[sdiy] Re: bit one/DAC fault tip

[Rude 66]

stephen,

 thanks for the tip. all my cleaning didn't help, so next suspects are the
4051 or the power supply.

how does one measure this problem? oscilloscope?

thanks!

ruud


----- Original Message -----
From: "Stephen Lenham" <lenham at clara.co.uk>
To: <synth-diy at dropmix.xs4all.nl>
Sent: Monday, February 09, 2004 5:06 PM
Subject: [sdiy] Re: bit one/DAC fault tip


> Hi Ruud/list,
>
> I can suggest another possibility. I had a very similar fault on a
> Sequential Sixtrak but it could theoretically affect any 80's
> micro/DAC/4051s-based synth.
>
> When I got the Sixtrak it played, but all the notes were "wrong" -
virtually
> random, but repeatable. The sounds were iffy and when trying to set
> parameters their value would jump, sometimes by a substantial amount.
>
> To condense a LOT of head-scratching into a few words, the DAC was being
> driven correctly but had an internal "stuck bit" (in my case, bit 10 of
the
> 12-bit device was permanently high).
>
> This seems consistent with what you have observed. Pretty much
*everything*
> in that sort of synth rests on the DAC - pitch, software envelopes (don't
> know if the Bit has those) and - less obviously - the reading of panel
pots
> (DAC value is compared with pot voltage to create a crude ADC). So if
thing
> seem to be basically working but with the "wrong values" everywhere, it
> might be worth checking the DAC (and its input latches, etc.).
>
> Your sysex patch dumps may well be arriving just fine, but if they can't
be
> properly converted into analogue control voltages...
>
> With this fault the severity of the symptoms could vary substantially
> depending on which part of the DAC is dead - I was lucky in that it was a
> high order bit so the effects were very pronounced. If you have software
> envelopes, program a long release and listen - any jumps in the volume as
it
> decays would confirm this problem.
>
> Of course, it may be something completely different ;-) But perhaps this
> will be useful to somebody. Either way, best of luck fixing it!
>
> Steve L.
>
>
> Rude 66 writes:
>
> >
> > ok, here's an update on the bit:
> >
> > cleaned all connectors. didn't make a difference. re-soldered the output
> > board. didn't make a difference. after playing around a bit more, i can
> > describe the problem a little better: something is modulating
everything.
> > the oscillators sound almost like they're frewuency modulated: on some
notes
> > you have a pure tone, on others you have what my mono-poly in fm-mode
sounds
> > like.
> > and the 'sound staying on after the eg has closed' is more apparent when
you
> > open the filter. when full open there's only a slight drop in volume if
the
> > key is released with the release at zero. the more you close the filter,
the
> > more it dies out. also if you first select a patch, you don't hear the
> > sound. it's only after it has been triggered that it won't stop anymore.
> > there are some things left to try: i haven't taken the chips out of
their
> > sockets and clean them.. (and there are 6 CEM filter chips..) and i
haven't
> > re-soldered the 'analogue board' yet. though maybe it could be a digital
> > problem.. what i find strange is that the sounds i send it via sysex or
tape
> > save are corrupted right away.
> >
> > ruud
> >
> >
>
>