[sdiy] Arp Quartet Constant Sound

[Nathan Trites]

Hi Jacob,

The String and Brass settings likely use a different mix of octaves than
the other two, which is why you lose the sound when going to those modes.
The TDA clock divider chips in here have a VCA input to control the level
of each output. Find the key that you're having the issue with and check
the resistor/diode/caps going from the key's switch to see if the signal is
stuck high. There's a good change that's happened, so you'll want to check
out those components.

If it isn't high, it is important to keep in mind that when those clock
divider chips begin to fail, certain notes can become stuck open, and get
that slight volume change with the key. You'll want to make sure the PSU is
running at expected voltages too - if the power rails aren't where they're
supposed to be, you can start seeing similar issues.

Best,

Nathan

On Mon, May 13, 2024 at 9:23 PM Jacob Watters via Synth-diy <
synth-diy at synth-diy.org> wrote:

> I have an Arp Quartet that is making a constant sound of the B note near
> the middle of the keyboard.
>
> It only makes the sound when Strings or Brass are turned on, and does not
> do it for Organ or Piano.
>
> Also, the tone stops when the Percussion in the String section is turned
> up.
>
> The key contact is not stuck to the power rail - so the key is not
> physically stuck in the ON position. Also, I can hear a very slight volume
> change in the sound when I press the same B key on the keybed. I think that
> this indicates that the envelope is being triggered properly.
>
> I tried swapping the B divider chip with the one next to it, and the noise
> continued on the B. Removing the chip entirely stops the noise, but then I
> have no B notes.
>
> The service manual is not the easiest to read. I think that I should be
> looking for either a short on the board with the keybed and dividers, or
> something on the F Board where only the strings and brass are filtered.
>
> Does anyone have experience with this string machine? Can you maybe point
> me in the direction of what to check?
>
>
> Thanks for the help.
>
> Best regards,
>
> *Jacob Watters*
> JacobWatters.com <http://jacobwatters.com/>
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