I recently purchased an REA modulation monitor. It works great! I wanted to connect it to my 75A-4... in order to monitor the modulation of reasonably strong received signals. I already have a precision AM detector (i.e., precision rectifier + low pass filter) connected to the 455 kHz output amplifier stage (using an emitter follower) of my 75A-4, and it was easy to increase the level of this dc-coupled AM detector to a high enough value to drive the REA modulation monitor.
However, the AVC circuit in the 75A-4 (a.k.a. AGC circuit) is a peak detector. This makes sense for SSB and CW, but that type of AVC circuit tracks the modulation on an AM carrier. One obtains a better detected AM signal for both listening and for driving the modulation monitor by using an average (carrier) level AVC detector (i.e., a precision rectifier followed by a low pass filter with a high frequency cutoff of less than 25 Hz).
Ref:
http://mysite.verizon.net/sdp2/id17.htmlI modified my existing precision AM detector by adding an extra stage that inverts the detector output (the 75A-4 uses an AVC control signal of approximately minus 5 volts), and also performs a 0.7Hz low pass filtering (0.22 second time RC constant) function. I inserted this new AVc control signal into the "AVC Test Point" jack on the chassis of the 75A-4.
It works like a charm!
With the 75A-4's AVC switch set to "off", the AVC is controlled by the signal I inserted. The output of my precision detector now tracks the average (carrier) level of the received AM signal... and not the modulated envelope of the AM signal.
To use the 75A-4 on SSB or CW... disconnect the new AVC control signal, and turn the 75A-4's AVC switch from "off" to either "fast" or "slow"
Best regards
Stu