Ok, below is a generic
layout for the
EDI ramp generator. You can see the 12 volt winding in the upper left
hand corner. It feeds into the bridge rectifier and then goes through a
diode before hitting the main filter cap, which supplies
power to the rest of the
circuit. Note how unfiltered
power feeds down to 1/4 of the quad op amp. This part of the
circuit is the "zero
voltage cross detector" and produces a pulse whenever the AC waveform starts. The output waveform is a
square wave with a 1% / 99% duration and a frequency of 120 cps. It goes through a 1k
resistor, then a
diode, and then into the 2nd 1/4 of the quad op amp. This part of the
circuit produces the ramp and is the heart of the
dimmer. There is a little notation that the thermal shut-down shorts the output when it kicks in (right after the
resistor) which turns the
dimmer off when overheated.
The upper right shows the four stages of ramp generation. Steps 1 and two happen real fast!
1) The ZVCT goes high, using the 2N4401 to discharge the ramp
capacitor. (.1 mfd)
2) When the cap is discharged the ramp output goes way high. (about +14 volts)
3) The ZVCT goes low and the output of the generator drops to the high-end set
point, 7.6 volts.
4) As the cap charges, the output curves down to the low end set
point of 2.6 volts. Then the whole thing starts over!
So, what is not shown is that the ramp output feeds the other two sections of the quad op. These also get the control board output. When the two voltages cross, those sections produce a pulse which is buffered and drives the opto-isolators. Failures in either of those two circuits would only take out one
dimmer channel.
If the .1 mfd ramp
capacitor had any leakage or was open, you would not get any ramp. Caps can get flaky and noisy, so short of setting up a bench
jig, that would be a cheep try. Next in
line would be the quad op-amp. It may fail shortly after
power is applied due to a substract crack in the
circuit, which would explain why it works after
power is removed. It's a cute little
circuit to watch working on a scope!
EDIT: Just saw your post. If the duration can be that long before the second
channel goes out, then it sounds more like a thermal problem with the quad op-amp.