Hi,

I'm having problems with the autotune on an OB-Xa. The problem is that when pressing autotune, sometimes all voices tune, but equally often one or more (often not more than 3 or 4) refuse to tune in the sense that the machine flashes the corresponding voice number on the patch buttons. However, it seems as if the machine in fact does try to tune the voice; I've tried trimming one voice purposely off by one semitone, cancelling the autotune voltages using the internal slider on the underside of the front panel (the right hand one, can't remember if it's SW1 or SW2), pressing autotune, noting that the voice in question is flashing on the panel, yet when playing the machine it seems fine.

I've tried analyzing the signal produced by the voice cards when autotuning (one way is to short the base and emitter of Q8 on the lower control board which makes the autotune process audible), and also looking at the signal going to U118 on the upper control board (the shift register which is part of the autotune periodmeter), but I can't see anything odd there, in particular, it looks the same no matter whether the machine says it can tune the voices or not.

I've read success stories along the line of 'I recapped all the voice boards and now my machine works fine', but I'd really like to find the offending components rather than just do a shotgun operation.

My latest idea is that something is wrong in the D/A-converter circuitry and some of the low order bits are faulty, as when listening to the autotune signal (and slowed down by a factor of 8 or else it goes very fast) it appears that the tuning takes place in fairly large discrete steps, in fact for long periods on some oscillators there's no perceivable pitch change at all). I haven't looked more into that yet.

The other idea, related to recapping, is that the two decoupling capacitors decoupling the +/-15V lines on the voice boards are not doing their job properly so that the oscillators tend to sync to each other when close in pitch via to the supply lines (most likely the +15V one). But I can't detect this happening to any great extent when just listening to a voice and twiddling the detune knob for instance.

Has anyone else had this problem and would like to share any experiances

/Ricard