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

Bob Weigel sounddoctorin at imt.net
Tue Nov 4 07:00:38 CET 2008

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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