[sdiy] Kawai/Teisco 100p

[Rutger Vlek]

Hi guys,

I'm working on the aftertouch section of a Kawai 100p (= Teisco 100p) and I run across a small transistor circuit that is undocumented in the service manual. The aftertouch system is basically a carbon rubber that generates a lower resistance path the harder it gets pressed. 

The service manual describes scanning of the aftertouch bar as follows. One side is supplied with +15V. The other side is attenuated a bit by a 2-resistor voltage divider (both 3K3) and then fed into a non-inverting opamp (2K7 input impedance). From here it's passed further into the synth.

When I opened this synth, I saw a small transistor circuit near the aftertouch bar that is not specified in the service manual. It taps off the +15V supply that is passed to the aftertouch bar and uses it to feed a BC413 NPN transistor (pin 1, which I assume to be the collector). The middle pin of the NPN transistor goes to ground via 58K. Instead of feeding the return signal from the aftertouch bar to the voltage divider (as per service manual) it is wired directly to the middle pin of the transistor. And the funniest part: the wire that feeds the voltage divider (and thus the entire synth) with aftertouch signal is connected to....+15V, rendering aftertouch useless.

I suspect this last connection straight to +15V was made because of a malfunctioning aftertouch bar (corrosion on contact material), which keeps the aftertouch signal permanently high. I already resolved the corrosion and now measure sensible resistances across the bar. However, I'm still confused about how this system is intended to work. So my questions to the list (hopefully synth tech's experienced with the 100p) are:

1) Is this transistor-resistor circuit of the original 100p, or was it added afterwards as a modification in an attempt to resolve aftertouch problems?

2) If it is original, what is the pin-out of the BC413 and how should it be wired? With only 1 transistor and 1 resistor, there are not much options, and I suspect it should have worked as an emitter follower. Am I correct in assuming this?

Regards and many thanks for any insights you can provide,

Rutger