The reverb tank in your amp is most likely picking up stray magnetic fields from the power transformer in your amp, OR, you may have a bad cable going from the reverb tank to the return to the amp. This has been a problem with all spring reverbs since their inception. How well the amp is engineered and built will determine how obnoxious it is.
Are you familiar with spring reverb tanks in amps? I have several Fender reverb amps, but they are tube ones. I jam with a guy who loves his M80 combo. I do not recall how the reverb tank is mounted in that amp. If your amp has the tank on the "floor" of the cabinet, it should be in a vinyl bag, with (2) cables going to it.
1 cable is going TO the tank ("Send"), 1 is coming FROM the tank ("Return"). You may have a bad cable and/or connection from the tank to the amp. IF you DON'T have the bagged tank at the bottom of the cab, you will probably find that the spring tank is up in the chassis of the amp itself, and that ain't good because the power transformer is too close to it and is causing the buzz. Then again, the connection to the tank could still be bad. That will most likely need to be taken to a tech.

buzzing

hey SIAB

I remember having the same problem with an old fender amp I think it was called a Reverb II ?

It would drive me bonkers with the buzzing from the reverb, of course after trying everything I lost hope and tried the
"Aurther Fonzarelli" technique (Happy Days). It did actually fix it.

All jokes aside, I ended up taking the unit off the side and layed it in the back of the amp. I am not a amp tech so I can only guess the way it was sitting it was picking up a ground loop from its coils inside and after I layed it down in the back it was out of that loop?

I am not familiar and I don't recommend hitting your amp, or even if the M-80 has a similiar reverb unit it could be just dust build up on the coils??

Sorry for the novel, I just remember how pissed off I was with that unit, good luck!!~

I had a Marshall JCM800 combo that had a similar problem that when smacked on its top would temporarily correct the buzz. Turns out that Marshall had soldered the "amp end" leads going to and from the tank right to the circuit board instead of RCA plugs that permit you to easily replace the cables when they go bad. Had to pull the chassis and re-solder the connectons...kind of a pain.
I happened to run across a solid-state Fender amp from your M80's era. The reverb had no vinyl bag ( it is used to isolate sound from the amp returning to the tank), but it was mounted at the bottom of the cab.
Pappa has a good point- if the tank isn't as far from the chassis as possible, re-mount it so that it is. You might also do something that I have seen clubbers do. If the tank isn't bagged, pull it out, wrap some aluminum foil around it (be careful not to let the foil touch the tanks' innards), then wrap that with a small towel. This may help keep both sound and magnetic/RF interferences away from the tank and its internal pickups.

Stray fields and that reverb pan can be a real problem. Stupid suggestion, try rotating the amplifier/box 90° or so. Try moving it around the room, and rotating it. You may be picking up an AC power feed, from the wiring around or going into your habitat? I don't believe there is any problem between the unit and the amplifier as it was designed as an integral piece. If that fails, try replacing the reverb pan with something like an Alises micro-verb and dial in what you want. Noise is gone! Impervious to shocks! More fun! But will be lacking that original pop/sproing effect.....

The AC fields are a huge problem. I had a problem with an MCI studio analog recorder, many years ago at NBC radio in Washington DC. The playback head had a small frontal hum Shield, that you would extend to prevent any hum. It didn't work! The noise would go away if you were to rotate the machine 90° but we could not use it that way. I contacted MCI and told them we needed some "mu metal" shielding around the complete head (Ampex and Scully had them). They said they did not have anything like that available and the problem was only the cabling. Bull crap! They replaced (shipped us) the cabling and nothing improved! I said we needed the mu-metal shielding! They had never considered that as they thought their design was too superior to require that. More idiotic electrical engineers that didn't live in the real world! They finally sent me some pieces of mu-metal which I bent, around the sides and bottom of the head. I was told by the station's chief engineer that you could not do that to mu-metal without having it "reanealed", as it would not be effective where it was bent. More bull crap! What I did work like a charm! After MCI came to Washington DC from Florida, they went back to their shop and build some "official" head cans for us and I am sure others that were having similar problems. Those old reverb pans were never a high ticket items to begin with. In 1970, I bought a few of those to create my own reverb unit at 14 years of age! One other thing you can try is similar to what AKG did with their spring units. Purchase a second spring pan, connect both inputs together in parallel. Rewire the second units output reversed phased to the first one and connect them together in parallel. The out of phase wired outputs may cancel all of the hum? It will also give you a cleaner and denser reverb? A trick that AKG called " Tortional Transmission Line" or TTL. And thus, the BX20E was born.

Now I don't know where in the world you may be? Nor do I have any idea where you could actually obtain some mu-metal? If you can find some, you may try constructing a small shield around the pickup transformer device in the spring pan? It may be your only hope? Welcome to electron world!

Good luck!
Remy Ann David

There have been TONS of Fender amps that have been unleashed onto the general public with design flaws that only bit them on the butt later on down the road! One of them is putting an unshielded reverb tank in the chassis too close to the power transformer!
At this point, the original poster has not replied about the placement of the tank, nor of the condition of the cables going to it.
I have not had the pleasures of working with Mu-metal, mainly due to Stephen St. Croix's articles in MIX warning about how dangerous it is to work with (easily slices up one's fingers) without the proper tools or training.
My suggestion that aluminum foil be used should be modified to COPPER foil (like what's used in the pick-up cavities in electric guitars).You can buy this from any number of guitar parts suppliers (Stu-Mac, Carvin, Torres Engineering, and a bunch more). Just make sure to solder a ground wire from the foil to the ground lead of the RETURN line to the amp. You might also contact Dan Torres at Torres Engineering (http://www.torresengineering.com). He may very well have a solution or suggestions to your problem...