[sdiy] roland 909 BIG problem

[Colin Fraser]

> -----Original Message-----
> From: owner-synth-diy at dropmix.xs4all.nl
> [mailto:owner-synth-diy at dropmix.xs4all.nl]On Behalf Of ultrahype
> Sent: 09 September 2001 06:04
> To: diy
> Subject: [sdiy] roland 909 BIG problem
>
> im using MY 909 and all of a sudden the KICK just starts oscilating
>
> it wont stop but the tone goes up and down as it plays or
> when you hit the button for the kick
>
> high and low , stop the machine , and the tone just keeps going
>
> tune it off and on the tone stops, but as soon as you hit the
> kick it starts and just keeps on going till you turn it off
>
> any ideas
>
> NO 909 kick ?? a fate worst than death...please help

Sounds like the decay pot (or circuit board connection to it) is broken.
The envelope shape of the bass drum is created by the trigger pulse charging
a capacitor, which then discharges through the decay pot and one other
resistor.
The vca level of the bass drum is controlled by the voltage on this
capacitor.
If the decay pot is open circuit, the capacitor will never discharge and you
likely get the symptoms you describe.
If you take the back off the 909 and locate the pins of the decay pot (there
are 3 pins for each pot just below two larger mounting pins), put a
multimeter between the centre pin of the pot, and the earth pads that the
mounting pins are soldered to. The resistance between the centre pin and
earth should be between 47k and 1047k (1M + 47k) as you turn the pot from
one end to the other.
The resistance from the cntre pin to either outside pin should vary from 0
to 1M.

Colin f