If you want to use the service menu to set the S-meter on a TS-50 (A5
for S1, A6 for S9, and A7 for full scale), what frequency is the
receiver operating on?

Curtis-

I've had my TS-50 ever since they came out and didn't know you could do that!
I did check it once, and found that the "standard" 6 dB per S-Unit was not
observed. Each S-Unit was different.

I would assume the receiver would be operating at the frequency it was adjusted
to prior to entering the service menu. Many times a service manual will
recommend making adjustments of this sort at 14.000 MHz, but I think you are
expected to set frequency with the tuning dial before starting the alignment.

73, Fred, K4DII