The PT-80 turned out to be perfect for someone with synth-diy background to modify; The drummachine consist of analog hihats and snare, bassdrum and congas are squarewave outputs that have a pre-set amplitude decay routed trough a analog filter (?). Ouput for chords and melody consist of two audio outputs, VCAs and CV outputs to control a pair of VCAs. Each sound except melody is routed via a resistor to one accompany mixing section which in turn is controlled by the panel "Accomp. Volume" slider. The melody is mixed with the rest after this section and the whole mix is routed through the "Main Volume" slider.

The top circuitboard seems to consist of decoders for the various keys, leds, keyboard and ROM-pack interface. The lower circuitboard contains the analog parts and the powersupply.

The cable inbetween the two circuitboards is pictured below.



My measurements yields the following (from left to right):

Pin	Data	Description
1	+5.7V	Raw battery power?
2	10mV	?
3	0.8mV	GND?
4	5.0V	Regulated power?
5	5.0V	Voltage output routed via ON/OFF button
6	?	Digital noise output?
7	1.6mV	?
8	Snaredrum	0.3mS trigger
9	Hihat, cymbal	CHH trig 0.3mS, OHH trig 291mS HIGH -> equal length LOW?
10	?	?
11	?	?
12	Bassdrum, conga	Square wave with amplitude modulation
13	Chord VCA CV	Digital VCA CV envelope out for chords
14	Chord audio	Chord audio squarewave output (amplitude controlled by VCA)
15	?	?
16	?	?
17	Melody VCA CV	Digital VCA CV envelope out for melody
18	Melody audio	Melody audio squarewave output (amplitude controlled by VCA): Flute - equal duty cycle Piano - HIGH signal being 1/3 of LOW, stops ~0.5 seconds after VCA fadeout Clarinet - PWM modulated Harpschicord - ? (don't remember, must look up again) Trumpet - HIGH signal being 1/10 of LOW Organ - Weird PWM modulation Celesta - Equal duty cycle, stops ~0.5 seconds after VCA fadeout Violin - LOW signal being 1/8 of HIGH

On the analog board, each resistor to the mixing section(s) can be identified by the following markings:

Label	Description
SD	Snaredrum "punch" filter output
SDN	Snaredrum noise filter output
CL	Congas filter output?
BD	Bassdrum filter output
CYN	Cymbal/Open Hihat filter output
HH	Closed hihat filter output
CHR	Chord VCA output
MELO	Melody VCA output

My reverse-engineering of the main circuitboard yields this *very preliminary* schematic.