[sdiy] Bizarre Xpander Tuning Problem

[Roman Sowa]

Some other thoughts about this:

I would look for broken trace driving addres lines of 4051 in the 
voices, check at the chip pins if address is nice rectangle.

Same thing with lines driving 74174 latch controlling vaweshape in the 
voices (UX02?), maybe there is some shorted or broken line. Checking if 
all 6 lines can go to both clean logic level at various patch setting 
will tell

Check if you can get the same amplitude at output with a patch using 
only VCO1 and only VCO2. It's worth checking if VCF and PW are working 
as expected, any fault here could suggest problems with addressing 4051

Check if you get nice clean square all the time during autotune at LM311 
output which provides frequency measurement source to the micro (U108?).

Roman

W dniu 2021-08-10 o 23:59, Tom Wiltshire pisze:
> 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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