their products. ?These have a pink-0colored appearance with yellow stenciling.? They have a high failure rate today. ? Paul ? From: [email protected] <[email protected]> On Behalf Of Floyd - K8AC
Sent: Friday, August 9, 2024 9:23 AM To: [email protected] Subject: Re: [HallicraftersRadios] SX-117 AVC pumping ? Finally resolved the AVC pumping problem.? As I mentioned, AVC pumping by off-frequency signals occurred only with the selectivity switch in the 2.5 position.? So, looked for what's different with the selectivity switch in that position.? That switch, S4, has four sections that switch resistors into the ground leads of T6 and T8 (50 KHz IF transformers) and switch capacitors between the coupling networks of T5/T6 and T7/T8 and ground.? Those actions change the bandwidth of the 50 KHz IF.? I checked the value of the resistors and capacitors and none of them were close to the values shown on the schematic.? After replacing the 220 ohm resistors and the? .01 uF capacitors, I realigned the IF at 50.75 kHz.? The result is that the AVC pumping has been eliminated - strong signals outside the passband no longer desensitize the receiver.? ? ? 73, Floyd - K8AC |
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?Sorry, can't provide any photos.? I don't know if the values shifted over time, or if they weren't right from the beginning.? The point is, when the values of the four parts match the schematic, the pumping of the AVC by signals out of the passband doesn't exist.? I'll leave it to others to explain exactly why that problem occurs when the components are not up to spec.? The SX-117 is now a completely different receiver.
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73, K8AC |
Floyd - K8AC
Paul Christensen