ROM mod

As the 707 is PCM sample based, the sample sounds are stored on a ROM chip. with the HKA design ROM mod you get PCM samples of the 707, 727, 808, 909, Linn, LM1, DMX and 7x7.

Hihat Decay mod

Replace resistor R126 on pcb with 1M ohm potmeter for open hihat decay

Replace resistor R123 on pcb with 100k ohm potmeter for closed hihat decay

External audio input for open hat / closed hat

Locate capacitor c69 (1uf). it's a polarized electro capacitor, so it has a "+"" and a "-"" side. The blue stripe indicates "-". The "+"" side leads to resistor R125.

Take out this c69. lead a wire from "-" side of original capacitor position on the pcb, to a normally-closed jack socket. Continue from the other pin of the jack socket (which is a closed circuit because it's normally-closed) through the capacitor back to the original capacitor c69 "+" hole in the pcb.

Normally-closed makes sure that when nothing is plugged in, the original hihat sound is audible, because the circuit is still closed. With something plugged in, extenal audio source takes over.

Wire ground for the newly installed jack socket to any other ground point on the pcb (like ground of other jack output sockets).

Pitch mod

Either figure something out with individual 555 timer chips per voice, or buy the circuitbenders.co.uk pitch mod diy kit. This I highly recommend though soldering this one requires good equipment, steady hand and patience.