I copied This from http://electro-music.com/forum/topic-37316.html
 
"Three mods i'll cover in this is;

-Open-hat/Closed-hat decay's (Long/Short whistle)
-Ext. Audio Input for Hihats (Whistle)

You will need (aside from you standard electronics tools) a;

- 10k Potentiometer
- 1meg Potetiometer
- 1/8 inch normally-closed phono female socket
- 2.2k resistor (see note)
- 10k resistor (see note)
- Patience

Luckily for us hackers, there is plenty of space to mount Pots &
sockets in these machines (unlike the 606, for example), but try
and get the smallest ones possible.
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Closed-hat decay (short whistle)::

Locate on the main board Resistor "R123" (10k). This resistor sets the decay time of the Closed-hat (short whistle). You need to desolder & remove this resistor from the board. Where you removed "R123", replace with a 10k potentiometer & a 2.2k resistor in series.

*note

The 2.2k resistor in series with the 10k pot sets the "minimum" decay time for this instrument. Experiment with slightly shorter resistors to fine tune the minimum decay time (eg. 1.8K), too
small of a value though, and the instrument will not sound.
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Open-Hat decay (Long whistle)::

Locate on the main board Resistor "R126" (1 meg). This resistor sets the decay time of the Open-hat (long whistle). You need to desolder & remove this resistor from the board. Where you
removed "R126", replace with a 1 meg potentiometer & a 10k resistor in series.

*note

The 10k resistor in series with the 1 meg pot sets the "minimum" decay time for this instrument. Experiment with slightly shorter resistors to fine tune the minimum decay time (eg. 5k), too small
of a value though, and the instrument will not sound.

Also, the Open-hat/Closed hat are identical sample wise, the only differnce is the decay times of the VCA. So by turning the decay time of the Open-hat right down (to 10k or so), the Open-hat will
sound the same as the Closed-hat."
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To: tr-707@yahoogroups.com
From: roligafest@...
Date: Thu, 5 May 2011 14:33:32 +0000
Subject: [tr-707] TR-707 OPEN-HAT & CLOSED-HAT decay mod