[sdiy] Bizarre Xpander Tuning Problem

gm montalbanogm at gmail.com
Wed Aug 11 02:43:48 CEST 2021


Correct, there are two 4051s for the demultiplexed control voltages for 
each voice (similar to the setup on the Chroma voice cards).  The only 
thing I can see that they have in common is the S/H signals -- 
everything else is separated. The S/H signals look good;  all related 
ICs have been replaced, with no effect.

If the outputs of the 3372 VCF/VCA chips were tied together (as they are 
on the Prophet 600), I could see how one voice could affect the others. 
But in this case, the outputs are separated, so that's no help.

Will keep digging.

~G

On 8/10/2021 2:59 PM, Tom Wiltshire 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> 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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