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

Bob Weigel sounddoctorin at imt.net
Tue Nov 4 21:18:17 CET 2008

Yes Z6 and Z11 we're talking about.  NAND right. I was remembering Z8 
for some reason.  No matter we have open collector outputs and the right 
signals are on the inputs.  There's nothing but low voltage noise on the 
outputs at any time.   SOME of the keys on a particular octave buss will 
produce a tiny change in voltage on the appropriate output.  But not 
enough to invoke a state change in the 74150's input.  And on 10X probe 
I still get close to 0V on all of the inputs at all times. So there's 
just zero activity there so it's the point of trouble whatever the case 
I'm sure.

Benjamín Velasco wrote:

> Bob, i also succesfully repaired my two Pro Soloist recently, after 
> very hard work. I didn't have the problems you describe though.
> Looking at the schematics, you are talking about Z6 being the 7403 
> right?(they are NAND O/C btw)
> Aren't the inputs of the 74150 negated? Maybe you don't need the 
> pullup resistor, as you need "ground" at the inputs to select data.
> cheers
> Ben
> 2008/11/4 Bob Weigel <sounddoctorin at imt.net 
> <mailto:sounddoctorin at imt.net>>
>     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...
>         Christian
>         On Mon, 03 Nov 2008 23:00:38 -0700 Bob Weigel
>         <sounddoctorin at imt.net <mailto:sounddoctorin at imt.net>>
>         wrote:
>             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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> -- 
> Benjamín Velasco Ossandón
> Cel. 77599598
> Tel. 3333791
> Of. 4139139

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