[sdiy] ARP Solina/String Ensemble help?

[Nathan]

Hi Eric,

By "two highest gate ICs" do you mean the 413 and 414 (the highest octave
for 4' and 8' respectively)? 413 is only used in the Violin, and 414 in the
Viola.

Since you still get the bleed when only one of those is installed, and
presumably if one of those switches is on and the other isn't, then the big
thing they have in common are the RC trigger networks ("sustain circuits")
for each note before they go to the gate ICs. If any keys have a
bad cap/resistor in the trigger path, or a bad gate IC, then the gates on
the chip could stay partially open. Every time you play a key and the main
VCA opens, you'll hear that droning. Issues with the sustain voltage or bad
caps on other notes can also interact with that.

I'd definitely check the caps on that octave. If you check the voltage at
the signal side of the 15uF cap for each of those keys, you may find that
some are higher than others, or the droning disappears when you probe.

Best,

Nathan

On Wed, Apr 16, 2025 at 2:24 PM Eric Frampton <eric at ericframpton.com> wrote:

> Hey y’all -
>
> I’m back at my never-ending Solina project, and am down to one last (big)
> issue.
> This is a Series 33, so 1975-ish with the Gate and Trigger jacks in the
> back and the Chorus switch on the front.
>
> When I play a key, I hear not only the selected waveform from the key, but
> also some or all (hard to tell) of the other keys droning quietly in the
> background. If I play multiple keys, the droning is louder. The drones
> follow the VCA envelope.
>
> If I scope the output for the highest gate chip for the highest 10 notes,
> and play the top C and work my way downward, I get the waveform for the C
> down to D# (as you’d expect), then silence/no waveform, then starting at
> the G (G2?) that is the top of the bass/cello section, I start getting
> lower amplitude waveforms all the way to the bottom C.
>
> For troubleshooting, I removed and socketed the two highest gate IC's. If
> I remove those and do the same test, I get silence on those top 10 notes
> (as you’d expect), then no added waveforms at all going down the keyboard.
>
> The added waveforms I’m hearing seem to be from the main Gate board, and
> not the Bass board, based on pitch. They show up in all four 4’ and 8’
> sounds (viola, violin, trumpet, horn) but not in the Cello or Bass - those
> two seem to be fine.
>
> i’ve got trimmer on the Gate board adjusted for 15V. If I lower it (say,
> to the point of voices not starting to speak), the drones are still there,
> they’re just lower in volume.
>
> Here’s a link to the schematics:
> https://manuals.fdiskc.com/flat/ARP%20Solina%20Schematics.pdf
>
> any ideas?
>
> thanks,
> e
>
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