My cr1604 sounds distorted at times on all channels and sometimes on just one channel. What could be causing this?

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.

have you checked the voltages after that?

Check the power supply voltage and look for a hot component. If it takes a while to bring on the symptom it's probably something getting hot.

This should be a fairly reasonable repair. The main problem is finding the source of the hum. Since it is effected by the sliders, the source is in the mic pre-amp sections. Since all channels are effected, I would suspect a power supply problem rather than a problem with the electronics. Please send a note to [email protected] asking her for contact information for me. I can help you out. These are decent mixers (I own a few) and should last for many years. Yours just needs a little TLC.

Dan

It sounds like one of the output devices is going bad, ie., output transistor, capacitor, resistor or maybe just a bad connection on one of the above menitoned items. One other problem which causes this would be the main or sub volume slider or pot is wearing out and needs to be replaced.It would definately be worth your while to have a technician trace down and fix the problem on this board. Hope this helps.

If the hum is observable in headphones, then it is likely a power supply problem, usually a dried out electrolytic filter capacitor.

initially you want to think bad solder joint, but it think its a bad mosfet or rectifier or diode. it would be hard for an untrained person to identify your problem let alone an experienced tech. it may be time to move on to another amp. in my shop it would be at least 65 bucks for the repair. so where did the "heat"/energy go from that point and what parts were affected. there may be a domino affect. burn one part, not usually.
yostamplifier.com

It sounds like a job for a technician. There are high voltages in the chasis and the potential for a serious shock to the unwary. 

That said, I can suggest several possibilities.

First, make sure the controls aren't causing the problem. Contacts can build up dirt and oxides and block the signal. Try twiddling the controls and see if the problem comes and goes, or if you hear scratchy noises. If symptoms are found, buy a contact cleaner spray, take out the chasis after unplugging and letting it set for an hour, being very careful not to touch any exposed conductors inside. You'll see some holes in the control bodies; squirt cleaner for 2-5 seconds and work the controls.
Also see if the problem happens without the footswitch, and try the channel switches several times - same problem, dirt and oxidation.

Other possibilities - cathode bypass capacitors drying up, and it looks like the amp uses some opto-couplers at various stages. 

Without hearing the noise or seeing it on a scope, I can't say for sure, but this type of noise can come from a number of sources. If there is a high signal level on one bus, you could be getting some cross-talk from that channel onto the channel with the higher gain setting. Try to track this down a bit by experimentation. For example, do you hear this with NO connections at all, but with the gain turned up on one channel?

Please try a few things to narrow this down a bit and I'll try to provide you with some options.

Thanks,
Dan

If all other functions work for those channels, the most likely problem is the fader itself. These can fail and operate more like switches rather than faders. Without witnessing the behavior, I can not state that for sure. While these mixers are not the hardest to work on, they are time consuming. The faders can be purchased from Mackie directly. If you need to have this repaired, any shop that repairs audio gear should be able to erpair this for you. Where are you located?

Dan