Vintage Synth Repair

On Saturday 19 April 2008 16:47, Philip wrote:
> Good info Roy...

Thanks.

> Brendan, the circuit looks quite simple compared to a lot of things i've
> seen so you should be ok! I actually have a thing called a Fluke
> 9010 microprocessor trouble shooter..see:
> http://artfromny.com/fluke%209010.htm it's great tool for helping with this
> sort of problem..takes like 2 seconds to test the data bus..ram etc..
>
> Ignore the prices this guy charges, you can get hold
> of these systems for very little money now..

Looks nifty,  but it'd have to be *way* cheaper than what he's talking about 
there for me to even think about it.  :-)

And then there's the time spent hooking it up and such.  Sometimes a scope is 
not all that much slower...

> --- "Roy J. Tellason" <rtellason@...> wrote:
> > On Saturday 19 April 2008 16:00, eggwheatis wrote:
> > > If it were me and I know nothing about this synth
> >
> > by the way...I'd
> >
> > > start by looking at the PSU with a scope....
> >
> > Yup,  I looked through the diagrams pointed at, and
> > the behavior described can
> > only be a matter of the last couple of them in
> > there...
> >
> > First thing I'd do is to check the power supplies.
> > The +5 needs to be within
> > a quarter volt of +5 or things won't quite work
> > right.  A scope may show
> > *some* hash on the power supply pins,  depending on
> > how good the actual
> > design is,  and some of that may be normal.  Not if
> > there's a lot though.
> >
> > > then assuming this is perfect I'd be looking
> >
> > around the CPU to see what
> >
> > > activity I have going on again with a scope or
> >
> > logic probe referring back to
> >
> > > the schematics, reset lines..clock, ports etc etc.
> >
> > I'd then check the
> >
> > > circuit around the rom and ram etc..chip selects
> >
> > etc etc write enable
> >
> > > blah blah..theres so much it could be..you only
> >
> > need one dead node in
> >
> > > the logic circuit and the whole thing won't work.
> >
> > Yup. A scope is *much* better for this sort of thing
> > than a logic probe.  I'd
> > start out by scoping the data bus pins,  DO-D7 on
> > the Z80.  Look for valid
> > logic levels and transitions between them,  but not
> > much time spent in the
> > middle (though I have seen designs like the c64
> > where stuff regularly did
> > spend short bits of time in the middle.
> > Nanoseconds...
> >
> > If there's a bad chip on the bus somewhere and it's
> > messing things up to give
> > the symptoms described this will be one way to find
> > it.
> >
> > Next is scope the address bus pins.  Depending on
> > where the software is taking
> > things there may or may not be activity on all of
> > them.  A logic probe with
> > the ability to "catch" very short pulses might be
> > handy here as well.
> >
> > Here's a trick you can try with that Z80.
> > Temporarily solder a bit of wire
> > across all of the data bus pins and to a ground
> > point somewhere.  This is a
> > whole lot easier if there's a bus buffer in the
> > system,  but the diagram
> > doesn't show one.  What this does is it feeds the
> > chip NOP instructions,  and
> > it just keeps on cycling through _all_ of the
> > addresses and keeps
> > on "fetching" them.   You should then see a square
> > wave on each address bus
> > pin,  with each time you go to the next higher bit
> > the frequency is going to
> > be half of what it was.  If you don't see this
> > somewhere then there's a
> > problem,  as something ks making one of those lines
> > get "stuck".
> >
> > I don't know these instruments and don't know what's
> > likely to be socketed or
> > not.  From what I can see you should be able to
> > remove the 68B50 and maybe
> > the 8253 as well,  as the former is just for MIDI
> > and I'm not sure what
> > they're doing with the latter.  You should also be
> > able to remove one or both
> > RAM chips and see if there's any change in behavior
> > -- if there is then you
> > have a problem pinpointed.
> >
> > If you don't find anything in particular this way,
> > start looking at the
> > outputs of the decoders,  the memory address decoder
> > especially -- a "stuck"
> > output on any of the decoding circuitry can cause
> > some bus contention,  check
> > the outputs of the 74LS138s and also the outputs of
> > the gates that connect to
> > those outputs,  and figure out what they _should_
> > be.  Like that ROM select
> > line shouldn't be low _all_ the time unless the CPU
> > never gets there.
> > The "feed it NOPs" trick above will help you get
> > there.
> >
> > > I'd say the rom is ok if you still have a problem
> >
> > with the test rom..
> >
> > Maybe.  Or maybe there's a fault which has done
> > something to one of the bus
> > lines and has ended up trashing them both.  One
> > hopes not.  :-)
> >
> > > I don't see any benefit in bypassing the digital
> >
> > side even if it were
> >
> > > possible..what with the garbage on the screen
> >
> > there is quite clearly a
> >
> > > logic problem.
> >
> > Yes,  and in an instrument like this I don't see how
> > it's possible,  or at
> > least not practical.
> >
> > --
> > Member of the toughest, meanest, deadliest, most
> > unrelenting -- and
> > ablest -- form of life in this section of space,  a
> > critter that can
> > be killed but can't be tamed.  --Robert A. Heinlein,
> > "The Puppet Masters"
> > -
> > Information is more dangerous than cannon to a
> > society ruled by lies. --James
> > M Dakin
>
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-- 
Member of the toughest, meanest, deadliest, most unrelenting -- and
ablest -- form of life in this section of space,  a critter that can
be killed but can't be tamed.  --Robert A. Heinlein, "The Puppet Masters"
-
Information is more dangerous than cannon to a society ruled by lies. --James 
M Dakin