The ungrounded Terminal 2 may also apply to the commercial version AR-88F receiver. The SC-88 is one of the last of the AR-88 versions produced with most receivers built between 1949 and 1950. The SC-88 receiver has the later feature of the Crystal Phasing control on the front panel with smaller knobs for the RF and AF Gain. Replacement of the Mixer tube with a 6BA7 nine-pin tube (it was the Mixer tube used in the Collins 75A-4 so it had to be good.) Additionally, all of the remaining 6SG7 tubes were replaced with 6BA6 tubes. Loosen the bearing lock nut and back the screw out slightly. Since the RF Gain controls the bias voltage, if we remove the ground connection at the potentiometer, the RF Gain bias will go to maximum negative voltage thus simulating turning the RF Gain all the way down.
This A-B/IRC resistance drift problem is found in every type of WWII electronic gear that used A-B/IRC resistors, not just the RCA gear. From the AR-60 analysis it seems likely that RCA was continuing to imbed information into their serial numbers with the AR-88 series. There are many other colors that show up from time to time, especially cream and very light gray both with black nomenclature, but without a physical inspection it is impossible to tell if those colors are original from the factory. There are some British Military manuals available that give excellent information and also a slightly different approach to alignment. With foaming plastic cleaner and a cotton cloth that is «just damp» no damage to the dial lettering will occur. The Diversity IF Gain control allowed the operator to «balance» the three receivers in the rack using a test signal (usually the incoming signal.) Once adjusted, each of the three receivers responding with the same gain to the same signal which further enhanced the diversity effect.
Later production CR-91A receivers dropped the «C» prefix. You will then be viewing the sweep signal passing through the entire IF section of the receiver. The SX-28 has double-preselection above 3mc and six positions of selectivity but it’s ham band calibrated bandspread is unique to all of the receivers mentioned here. Combining the multiple detector outputs was simple since each was basically a diode and their outputs could be tied together.