No volume control with 2 faders on Mackie CR1604 mixer

I doubt the issue will be the regulators if you have the correct voltage on them.
Check where the hum is coming from.
What frequency is the hum; 50/60HZ = mains hum = bad ground. 100/120HZ = power supply hum.
Affected by a control somewhere maybe.

Plug either a powered monitor or amp and then speaker into one of the aux sends. Now adjust the monitor mix using the appropriate aux send pot on each of the channel strips you want to send to the monitor. Think of the aux send pots as faders for the monitor mix the same as the main fader for the ain mix.

have you checked the voltages after that?

Use the main left and right... the CR outputs are INTENDED for running control room monitors.

There are cables that bridge the sets of channels together. You MAY have a bad connection of one of these. The strips of channels I believe are in banks of 8 in the Mackie's. The cables should be accessable with removing the back of the unit.
If you have had a spill of liquid into the faders, then you know what the problem is and requires extensive work to replace faders. You could try putting a bit of CRC226 into the middle of the fader slots if the faders are noisy. You can get the CRC226 at the Home Depot in the electrical dept. Use ONLY this exact product.

If the faders have become noisy, they may be changing due to poor contact. Try putting a small amount of CRC226 in the center of the slot of each fader and then move fader knob to top and bottom. CRC226 is available in the electrical department of Home Depot in a small can with spout... about $2.70

If you havent ground looped the chanels. Most of the time the chanels need to be cleaned.

Removing all the knobs from a section of the board where the bad fader is can be a hassle.and sometimes some of the nuts from jack or screws from some connectors.

You are probably better to order the part from Mackie to get the exact part. Most are made by ALPS and I get ones for some equipment from DigiKey.com or Mouser.com. However, it does take a bot o knowledge to get the correct part and sometimes requires cutting the shaft of one that is too long.
One MUST match the mounting holes, resistance value, length of throw AND the "taper". Yours will probably be a standard "Audio taper" for the main output.

You may be getting a small amount of cross-talk. How high are the gain settings and sliders when this occurs? Also, can you describe the overall setup that you are using? I'll check the prints, but you should not be getting any volume unless the gains are maxed.

Dan

You can purchase the channel faders directly from Mackie Service Dept., you can replace them yourself if you're good at soldering - HOWEVER, be advised that disassembling the unit to get at the faders to replace them will require a lot of time and patience if you've never done it before. If the faders are not moving smoothly, it's possible that the wrong type of cleaning spray was used on them and there's a possibility that they could stop moving and controlling sound.