[sdiy] SCI Pro One 15+ power rail

[Tom Wiltshire]

I copied the rest of the list in, Olav. Don't know if it made it, but I tried!

I can see two ways of approaching this. Either trace the signal from the sources out through the synth to the final output to find how far it goes, or alternatively, track back into the synth from the final output until you get to something that's making noises.
Personally, I like to go forwards, and I like nice positive results like seeing/hearing a sound from my probe. I'd start at the VCOs. Do the VCOs make sound? If so, then we know there is sound in there somewhere. Next up are the VCO summers, both U105. Is there sound coming out of the mixer? Easiest place to test is the CW end of the Mix pots.
From there we go to the filter, so put the Res to zero and the Cutoff to max, and check whether there's sound at the output of the filter. The U110 TL082 buffer might be convenient for that. After that there's only the VCA and the output driver (a 5532).

Since there's got no sound coming out, presumably Csaba isn't going to get that far. One of those tests must fail. How far he does get will tell us a lot.

HTH,
Tom


On 30 Aug 2013, at 03:32, Olav Martin Kvern <okvern at ix.netcom.com> wrote:

> 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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