[sdiy] Some OB-8 Questions....

[Magnus Danielson]

From: Gavin Muir <gmuir at sympatico.ca>
Subject: [sdiy] Some OB-8 Questions....
Date: Tue, 6 Dec 2005 07:55:54 -0500
Message-ID: <E0BF28BA-1564-4EC1-89AE-1F436E235730 at sympatico.ca>

> Firstly, best of the holiday season to everybody as it quickly  
> approaches!
> 
> Doing some more "cleaning up" on my Oberheim, and I have a couple  
> questions....
> 
> I am having some problems with the auto-tune routine, which regularly  
> is failing voice 8. The problem is, voice 8 (both oscillators)  
> appears to be functioning just peachy. I was trying to trace back how  
> the signal from voice 8 finds its way back to the processor for  
> tuning, figuring maybe something is amiss there, but seem to be  
> getting myself more confused (particularly as the "oscmux" circuitry  
> from the output resides on the other voice board, and I am unclear as  
> to where the top voice board feeds into that!).... Does anyone know  
> anything about the auto-tune routine and what it is actually looking  
> for (and where)???

Not very peculiar at all!

Each voice has an output (see Sheet 1 of 3, 1682B) which feeds both the pan-
pots but also the op-amp U504. The details is a bit more involved. The upper
board does not have U504 mounted, but the lower board has. The upper board
feeds its contribution through the C5 connector pin to the lower board where it
is summed up just as the other 4 voices of the lower board. The output of the
op-amp is feed to the A12 connector pin (OSCMUX) which goes to the flat-cable
over to the main board and it there shows up as the H12 connector pin (OSCMUX
again) and goes throug U37 (see Sheet 1 of 4, 1679B) which amplify it into the
OSC signal which is then fed into the 8253 (U16) which acts like a frequency
counter. The AUTOST at U4 triggers the measurement and AUTODNE# hooks the
second counter in the U4 and also directly CPU senseble through U5.

It is a bit confusing with connectors changing denotation, but once you know
it, the magic disappears.

> Secondly, I have resurrected a nice 61-note Pratt-Reed keyboard from  
> an old Xa (much prefer the feel, and my panasonic mechanism was STONE  
> DEAD!), but I must admit to being a bit dim when it comes to cleaning  
> and maintenance of a pratt-reed mechanism.... I am getting a lot of  
> ultra-staccato multi-triggers on many notes.... Is there some  
> additional debouncing that needs to be done, or is this (pure and  
> simple) a maintenance and cleaning issue?

Clean up your act! ;O)

Yes, maintenance and cleaning is booring but necessary. With a trimmed up OB-8
you surely need it.

Cheers,
Magnus