[sdiy] Bizarre Xpander Tuning Problem

gm montalbanogm at gmail.com
Wed Aug 11 02:46:27 CEST 2021


Not a Japanese unit, but all of those ICs (and more) have been replaced, 
with no effect.
Will look at caps tomorrow -- but I'm still not sure how they could 
affect ONLY the VCO2 on each voice, while all other demultiplexed 
control voltages are good...
There has to be something in common for only those oscillators (input or 
output), but I can't find it yet.

  Thanks for the ECOs -- those will come in handy now & in the future.

~GMM

On 8/10/2021 3:32 PM, Peter Pearson wrote:
> These aren't elegant repair suggestions, but I hope you find them 
> helpful.  My general start for Xpander repairs are:
>
> If your Xpander is the Japanese version, it's probably got leaky 
> corroded electrolytic capacitors on the voice board. Swap them all out.
>
> Replace all 4051s (especially U805) and chip select ICs. Replace the 
> LM311 tune comparator (U108)  Press down on all the other socketed ICs 
> to make sure they're seated nice.
>
> Replacing the polystyrene caps in the DAC (C805 and C806) has been 
> useful in the past.
>
> A list of factory ECOs for the Xpander is here and can also prove useful:
>
> http://www.electrongate.com/obfiles/Xpander/index.html 
> <http://www.electrongate.com/obfiles/Xpander/index.html>
>
> Best of luck with it!
>
> On Tue, Aug 10, 2021 at 6:03 PM Tom Wiltshire <tom at electricdruid.net 
> <mailto:tom at electricdruid.net>> wrote:
>
>     Your email makes me realise that I don’t understand the Xpander
>     schematics as well as I thought I did.
>
>     On the page labelled “pg55” of the service manual (page 54 in my
>     pdf) there’s a pair of 4051 switches which deal with general CVs
>     for the voices. The first of which is the VCOF2 signal which
>     presumably is the frequency modulation for VCO2. If that was bad,
>     VCO2 would go out. But it seems to me to be a “per-voice” CV, so I
>     can’t see how any single fault could take out all the voices at once.
>
>     Clearly there are assumptions here about how the synth works and
>     how the firmware treats the hardware when it’s doing the tuning
>     routine, which is what I meant about not understanding this thing
>     as well as I thought I did.
>
>     Thoughts, please?
>
>     Tom
>
>     ==================
>            Electric Druid
>     Synth & Stompbox DIY
>     ==================
>
>
>
>     > On 10 Aug 2021, at 19:37, gm via Synth-diy
>     <synth-diy at synth-diy.org <mailto:synth-diy at synth-diy.org>> wrote:
>     >
>     > I have here an Xpander that will either pass or fail Tune, but
>     will always have the VCO 1s in perfect tune & scale, while all six
>     VCO 2s will be out of tune & out of scale.
>     >
>     > Did the usual replacement of all DAC components, and the ICs
>     involved in the tuning routine;  no help.
>     >
>     > Am struggling to find what all six VCO 2s have in common that
>     might be causing this;  any pointers would be greatly appreciated.
>     >
>     > ~GMM
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