[Hallicrafters] SX-101 Mk III Overload / Desense

[Andrew, Jim - BD]

Hello All,

I have been restoring an SX-101 Mark III. The good news - this is a hot receiver, looks great, and with a small exception everything works. The bad news - when I tune through a strong signal ( like the crystal calibrator output) something overloads in the receiver and the signal goes to near zero. I can recover the signal by simply tuning away (10-20 kHz) or by turning down the RF gain. The problem is noticeably worse on LSB than USB even though I recalibrated the BFO to 50 kHz and signal strengths are similar for both sidebands.

At first I suspected an AVC problem, but the event happens both with and without the AVC engaged. It sounds precisely like what happens during alignment of the 1600 and 1700 kHz 2nd Converter Oscillators so I have gone through that procedure several times, only to find I have aggravated the situation.  Adjusting the crystal activity slug while the signal is muted does not seem to help.

Have only peaked the 50.5 kHz IF by ear - wondering if that might have a role in this. I have a replacement 12AT7 2nd Converter Oscillator tube on order, but in order to properly troubleshoot I would appreciate comments anyone might have on the circuit theory, either in this stage or elsewhere, that might cause such a problem.

Thanks in advance for any thoughts or similar experiences you might have had.

Two steps forward, one back. At least the tubes keep the shack warm.

73,

Jim, K5UI