Mark III Ground Issue

[gyver]

I am having a really strange ground issue with my Mark III. I took it into get looked at when I first noticed it and they could not find anyting wrong, but thought it might be the power cord so they replaced that. I brought it home and it was fine for two weeks and then started up again. I plugged it into a different outlet and it was fine for a week and then started up again. I then plugged it back into the original outlet and it was fine for another week and then started to act up again. I have tried the same guitar through other amps with no problem except through the Mark III and several guitars through the Mark III all with the same problem so I am sure it is something with the amp. It will go away if I unplug the amp and plug it back in but it will not go away if I simply turn the amp off and back on. All of the outlets in my practice space are properly grounded. The technician I took it to is a retired electrical engineer who has been working on amps and guitars for 30 years and is well-respected in the area and he cannot figure it out. Does any one have any ideas? This particular Mark III is a blue stripe with no reverb or EQ.

Thanks,
Gyver

 

[Rob Lockwood]

Are you saying that if you just unplug the amp and immediately plug back into the same outlet without moving/disturbing the amp, leaving it for a period of time or doing anything else that could affect it the problem goes away for a week? Just thinking that if you are physically moving the amp to the different outlets the movement could be doing something, maybe vibration affecting a bad connection.

What is the symptom of the grounding issue, is it acting as though there is no ground? When it is playing up, without touching anything else you could do a resistance measurement between the amp chassis and a known good ground, or even use a jumper cable with croc clips to touch between the chassis and earth to see if the problem clears. Dont touch anything metal while doing this you dont want to put yourself in the path between the amp and earth!!!

There is a grounding switch on the Mark 111. Does moving that switch have any effect on the problem?

 

[gyver]

I first noticed it because I was getting mic shocks while playing. The amp became very noisy as well. When I touched the strings it went away but when I took my hand off of the strings it was a mess of noise. Initially I tried several different guitars through it and they all had the same problem, but when I plugged into a different amp the problem went away. I did try the grounding switch and the problem remained although the level of noise changed when I switched to "A" or "B." The mic shocks were much worse on A and B.

As far as unplugging the amp I do not move anything and I unplug it and plug it back in right away. I have done this a few times and the problem always goes away, but simply turning the amp off and back on does not work. The amp will be working fine for several days and then I will go out to play it and the problem is back as soon as I turn it on. Sometims it is working fine and the problem will arise while I am playing.

The amp was fine for two months after I got it last year. The problem popped up a month or so after I replaced all of the tubes in January (Mesa Boogie Tubes) - could it be an issue with a bad tube? The amp technician said a bad tube could cause the noise issue but should not create the mic shocks.

Thanks,
Gyver

 

[Rob Lockwood]

That is very strange! It does sound as though the amp chassis is losing proper connection to the ground of the outlet, either completely or partially. Very seriously I really would not be using this amp at all except for careful testing under suitably protected conditions until it is fixed. People have been killed by these sorts of grounding faults.

When the power cord was replaced, was the plug on the end of the cable replaced too. Seems unlikely but I wonder if there could be a problem with that making a good connnection?

Have you had the grounding system of the house tested? Just because the outlet is connected to a gound wire doesn't guarantee that the ground connection is ok throughout. I have heard of metal stakes being used to make the ground connection losing connection when the earth around it dries up.

I think that unless you are confident that you can safely test the amp you need to go back to your tech guy and see what he can suggest, If nothing I would try someone else because this could be a serious safety problem and it should be traceable. If the fault is in the amp the tech should be able reproduce the problem in the workshop by leaving it plugged in and switched on then the rise in voltage of the chassis should be measurable (If he can't reproduce it that suggests a problem with the house wiring). If he can reproduce the fault then it must be possible to find the cause but it may involve poking around inside the amp with a meter with it it powered up so its not something to try unless you have the necessary skills.

 

[dgr888]

Makes sure your tech has checked the .05 600v cap that is on the switch itself as this will cause problems like this.