[sdiy] SCI Pro One 15+ power rail

[Olav Martin Kvern]

Tom Wiltshire wrote [just to me, as far as I can tell]: "Olav is right 
to question the Mode and Repeat/Ext switches. My Pro-One has some dodgy 
switch contacts, and if these switches don't close properly, it's easy 
to get the synth in an unplayable state. I occasionally have to waggle 
mine back and forth a time or two.

"I also second his analysis of "probably not the DAC". To say the DAC is 
at fault from those symptoms is a hell of a leap. It could be any of the 
VCA, the Gate circuitry, the envelopes, the output, the oscillators, etc 
etc. You need to narrow that down a lot more."

My thinking is that the external gate goes almost directly to the CEM 
3310 envelope generators (through two gates of a 4049 and across the 
Drone switch)--assuming that the VCO mix pots are turned up all of the 
way, that the filter is fully open, and that the Sustain on the VCA ADSR 
is turned all of the way up. In fact, turning on the Drone switch ought 
to produce some kind of sound--it just connects the 3310 gate pins to 
+5V. If it doesn't, you've got to suspect something very basic (ground 
fault, or a problem with the output amplifier) or something more 
seriously horrible.

Thanks,

Ole

On 8/28/2013 3:34 PM, Olav Martin Kvern wrote:
> Hi Csaba,
>
> re: "...still no sound with the keyboard played or from the CV/Gate 
> [...] Not sure what the problem is but I'm suspecting the DAC"
>
> If you don't get anything using CV/Gate from the external inputs, the 
> DAC is probably not the problem. I'm hoping that's good news. When you 
> have the middle Mode switch set to Repeat/Ext and provide a gate 
> through the Gate input, does it trigger the envelope generators? Does 
> the Gate LED in that section light up?
>
> If (at some later point) you do find that there's something wrong with 
> the DAC, the AD558 is still in production (according to www.analog.com).
>
> Great synth--mine is still working fine. I walked miles through 
> orchards in 1983 to pick up my tech manual from the SCI office. I'm 
> sure the orchards disappeared long ago.
>
> Thanks,
>
> Ole
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