[sdiy] Arp Quartet ADSR issues

[Jacob Watters]

I recently repaired one that had a note that was stuck on. It turned out to
be the leg of a resistor that broke off. Replacing that fixed the issue. It
was difficult to see that the leg was broken until I applied heat to the
solder joint and the leg moved around freely.

Maybe try closely inspecting, and maybe even reflowing the solder, for all
components in the suspect areas.

*Jacob Watters*
JacobWatters.com <http://jacobwatters.com/>


On Sun, Aug 4, 2024 at 3:01 PM Ben Walker <ben at audio.computer> wrote:

> I’m fixing up an Arp Quartet that I got from eBay in a pretty bad
> condition. I’ve replaced a bunch of chips that were fried, repaired a
> broken ground trace on the filter board, and now it’s making some good
> noises. But there’s still something not right, and I can’t get my head
> around how it’s supposed to work from the schematic
> <https://www.polynominal.com/siel-orchestra/ARP%20Quartet%20Service%20Manual.pdf>
> .
>
> - The brass sound works, but the attack is always short and the decay
> always (very) long
> - The piano sound works
> - The organ sound doesn’t work (no output)
> - The string sound only works when “percussion” is engaged.
>
> There’s obviously something wrong with the ADSR circuit. There are a
> couple of parts of that circuit (Board B, pp. 3-4 of the schematic) that
> are switched in and out by a 4016 quad switch, which explains why some
> sounds work (brass, strings + percussion and piano all enable the same bit
> of circuitry).
>
> The KYBD GATE signal that is sent to the keyboard bus bar sits around +5V
> for the working sounds, and close to 0V for the others. So there’s no
> voltage hitting the input pin of the TDA1008, and therefore no output.
>
> The ADSR signal just sits at +6V no matter what the setting or whether a
> key is pressed, which might explain the lack of working attack/decay
>
> Does anyone have experience with the Quartet, or is anyone able to
> interpret the schematic and take a guess at which part is not doing its job?
>
> Thanks,
>
> Ben
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