Volume ports voloume pots

It seems likely that the pot (volume control) is dirty. with a slider you can normally get fluid down the side of the knob, your's is a rotary control and you sometimes can do it round the shaft of the pot but more often have to spray it in where the connections go into the pot. You'll need some contact cleaner spray which you can get online or from an electronics store, get it onto the pot track and then take the pot to each end of its travel a few times, this'll probably sort it out until next time. If you're taking it apart it might be worth just getting a new and better pot and soldering that in if you are hand with a soldering iron. The ones in Mackies nowdays are pretty sh1t.

Just spray a few shots but you have to take the top cover apart, then take out 3 philips screws in the pots module, take off caps and then you should be able to give'em a good cleaning spray shots.

Usually with volume pots,the ratings dont matter. you can really buy then anywhere like guitar center,micro center and online. they're really cheap and well worth it.

I suspect you may have either a circuit board crack or the volume pot is bad or soldering of it is bad. Vibration will make things come and go. Look for broken solder around the pot leads first. Then look for a cracked circuit trace on the board near the ground end of the volume pot. If you put cleaner in the pots NEVER use Deoxit or Blue Shower. Use CRC226 which is available at Home Depot in the electrical dept.

Pots are dirty or worn. Use an electrical control cleaner made for volume controls or have them replaced.

It is not clear if the volume knob you are talking about is in the amp or the guitar... SINCE you said you verified the amp is OK I have to assume the volume control you are talking about is in the guitar and my answer is based on that...

I SUSPECT that the resistance element and/or the wiper in the volume pot is damaged. One other possibility is the soldering of a wire to the volume pot may be bad. If you are competent with a soldering iron you could replace the pot yourself or take it to a competent guitar shop for repair. I would myself suggest you open the electronics and look for anything obviously wrong. I have ALSO seen one other problem where one lead of a pot is touching the CONDUCTIVE paint in the cavity where the controls are. If the shaft nut becomes loose and the pot slightly rotates this can happen, grounding out the signal.

If the volume pot is scratchy noisy, FIRST try squirting a bit of CRC226 into the pot. CRC226 is available at Home Depot in the electrical department. There are other CRC products there so be sure to get the 226 type. About $2.75 a can.

The product is also good to use in slide pots on mixers. Use sparingly, it doesn't take much. For rotary pots you often can use the spout to spray into the slot where the terminals go into the pot.

It sounds like (pun intended) the volume pot just needs to be replaced. A good pot is worth about $5 and then you just need to solder it in. That tends to be the tricky part if you can't solder. It takes longer to heat up the iron than it does to do the job. I'd assume this happened either after it had sat for quite a while or after a move.
What can happen is the wiper or the resist inside the pot breaksor even the wire to the pot breaks and there goes your volume control. Depending on what model of amp you have you don't want to run it like this for too long. You can't fix the pot but you do need to replace it. I would assume it sounds like it is either full on or doesn't change the volume at all.
This is one of those maintenance things you should check for from time to time. you may notice that the pots get noisy or inconsistent for a while and then they do this kind of thing.
The attenuator will kill the volume but that doesn't actually fix the problem you have. When the amp is running properly you should be able to get quite a low volume. If you need to run it harder so that it gets into the distortion range then a attenuator will drop the volume down to a practice at home non eviction volume ;-]

First try turning the volume control down before turning switch to operate.

IF this seems to fix it, IF the unit has a tank reverb, the vibration of the reverb springs MAY be causing the problem. The solution then is to reduce the volume before applying power WHICH is a good idea anyway... from an electrical engineer.

If you can't clean the pots or if you just want to try replacing them, try using
500k pots again, in case one or both of the pots is worn or dirty or
defective. If that doesn't work, and cleaning or replacing the
switches doesn't work, then the problem is probably elsewhere.
You could put a 500k resistor (with a sufficiently-high power rating of at least half the power rating (Watts) of the pot) in parallel with the pot, or, to cover all bases, put a 500k resistor from each of the two outer lugs of the pot to the middle lug. That should change it into the equivalent of a 250k pot. You could even just temporarily clip the resistors on, with alligator-clip leads, if you're very careful, just to test it.

Good luck,

The dude.