TR-606 DIY part 1: OPEN AND SHUT CASE
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Next on the list were the hihats.  The hihats work by putting six high
frequency oscillators through a bandpass filter, then through a VCA, and
then highpass filtrating the VCA output.  The open and closed hi-hats are
generated by two envelope generators connected to the VCA.  These two
generators have different decay times - one for open and one for closed.
The most useful control I could find was adjusting the cut-off frequency
of the bandpass filter, and this is very simple to achieve.  Resistor 223
(560 R) controls the cutoff frequency (see fig 3, hh1.GIF).  Replace R223
with a 47R resistor and a 4.7K pot (see fig 4, hh2.GIF).  You will now be
able to sweep the hi-hat filter up and down to your heart's content.

One problem I did encounter was that the output tended to clip a lot, so I
overcame this by turning the gain down on the drum mixer, by changing
resistor 189 from 47K to 22K.  This resistor is next to IC14, which is next
to the headphone socket.