Newb. Glad to find this forum ! Fender M80 Bass Combo

[phatt]

Very likely a mechanical problem. I'd check that pot, sounds like it's failing.

Possibly the wiper is now worn and making intermittent contact with the resistive track inside. You can squirt them with a spray of electrical cleaner but in my experience ,, better to replace them while you have it apart. The can of spray may cost more than a new pot

Pots can also fail right at the lugs. Those tags are a press fit crimped directly onto the end of the internal pads. Even small stress on them can cause problems.
The fiber base plate can also crack which of course breaks the thin carbon track right at the join of lug to track.
Also check the FX loop area,, in good light.

In regard to increased hum while hand is near the input.
If the input socket is not grounding then yes with volume up you most certainly will be microphonic to even your hand.

The Hot end of Input sockets for guitar amps are always ground switched when cord removed as they are high Z inputs. So maybe the input socket you replaced has not been correctly replaced, you may need to recheck it.
Phil.

[dogface]

Thanks phatt, I'll keep picking around. your help is much appreciated.

[g1]

If the volume is up, even with a grounded input you will get hum waving your hand around the input area of an open chassis.

[dogface]

Well fellas, looks like I've got it with your help. I went through all the suggestions and just couldn't find anything that seemed like a physical ground issue. New that is.
I went back to my original fix, the point where the power board grounded to the chassis. Seems what I did was an improvement, just not enough. i removed the screw and the brass nut i had put under it to try to bridge the gaps of poor contact. I fired up the soldering iron and flowed a perfect ring around the screw hole, put everything back together, and all seemed good.
No more crackles, cut outs or fuzz tone. I noted that one leg of the filter caps are in a wide path on the board very close to the ground point. Forgive my primitive terminology, anyway it works. I still cant believe how loud this little guy barks. with the 4 ohm Black Widow i replaced the original 8 ohm speaker with, I get all 160 watts and it is a pretty efficient speaker.

I tried to find some specs on the Fender/Celestion 8 ohm speaker that is original equipment. The most I could find is that it is sold as "Fender 15" replacement speaker" no specs. It was used in other amps and cabs of the era ~ 200 watts or so, so I guess it is a pretty typical middle of the road 15. 200, maybe 300 watts ? Anyway, I may put that back in the Peavey cab I stole the Widow out of and experiment with my slave amp idea with that.It will be plenty for that.
THANK YOU  !!!!

[LateDev]

A lot of amps used Celestion Speakers and those speakers were common to Marshall as well. I know there was a round wire and an elliptical wired speaker done at that time. The elliptical wired speaker was more efficient given that the air gap could be smaller for the same number of turns. I would presume that would have been perfected now with a flatter wire used on high efficiency speakers pro speakers of all types.

lol I have 2 x 12" Celestions in the hi-fi. I did have RCF speakers in there, but they were way too good for hi-fi 

Congratulations on getting it sorted, there is no worse problem than noise on any amp and is one of the harder things to successfully track by remote.

[J M Fahey]

elliptical wired speaker

Just curious as to what that means.

[LateDev]

The coil wire has an elliptical cross section.
This of course means that the wire can still have the same current carrying capacity as a round wire with the same total cross sectional area, and the same number of windings as a round wire will fit in a smaller gap, or you can increase the number of windings for the same gap.

Speaker efficiency is increased by increasing the maximum amount of magnetic flux.

There are other factors to consider with Celestions from the older speaker range, as in they had a cambric edge surround which stiffened the movement and some manufacturers preferred a metal dome, rather than a fabric dome over the centre pole. I can't remember but I think Fender may have had the metal dome.