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So the hum in question might just be 60 cycle leakage instead of the originally stated 120 cycle?? I guess that this is possible but what is the explanation for the loss of the hum when SW10 is in the Bass (boost) IN position?

I believe that the SX-28 schematic is correct after all.? With SW10 in the Bass (boost) IN condition, CH2 and C43 are shorted out and R37 & R38 make up the plate resistance.? In the Bass (boost) OUT condition, CH2 and C43 are in circuit but only R38 is used as the plate resistance.? A lot of control is accomplished by a simple SPDT switch.

It seems to me that the curves in Figure 11 of the manual are believable since the contribution at 1kc of CH2 and C43 are, as previously stated, clearly evident in the Bass (boost) OUT curve.? What the contribution of R35 for the midrange frequencies in either curve is not mentioned but clearly, the lower the resistance of R35, the lower the available midrange audio frequencies.

Perhaps part of the problem is that the SX-28 was made a decade before the high fidelity craze of the 1950s.? Today, over all tone is considered to be composed of bass, midrange and treble frequencies.? Treble frequencies for the most part are not available in the SX-28 since the high audio frequencies are limited to perhaps 3 to 4 kc.? Midrange frequencies are fixed by C40 in the phase splitter and by C42 and the R35 pot in the 1st audio triode section of the 6SC7.? Since the SX-28 is, in the end, a communications receiver, the loss of treble frequency control is not an issue.

Jim