Stuck solo pfl - Mackie 1604 Vlz3 16 Channel Mixer

Probably one of your PFL switches is stuck in ON.
If not, the digital decoder went kaput.
Also could be your +5V power rail, that one powers the digital stuff...
Good luck.

The "PFL" Stands fo r"Pre Fader Level". Just in case you did not realize. so when you press the "PFL" button , and the red light goes out?, You're saying that the hissing goes away and that you have clarity. Yes? and the volume presents no issues? Also correct? Part of that simply, is with the PFL in the on position. You are engaging the fader for one, which will add to an already boosted signal ( the the fader is turned Up?). also you may have your Clip set incorrectly. to set this you turn PFL off, then at your signal input level Dial (usually at the very top in-line with your fader. _ set the volume on your device, then input clip level on the board \ to where the clip light comes on. not a steady light, but a blinking light so make it steady, then back it off a hair. Okay? this whole time make sure your mains are down. or off at the faders. Once clip is set., PFL is in off position, gradually turn your mains up to level. Now, the PFL is for when you have more than one signal. say from an instrument on another channel. unless you have that? No need to use the PFL. If you do have an instrument in line,. then, before you turn PFL on,make sure that your fader on that channel is turned off. -turn on your PFL, then gradually bring that to level. never messing with the mains level.

I know that is a lot of info. but it is a process. that should eliminate any hissing you may have heard.
Good luck!!
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Mute Switch/Mute-Clip LED :

• This switch mutes all Aux, Group and L/R sends from the corresponding channel.
• This switch is equipped with a red LED that will illuminate when the channel is muted.
• When the MUTE button is out,the LED functions as a Clip indicator that will illuminate at 2 dB below clipping.
• Muting the channel does not prevent the PFL signal from being sent to the PFL/AFL mix when the PFL Switch (12) is in.
• This switch mutes its respective Group send from the Group channel.

Go to page 11 in this link :
Peavey FX Mixer Series Operating Manual

You can also try, Restoring Factory Settings, in the Utility Menu Options.
Go to page 16 in ths link :
Peavey FX Mixer Series Operating Manual

CAUTION:
Restoring the factory settings option resets your mixer's presets, reformatting factory presets and erasing all other user presets. (This option is handy when your venue has permanently changed and all-new presets are required.)
Once confirmed, the presets are not retrievable and will be permanently lost.
Should you select this option in error, a warning screen allows you to cancel the procedure.

Same problem on my Eurodesk SX2442FX..
how have you solved this problem?

Try headphones... do you get sound both sides (assuming your PAN controls are centered.???
Now do same with the main output cables discoonected... do you get headphones both sides???
Next solo or PFL a channel... do you get sound both sides???
Answer me these as they are tests that I can do some isolation using the circuit schematics. Make sure to answer all three conditions.

The Efx 12 has two outputs monitor out as well as mix out ( XLR sockets ). If the level bars are moving its an indication that the mixer is working. Have you tried headphones out as well as monitor out.
What is your amplifier and speaker setup- you have not mentioned it at all.
Even if any PFL switch is on you won't get sound out of the speakers.
PFL stands for Post Fade Listen and when any of the 12 channel PFL switch is on only the headphone out put will work.

This is likely a problem with the FX in the unit or the power to it. Does the FX display act correctly? Check that the headphone source "Main Mix" is the ONLY button down for the headphones. If not, make it so. If the clicking is still there, try the headphones in the aux send jacks... see if EITHER left or right can hear the clicking. Back in the phone jack, scan across the SOLO channel buttons down one at a time. Is there clicking? If not, the problem involves the mixing buss or further down the line. Make sure the SOLO buttons at the FX Aux sends are up.
What effect does the MODE button have in SOLO and in PFL? Is the SOLO LED off? Is the number in the FX window SOLIDLY on and not flashing?

Select desired tracks with the PFL buttons. This solos those tracks to the headphones.

The circuitry is a bit complex for that PFL light. The LED is driven by a transistor T38 type BC846 to turn it OFF by grounding out the current coming by way of teh MUTE LED or the MUTE switch (SW14) horts out the MUTE LED. There is a 1K limiting resistor from +15 volts feeding the two LEDS in series.

IF transistor T38 is open the PFL will be on. HOWEVER T38 will also be turned off if transistor T44 (BC846 type NPN) or if capcitor C90 2ufd/50v is shorted OR if resistor R253 is open OR is switch SW8 (PFL DPST )is not contacting the ground side of of the half tied to R247. If R247 is open that will also cause the PFL LED to illuminate...

AND if the signal coming in is at clippling level the PFL LED will light.

All that circuitry serves as a clip detector plus the PFL function.

Verify you are not sending clipping level into the channel first... turn the trim off. If not, you are in for major surgery as all the knobs for that section of the circuit board and the screws for the XLR's and the 1/4 inch jacks must come off as well as some support screws IF it is necessary to access the top side of the board... Maybe you will get lucky and the bad component MIGHT be on the bottom of the board.

Is the PFL button stuck down on these two channels ?