[sdiy] Oberheim TVS (original) digital keyboard module design

[Michael E Caloroso]

First off, the buffered CMOS may not work.  Some of the ICs in the old
Oberheim circuits only work with the A series or unbuffered.  Kevin
Lightner (RIP) pointed that out, but I never had a TVS on my bench.

The DAC is multiplexed between the two channels.  There is a 4016 on
each channel routing the DAC signal, which is enabled by the
XOR/NOR/NAND logic following the '174 (note that each channel has
different address lines driving the logic network).  The output of the
4016 is basically a S&H.  If this is giving you trouble the first
thing I would verify is the operation of the 4016 then that
XOR/NOR/NAND network.

I have a 1981 NS CMOS databook with both the 74C174 and 40174, and the
datasheet for the 40174 says that it is equivalent to the 74C174.

MC

On 7/21/15, Travis Thatcher <recompas at gmail.com> wrote:
> Recently I acquired a TVS in pretty remarkable physical condition and
> mostly fully functional electronically. When I got it everything
> worked except the keyboard did not actually play the correct notes. It
> did properly trigger on all notes. I added sockets for all the ICs in
> the keyboard module as its a known point of failure. After a few
> mishaps with some lifted traces burning out a 4016 and some other
> screw ups I have it 99% there.
>
> The last 4 notes are the issue now as they are triggering but again
> playing the wrong pitches. I assume I borked up either the flip-flops,
> the 4512 or the 4028. I'm ordering new chips for those now. I did
> notice that if I use a different 4512 it still plays wrong pitches for
> the last 4 notes but differently wrong pitches.
>
> My question is that of the design of the actual module. I sort of
> understand the addressing and general gist of it but I'm not entirely
> sure how the dac works here. The logic for the gates seems to make
> sense but from an educational standpoint I'd love it if someone
> familiar with these circuits could help me get a better understanding.
> Also - the parts I am using are CD CMOS series, CD4512BE, CD4028BE and
> CD40174BE. I'm specifically curious if the CD40174BE is a good
> replacement for the 74C174s that were originally there. I know all the
> supplies for the chips in this circuit are +9v rather that 5v TTL
> logic i'm used to with this stuff.
>
> Schematic can be found here:
> http://www.ersatzplanet.com/Pages/pdf/ObieSchemos/2V_Digital_Keyboard.pdf
>
> Thanks for any info - I'm really excited to have this thing fully up
> and running and I'd love to have a better understanding of the
> circuitry behind it.
>
> Travis
>
> --
> http://www.travisthatcher.com
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