Klaas Robers wrote:But I have seen already that the real resistor values are different from those in the diagram. For the first amplifier the resistors from the inputs to ground are 150 ohm and the feed back resistor is 47k...
The circuit diagram is far more recent(?) than the Robot 70 schematic I have. Using 1458 dual op-amps is a give away. There is a chance that over the duration of the production run the old uA709s were replaced by the 1458s requiring a new pcb layout. Maybe using dual devices they had one spare so they just shoved it in as a second amplifier. I can't see any reason for it otherwise.
The interesting thing is the actual demodulator is the same, including component values, as the Robot 70, with the exception of the 709s/1458s. It saved all those pesky compensation components. It was a proven design to them, why re-invent the wheel?
Keeping documentation current as minor changes are introduced in a production run is a pain. Re-print manuals, keeping track of serial numbers and so on. Many manufacturers don't bother except for major updates. The discrepancies you've noted are minor and don't really change the function or performance of the device.
I do notice that the output filter is different, I'll run a quick simulation and see what they were trying to do.
A little later...well, whatever they were trying to do didn't make any difference. The output filter is -3db at 770Hz and the overshoot/ringing is still about 20%. Not exactly what I would call an improvement.
Steve A.