[sdiy] Prophet 5 question

[Tom Wiltshire]

On 1 Apr 2009, at 23:44, Karl Ekdahl wrote:

>
> Hi list
>
> I found the culprit to be a single last tantalum i had not seen  
> earlier, of course it was the one that was dead.
>
> Right now the synth makes sound, but it's weird; the keys are  
> working very on/off, different keys will not work at different time  
> just to start working all of the sudden. I've checked the mechanics  
> and they all measure fine, i've also reseated the keyboard ribbon  
> cable several times. I'm starting to wonder if there's something up  
> with the keyboard scanning circuitry.
>
> Another problem i have is that oscillator A is very faint on all  
> voices, i can hear it if i turn down oscillator B all the way - and  
> it's the same for all voices. I'm guessing there's some mixing  
> circuitry common for all voices that i need to look at.
>
> Karl

Karl,

There's no mixing circuitry that would account for all the oscillator  
A's being quieter. Osc A and B are mixed in each voice before being  
fed to the filter. Consequently, I'd look at the CV circuitry for the  
mixer CVs. The two oscillator signals are mixed by a CA3280 wired up  
as two VCAs. The "oscillator amount CV"s for both oscillators are  
common for all voices, so this would explain the symptoms you're seeing.

Regards,
Tom