[sdiy] OBXa Memory problems

Michael E Caloroso mec.forumreader at gmail.com
Fri Mar 6 03:23:00 CET 2020

CPUs can go bad even though the synth "works".  I had a bad Z80 in a
Memorymoog where autotune wouldn't work although everything else did.
That was an evil failure.

Are you SURE the address/data lines are TRULY working?  Unless you can
record the activity with a logic analyzer, a 'scope doesn't tell you
that much.  I'd check the buffers for the address/data lines.  Look at
the R/W and enable pins too.  Check that the IC generating the enable
line(s) is not simultaneously enabling another line, that will corrupt
data out of memory.  Check the rails at the ICs not just the power
buss.  ANYTHING that touches those ICs.

You might have write-only memory.  I had a bad RAM IC on an OBSX.

Diagnosing corrupt data lines is what separates the men from the boys.
Not easy.


On 3/5/20, gm <montalbanogm at gmail.com> wrote:
> I'm currently dealing with an OBXa that was working, but is now
> exhibiting memory problems.
> The ADC is working:  all switches & pots can be used to set a patch.
> Holding down Write & selecting a patch location will let the patch play,
> but as soon as the patch is reselected or another patch is selected, all
> patch data goes back to random.
> Some givens:  all power rails are clean and correct; all address & data
> lines are present; ROMs, RAM and CPU have been swapped out with no
> change.  This unit is one of the early upgrades, with the piggyback
> memory board, running version XAC0 in 2716 ROMs.
> Since all ICs are socketed, I swapped out A117 and A115 and tested the
> memory protect switch -- switch grounds A117 pin 1 as it should when
> engaged, and releases ground when turned off.
> I have the feeling I'm missing something stupidly obvious here, and
> would appreciate any pointers as to where I can look next.
> Thanks.
> ~GMM
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