[sdiy] Why is TB-303 IC6 connected to both digital and analog voltage rails?

cheater00 . cheater00 at gmail.com
Sat Dec 14 17:00:53 CET 2013


Hi guys,

With decoupling the LED driver having been such a great success, I
decided to try and see if I can decouple the digital side of the
TB-303 for reduced noise. Tracing out the voltage rails on the layout
image, I've finally found a use for ALL the colours of my gel pens...
Jokes aside, IC6 is a huge riddle to me. You can find above the left
end of the huge horizontal IC (the CPU). Its upper right corner is
Vdd.

http://machines.hyperreal.org/manufacturers/Roland/TB-303/schematics/roland.TB-303.schem-6-left-150dpi.jpg

This image is a bit too bland to see the traces, so let me describe
it. It goes up and right towards D7, which is powered from the same
place D2 is powered from (not shown on the layout since it's the other
layer that only has like 2 traces). That seems to be the digital
supply. Then, the trace powering IC6 keeps going, under C9, and
towards D9 (right under the TUNNING knob), where it connects to the
analog supply. Why was it done like this?

Note that the left of D7 is mistakenly marked as battery backup. I
checked, it's not. It's connected to the 6V provided off the PSU. It
is 6V and is derived from the right side of D2 if you look at the
layout (it's under IC1, above the RAM chips). That goes via a trace
not shown on the layout, under IC6, and to the left side of D7. The 6V
is unregulated, the 5.333V analog rail is regulated.

You can see this weird arrangement in the schematic here in the upper
left corner:
http://machines.hyperreal.org/manufacturers/Roland/TB-303/schematics/roland.TB-303.schem-4.gif

I am told that one of the schmitt triggers in IC6 is is a power
supervisor - it keeps the CPU in reset until the digital power supply
is up. Does that sound correct?

The 5.333V supply also powers IC9 (hex type D flip-flop, VCO CV DAC
latch), IC11 (dual op-amp, VCO), and IC13 (dual type D flip-flop,
envelope).

I'm thoroughly confused. Does anyone have any tips for me on what is
going on in here?

Cheers,
D.



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