[sdiy] ARP pro-soloist oddity or..am I just totally missing it? 74150?

ChristianH 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.
> -Bob

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