Changes To Qs7/Qs8 Vco Circuit (All Main Pcb Revisions); Qsr - Alesis QS Series Service Manual

6.25 Changes To QS7/QS8 VCO Circuit (All Main PCB Revisions)

It was found that the Digital VCO could lock up if a slight negative voltage was present
on the +5V line prior to power up. While this sounds unusual, it happens!
Unfortunately, it wasn't our first guess. Originally it was thought that reducing the amount
of negative voltage at the control voltage input was enough. This was done by soldering a
120K bleeder resistor in parallel with C43. While this seemed to be the "fix" for a while, it
ultimately failed to fix all keyboards, and a further investigation was conducted. All units
exhibiting this fault should be checked that they not only have the smaller supply capacitor, but
this resistor has not been installed previously. If it found, it should be removed.
The drain on the +5V rail is sufficient to lower the charge across raw supply filter
capacitor very rapidly, but the -5V rail powers very few components, and consequently, the it's
raw supply filter capacitor holds a significant charge. Once the +5V supply is fully discharged
the conditions are perfect for the negative rail to "leak" to the positive rail via all of the devices
common to both rails. Eventually this charge will leak to ground too, and if power is cycled
quickly it can be significant enough to stop the VCO. The solution is to reduce the size of the
negative raw supply filter capacitor from 2200µF to 1000µF, hence reducing the amount of
charge it can retain on power down.

6.30 QSR

There are currently no changes this product.
Alesis QS Series Keyboards Service Manual V1.00 38 11/19/06