Help! w/Hum

[ronstephens]

So the UPS guy drops off my RM4 and Top Boost module (more modules coming tomorrow). I plug in the Top Boost, conncect the RM4 out to one side of my Mesa 20/20, which drives a single 12" Celestion V30. The lights are on. I set the preamp knobs to 12:00, with master volume off. Switch the Mesa off standby and... HUUMMMmmmmm! OMG!. WTF?

I fiddle with stuff. Swap cables (audio and power), wiggle stuff, etc. and this hum is unbearable.

It hums with no guitar connected. The Mesa does not hum with no input. The RM4 does not hum with the module removed. It looks like the Top Boost module is the culprit.

Could I have a bad module? I hope, when the other modules (Clean, JTM & Tread) are delivered tomorrow, they'll work as expected. But if they all hum, and this is "normal", I can't imagine anyone using this thing - especially for recording.

BTW: The TB sounds GREAT - even over the hum.

Any advice?

 

[kc2eeb]

Did you try the ground lift switch?
Are you plugged into the same AC outlet?
Is the AC outlet properly wired? (Get a tester at Home Depot)
Note: As long as you have a signal cable connecting the RM4 and the 20/20
you are NOT "ungrounding" the RM4 when you use the ground lift, because it is grounded through the signal cable to the 20/20.
What you are doing is eliminating a ground "loop".

 

[ronstephens]

Thanks for the suggestions.

Yeah, I tried the ground lift right away. Subtle difference, if at all.

At first I was plugged into my power strip. Then I tried a two-plug wall outlet. Then I tried a ground lift, one device at a time. No change.

I've also swapped out the cable between the RM4 and power amp. No change.

Is there another way to isolate the grounds?

 

[JKD]

Hum is definately not normal...you'll know whether it's a problem with the chassis when you get more modules to test.

I wonder if one of the cables you are using to power the units is faulty? Worth a shot....crap like this is cabling a lot of the time.

 

[ronstephens]

2 more modules came today. Same hum. Seems there IS a ground loop between the RM4 and the 20/20. I've swapped all cables but one - the 202/20 power cable, which is hard wired. It's definitely 60Hz.

I've never had a ground problem with the 20/20 before (isn't that what we all say, right before something breaks?)

Could it be physical proximity? The RM4 is right on top of the 20/20 in a 3 space rack. Maybe a transformer inducing 60Hz?

Any suggestions before I take the whole rig to a tech?

 

[JKD]

The RM4 ground lift should kill the hum..it seperates signal and chassis ground....

You can test your second theory by seperating the two pieces

 

[ronstephens]

Answering my own message...

It aint proximity. I unracked the RM4 and put it across the room from the 20/20. Same hum.

I put a ground lift on the RM4 (again) and - what? - this time it fixed it.

I re-racked the RM4, still with the ground lift and - fixed.

So I'm guessing, one uses the chassis for ground, the other is isolated from the chassis. I've seen that a lot in studio racks - somebody doesn't play nice because of different ground design philosophies.

Now I'm wondering: is there a way to fix this?

 

[JKD]

isolate the units from the rack? You can get little plastic 'shoulder' washers I believe...

..so long as the hum is gone, you're good

 

[ronstephens]

isolate the units from the rack? You can get little plastic 'shoulder' washers I believe...

..so long as the hum is gone, you're good

Well, since they still hummed when the were 20 ft. apart, chances are isolating them from the rack won't help.

I wonder if one of them has their AC wired out of phase?

 

[JKD]

I thought you said the ground lift on the RM4 fixed the problem?

 

[ronstephens]

It treated the symptom. It didn't fix the problem.

 

[JKD]

It treated the symptom. It didn't fix the problem.

Well..after isolating teh units from teh rack...I don't know any other way..except to re-design the 20:20?

The the ground lift switch is a god-send in my 1 amp, 2 rack pre-amp setup...it does exactly what it's design to do in preventing ground loops between pieces of kit in the rack.

When I added a second amp to that equation, i was forced to use an isolation transformer to prevent the ground loops between the two RM50s

 

[crankyrayhanky]

not sure if this helps.....

I run (2) rm100s and needed to get an ebtech to solve the problem....though before I had it, using a 2 prong adapter on one amp also solved the issue

 

[ronstephens]

Boy do I feel like a dumba**.

I took everything out of my rack to install insulators (so none of the rack gear would ground out on the rack rails) and noticed that, lo and behold, tucked away in a nearly invisible spot on the Mesa 20/20 is a GROUND LIFE SWITCH!

Reassembled the rack - BUZZZzzzzz!. Flipped the newly discovered switch and.. only the sweet hiss of all those class A tube circuits.

Life is good again.

 

[GtrGeorge!]

I know what you mean. I went thru what you did. My Rm4 and the other gear had multiple ground loops. To much ground, and potentials, is my guess. If I pulled the unit out of the rack..while keeping it still hooked upto audio and midi cables..the hum went away.
This was my proof that the rails in my rack were connecting one circuit to the other. I tried rubber washers and that didnt work...parts of the chasis were still touching..and to be honest..thats still not wise imho, i would still bewithin a 1/16 of an inch away from a dis-abling hum at a gig..if it suddenly happened mid-song.
I got my ac wires set up properly and all was/is good. I just have to make sure that there is essentially only one path to ground and the "bad" circuit is forced to use that one...and not try to piggyback to anyother.