[sdiy] OBXa Memory problems

Peter Pearson electrocontinuo at gmail.com
Fri Mar 6 03:33:10 CET 2020


Yeah I usually replace the chip select ICs on those as a matter of course

On Thu, Mar 5, 2020 at 9:25 PM Michael E Caloroso <mec.forumreader at gmail.com>
wrote:

> 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.
>
> MC
>
> 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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