The 2 and 7.6 volt
trim pots are only for the local controls on the front of the pack. They have no
effect on the incoming analog signal from your light board, other than if the 2 volt pot is set too high, the lights will not go to black. I am assuming you are using an external light board. If not, then you want to measure that
voltage at the test points on the board. (they should be labeled.) If not, the best place to measure is at the input to the comparator on the
channel. Each
channel has one, although the packages are combined as they use quad op-amps. Each one receives the DC input on one input and the ramp on the other. You are looking for the DC value.
The 220 mfd caps tend to dry out and that may
throw the ramp. If you scope the caps, a dry one will show a very large AC ripple. From your symptoms, we can be pretty sure the ramp generator is the problem. Although the quad op-amps do fail, the more likely failure is the caps.
Sorry to be so long to get back on this. Here's where it stands. I replaced one of the
diode rectifier packs and it appeared both sections of the
unit are working about the same. I don't really know if both at full
power or if both are at half
power, but they were about the same on the test lamp. The main control
power switch on the front panel then fell apart, the tabs holding it together gave way. The bronze contacts in the
switch had sustained mechanical damage so I had to find a new
switch to fit the panel. Standards have changed, so I had to do some mechanical mods to allow a modern
switch to fit the old hole. As a final insult, the plastic knob on the 2 volt
trim pot was dried out from the heat - it separated when I turned the pot to adjust the
voltage. Same with the 7.6 volt trimmer. These have now all been replaced. I have not found any test points on the
circuit card.
Tracing the circuits, and using the partial schematic you referenced in the other post, the 2vdc and 7.6 vdc trimmers are connected to the V+ (top of the filter cap) as follows:
V+ -- 5.6k Resist. -- 10k Trimmer (7.6 vdc) -- 10 Resist -- 10k Trimmer (2 vdc) -- Gnd.
I don't know the exact value of V+, but unless it is in the vicinity of 10.5 volts or so, I'm not sure you can get 7.6 volts out of the
circuit. Have I mis-traced the ciruit?
We have also reworked the
power to each of our two units to include the proper grounding required by code.
Re-reading your troubleshooting article, I'm not
clear on how to be sure the ramp generators and thresholds are correct. I plan to proceed as follows:
1. Set the 2 volt trimmer to 2 volts on the output of the
voltage follower op amp connected to the wiper of the pot.
2. Set the 7.6 volt trimmer to 7.6 volts on the output of the
voltage follower op amp connected to its wiper.
3. Set the LO trimmer to move the
base of the waveform from the ramp generator op amp to be 2 volts.
4. Set the HI trimmer to make the top of the waveform from the ramp generator op amp be 7.6 volts.
5. Repeat 3 and 4 for the other ramp generator.
Please let me know if you have any suggestions.