PLEASE do not take offense but some of the assumptions you are making and questions you're asking are making me unsureSo essentially connecting the 18v to that board will just end up with less dimming? Or everything partly dimmed all the time? We're essentially losing the top end of the dimming capability?
Well, it looks like all of us are way off base on the control voltage except for @RonHebbard. Those dimmers wantPLEASE do not take offense but some of the assumptions you are making and questions you're asking are making me unsure
you understand how dimming systems work but of course that's probably why you are here!
You are not connecting 18 volts to the demultiplexer board... the demultiplexer board is sending 18v to the dimmers!
If you don"t feel comfortable working on this equipment, please call in a technician who is.
On the diagram I provided earlier in this thread, the "signal flow" goes from top to bottom, so the control voltage goes FROM a console or demultiplexer TO a dimmer. Zero control voltage tells the dimmer to not output AC voltage to a circuit/fixture; 15 or 18 vdc tells the dimmer to output full AC voltage to the circuit/fixture. IF the control voltage for full output of the dimmer is indeed 18v, 15v will tell the dimmer to output slightly less than full voltage to the circuit/fixture.
The first issue is the 18 volts. This is an odd but not unrealistic number.
As @FMEng has been saying, you need to use your meter. Set it to the DC Volts mode with a range of 20 volts or higher.
To find the actual control voltage to the dimmers, take a reading on the 48-channel demultiplexer circuit board by placing the negative (black) lead on one of the two COM terminals on connector J1 and the positive lead (red) on an easily accessible output terminal of connector J4 or J5. Slowly bring up the fader on your main console that corresponds to this number terminal and see if the meter slowly goes from zero to 15 (or 18) volts as you move the channel fader to full. If the meter needle tries to go below zero or if a digital meter shows a minus sign, you may have a negative control voltage!
Assuming this goes as expected, you should then do the same on the houselight control system.
BUT, there may be extra wires with other voltages in this system that operate "take control" relays or light up fader scales or buttons. Please post some photos of terminal blocks / houselight control wiring before trying to measure this.
And don't get in over your head so you're around to report back!
Replacing the normal (~270 degree / open dirt collecting) trim potentiometers with small, sealed plastic, 10 turn, Bourns potentiometers then housing the Bourns pots in a Bourns, slip-on housing providing a tiny / tapered / funnel-like opening on one end for the insertion of a ~3/32" slot screw driver added years to the service life (and only annual trimming if you were REALLY picky) to 36 used JTM's ( A 24 dimmer wall-hung cabinet with a 12 dimmer wall-hung cabinet next to it) installed in Burlington, Ontario's amateur theatre in the mid 1980's through the early 1990's.All the Strand gear I have ever seen, a lot, is 10v control but will work on less, but this does not matter as all the Strand dimmers have trimming on the dimmers, which is just as well as they tend to drift around with age, they are wired -10v but most can work +10 by reversing the control wires on each module.