Connecting KC 87 to Commodore 1084S monitor
Both KC 87 and Commodore 1084S have RGB connectors with composite sync. Unfortunately, the composite sync signal from the KC 87 is much different to the standard and the monitor refuses to synchronize at all.
I built a small circuit that adjusted the KC 87 composite sync signal to the shape accepted by 1084S (and possibly other monitors too). The circuit uses LM393 comparator to detect horizontal sync edges. The reference voltage is taken from a potentiometer, so I could set it to the right values empirically. The signal from the comparator is passed to 7404 double NOT gates in order to adjust it to the TTL levels on the output (any other 74xx chip can be used too).
I tuned the circuit with the oscilloscope plugged into the input and output and found out that after removing the oscilloscope I had to add 1M resistor on the input (equal to the resistance of the probe input) or vertical sync was very hard to catch with the potentiometer.
The other finding was that the 5V power from the video output on KC 87 was not good enough. In my KC 87 it really rated around 3.5V and after connecting to the monitor, the voltage dropped even more. After switching to an external 5V power supply, everything started to work fine.
Here are the signals before (yellow) and after (purple) the circuit.
And some picture of the experiment bench…