[sdiy] TR-909 undersized DC blocking caps

[Richie Burnett]

Just thought I'd put this info out there as i've not seen it mentioned before...

A couple of years ago I did some repair work on a customer's TR-909 and
discovered that the DC-blocking capacitors in the final output mixer are grossly
undersized.  The impedance of the mixing busses (used to mix the individual
sounds into left and right channels) are around 1k1.  With the existing 1uF
coupling capacitors (C153, C154) this makes a high-pass filter with cutoff
frequency around 140Hz!  Considering that the fundamental frequency for the kick
and most of the toms are well below this figure, it does a good job of stripping
most of the bottom-end punch out of the stereo mix outputs.

Replacing C153 & 154 with 10uF components restored the bottom-end to the stereo
mix outputs so they sounded the same as the individual instrument outputs.

Maybe the bass-light stereo mix behaviour is well known to seasoned TR-909
users?  ...and maybe Roland did it for some reason?  Either way, i've not heard
it mentioned before.

-Richie,

(NB. The same arguments apply to the Introspectiv-9090 as it's C4 and C5 are
also 1uF)