Let me also add the following... In order for the system to function,
one would need the 2321E loop antenna, the special (balanced)loop
antenna cable set, and any of the 551-series servo amplifier/indicators.
Without these items, one simply has a LF/MF receiver. For listening,
simply connect A+, ground, headphones, and a wire antenna to the sense
antenna input. Input voltage (12v or 28v) is rear-panel selected. No
internal changes are required.