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EH 7550 - Deluxe Memory Man - 4 knobs - Red and Blue - Squelch switch - SAD1024 - Compander
Schematics for EH 7550. Note it is called "Memory Man Deluxe" on the schematics as opposed to "Deluxe Memory Man" for EH 7850.
As far as I know, this is the correct schematic for EH 1307 Memory Man boards.
Demystifying the SQUELCH
The SQUELCH switch connects pin 16 of the compander to ground through 470 kΩ. This side of the compander is the expander side which is the delay output. There is a visible cut in the delay output when engaged. I have read somewhere on the internet that this was an attempt to control runaway feedback, but I've never seen an official manual or EH explanation.
2025 update: a reader alerted me to a posting of the original manual. In the overview diagram, the control is labeled "NOISE GATE (SQUELCH)". The instructions say "The effect of the NOISE GATE is subtle. When ON it minimizes noise when no signal is present/ When OFF it permits a maximum amount of undistorted low level echoes but does not affect noise."
The earliest design features an OTA based noise gate implemented with a CA3094, so I suspect the compander revision kept "noise gate" as a feature specification.
Pin 16 allows access to the control current for the expander's gain cell. Grounding this pin will mute the expander's output while feeding it high voltage through a resistor should increase output. More info on this can be found in the NE570/571 application notes.
The EH 7811 circuit appeared to take this "squelch" idea and turn it into a "mute" function. For further study: NE570/571.
Calibration Guide
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