Tech: Noisy TZ clock optos

[beaver]

I have the original clock boards in my machine without the LED mod.
This past week, it has been acting up. I get an occasional credit dot
that goes away. So I decided to put it into clock test mode to see
what is going on. I hope this is useful to somebody.

With the motor running in slow, the 12 o'clock opto would flicker
constantly. As the minute hand goes around, the other optos would
fire briefly. Also, the CPU was making the beep sound constantly.
With the motor disabled, no 'X's in any optos. On a hunch that it was
motor noise, I disconnected the motor, and enabled the motor again.
The problems went away. I could trip each of the optos with a
corresponding beep sound when I passed my screwdriver in the opto
slot. Sounds like the motor noise was getting into the opto circuit.

I know that some consider the original TZ optos marginal in design,
and perhaps my motor is getting old and noisier. To fix the EMI
problem, I added a 1uF (non polarized) and a 0.1uF to the motor
terminals, and then a 0.01uF in parallel with each of the opto
receivers. I no longer get any noise in the optos, and it works fine
now.

Edward Cheung CARGPB26