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

Bob Weigel sounddoctorin at imt.net
Tue Nov 4 11:36:17 CET 2008

There's a note on my crumar DS-1 page about a problem with their 
design.. on at least some of those units they failed to pull up or down 
...can't recall...the input of a gate that was involved in switching the 
waveform.  On mine at least the voltage would gradually wonder after you 
made your selection and in some seconds it would start rapidly toggling 
between waveforms :-). 

Anyway these dont' seem to be pulling up at all so maybe it's just a bad 
74150. I'll try installing some pullups to see what happens..  I thought 
possibly something was left off the schematic or something since I 
haven't gone to the work to pull the board up to see where all the 
traces go (requires pulling all the trigger wires out of the plastic 
guides...), and thought someone might have seen the problem on this 
particular unit.  Thanks for the thoughts, -Bob

ChristianH wrote:

>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.
>Synth-diy mailing list
>Synth-diy at dropmix.xs4all.nl

More information about the Synth-diy mailing list