05-08-2016, 06:25 PM
(This post was last modified: 05-08-2016, 06:28 PM by ppppenguin.)
I don't have a layout diagram but here's the relevant fragment of the schematic. It's from hardware revision 3.0.1 but this part of the design is essentially the same in all revisions. R49 is the 2K2 resistor feeding pin 4 of the sound modulator chip. For completeness R33 should also be changed from 220R to 2K2. This feeds pin 4 of the vision modulator.
For some unknown reason the problem only affects the sound modulator in practice. The exact cause of the problem isn't fully understood. In normal applications pin 4 of the modulator is connected to a crystal but it is feasible, as in the Aurora, to drive it from an external clock. It seems that if you drive the pin too close to the 0V rail the chip can object, causing buzz on sound. Raising the resistor value, in conjunction with stray capacitance, reduces the drive to the chip.
PS: R33 and R49 may be 470R rather than 220R in some early units. In any case it needs to be raised to 2K2 if there's a sound problem.
For some unknown reason the problem only affects the sound modulator in practice. The exact cause of the problem isn't fully understood. In normal applications pin 4 of the modulator is connected to a crystal but it is feasible, as in the Aurora, to drive it from an external clock. It seems that if you drive the pin too close to the 0V rail the chip can object, causing buzz on sound. Raising the resistor value, in conjunction with stray capacitance, reduces the drive to the chip.
PS: R33 and R49 may be 470R rather than 220R in some early units. In any case it needs to be raised to 2K2 if there's a sound problem.
www.borinsky.co.uk Jeffrey Borinsky www.becg.tv
