[sdiy] Arp Quartet ADSR issues

[Ben Walker]

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
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://synth-diy.org/pipermail/synth-diy/attachments/20240804/592f3cd2/attachment.htm>