[sdiy] ARP pro-soloist oddity or..am I just totally missing it? 74150?
chris at chrismusic.de
Tue Nov 4 10:33:13 CET 2008
Open TTL inputs default to a somewhat high state, although it appears to
be frowned upon to use that as a feature. I always used explicit
pull-ups where needed. That was back in the days. Sigh, feeling old now...
On Mon, 03 Nov 2008 23:00:38 -0700 Bob Weigel <sounddoctorin at imt.net>
> I have one on the bench...haven't done one since mine which needed the
> potted VCF replaced so I had to build one on a board. But this one has
> a different issue. The clock is good, the dividers are good on the
> keyboard circuit board E. But then the clock signal from the f/f
> dividers is distributed to the 7403 open collector OR gate inputs. At
> first I went "Well..just a crumpled output" and I clipped the 7403 out
> and replaced it ..then looked up when I saw that the output hadn't
> changed at all and realized those are open collector I'd forgotten.
> Anyway here's the query. The outs of those gates are attached to the
> octave busses (There are 4 buss wires in the keyboard itself...3
> covering 1EA on the lower octaves and the 4th one is just for the high
> C). So they're completely open until you hit a switch. Then they
> attach to the input of the 74150 that corresponds to the NOTE you hit.
> (Eg. All the C's switch wires are tied together and attach only to an
> input of the 74150 data selector)
> So....if my question isn't obvious already...where the heck is the pull
> up? :-) I mean..I have a pro-soloist working that I could tear apart
> and puzzle over. But does someone happen to know if the 74150 has
> inputs that pull up the voltage normally? I don't see indication of
> that on the data sheet I have. But if that's not the case I'm a bit
> stumped as to what is supposed to happen there. It seems to be doing
> just what I'd expect it to do from the diagram;. nothing.
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