[sdiy] Oberheim Xpander Res fail

[Tom Wiltshire]

Hi Csaba,

You said that it doesn't tune the resonance, but that it doesn't report which voice, right?

If it's failing on the resonance in general (rather than on a single voice) perhaps it's the comparator that turns the oscillation into a signal that the uP can measure. There's a LM311 on page 57 of the service manual (pg 56 of the pdf) that looks like chief suspect for this job. This has inputs from all the voices, and the 'OSC' output from it goes to pin 14 U916 ( a 74LS367 on page 51) on the Xpander voice board processor page. That's a 3-state buffer and the signal comes out at pin 13, which is connected to D1, one of the uP data bus lines.

Still, if the comparator were totally dead, it shouldn't tune the oscillators either, so maybe you're right looking at the CV generation. But then again, if the voices *work*, the resonance CV generation has to be ok. The place to check Res CVs is U816 (a 4051) on page 53 of the pdf. That chip or the associated R/C networks could be a problem.

Perhaps you could try looking at the DATAIN* signal on pin 15 of the '367. That controls the buffer state. You could also check the MUTE signal on pin 6 of any of the 4051's that control the panning. Since you're not hearing the tune up routine take place, that should be working and going low.

That's about all I can find that's related.

Tom




On 15 Jan 2012, at 10:54, Csaba Zvekan wrote:

> Thank you Tony,
> 
> I just checked the Xpander again took Patch 07 "Spheric2" went ahead and adjusted VCF settings to self oscillate . Played all the notes from C1 to C8 and it seems just fine. Also played one note in several registers to compare them and they sound all fantastic :). 
> So I think my problem is not in the Voice / Filter section rather it is around the DAC. Or  it can be what ever is responsible for either generating the necessary voltages for the "Res auto tune procedure DAC (which I think is fine tune as the synth sounds perfectly OK). The other variant I can think of is the microprocessor section where it's sensing the 'oscillation break point' and simply can't . 
> Like Tom explained in an earlier mail . Thank you for that as well.
> 
> What I'm trying to figure out is what parts are involved in this "Res tuning " procedure.  I believe somewhere there in that chain lays my problem.
> 
> Csaba
> 
> 
> On 15 Jan 2012, at 10:42, Oakley Sound wrote:
> 
>>> Even the percussive patches with high resonance work excellent.
>> 
>> Have you checked that all voices go into resonance in the same way? That is, set up a simple self oscillating filter patch and check that all six voices can be heard and each can be made to oscillate over a wide range.
>> 
>> I had a T8 once that refused to do the filter tuning part of the calibration because one of the VCFs didn't oscillate over the full audio range. This was due, in the end, to a bad capacitor in the feedback of the filter.
>> 
>> Tony
>> 
>> www.oakleysound.com                    Oakley Modular Synthesisers
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