Mark I Transformer Hum

DGNGuitars · Nov 21, 2011

Hello all. A friend of mine brought me his Mark I from 77 to see if I could get it up and running. I build custom guitars and do high end repairs on all stringed instruments but generally stay away from regular amp repair, only for good customers. I have a basic knowledge of tube amps and can generally get most Fender/Marshall types up and running really well. I also built my own Marshall Type amps that I gig with on a regular basis. Since I am not too familiar with Boogie amps, I was hoping some of you would be able to help me out.

First off, it came in with half of the tubes missing. I put in a full set of new tubes and because of the age, I did the filter caps and bias cap as well.

After I finished the work, I fired up the amp. Immediately I got this low pitched hum. I shut it down after a little bit and checked around the power section. Sure enough there was a cracked grid resistor. I replaced both grid resistors and fired it back up again after carefully checking over the rest of the amp.

I can hear a faint hum when just the power switch is on, and then when I flip the standby the hum comes straight through the speaker. It is lower than it was before I swapped out the grid resistor though, but still there. It sounds like PT hum or heater hum, but both the 100ohm resistors on the heaters are fine. Also, I am sure the power tubes are still ok because the hum is there will no power tubes installed, amp on standby. The hum is also still there with the power tubes in and all of the preamp tubes pulled.

Anyone have any ideas? Does it sound like a bad PT?

Thanks in advance for any help you can give.

Dan

dgr888 · Nov 22, 2011

If it has reverb check out my post on the fix for it. These are notorious for having the tanks installed backwards.
dgr888

DGNGuitars · Nov 22, 2011

Thanks for the tip, I'll look into that. Would that make a difference if the tank is unplugged though? I do hear a small amount of hum when I turn up the reverb but it is higher in pitch.

Today in a regular shop setting the low hum isn't that bad at all and even when playing very low it can't be heard. I put the amp through its paces and it absolutely screams with nothing funky happening. I have read that these amps can be on the noisy side. Maybe that is the case here.

Any other suggestions?

chupacerveza · Nov 22, 2011

Is the ground switch centered?

Here's the link you need to troubleshoot these puppies:
http://www.mesaboogie.com/manuals/Mark I Maint-Repair.pdf

Best of luck, let us know what you find!

--Robert

DGNGuitars · Nov 22, 2011

I think this may be a case of nature of the beast. I friend of mine was in the shop today and said 2 out of 3 of his Mesa Boogie amps have the same quiet hum. Like I said in my last post, other than that, the amp absolutely screams.

Robert, thank you for that link. I had stumbled upon that last night in my searching and it was very helpful. I know some amps are noisier than others, this may be one of those. The reverb could certainly use the upgrades and mods because when I turn it up I get a little of that buzz.

Basically, all of the voltages are correct, the bias is dead on and everything looks good inside. Since my friend hasn't had it running in close to 15 years, I doubt if he will remember if it was slightly noisy or not. I'll have him play it am see what he thinks.

By the way, I think I am feeling a Boogie coming my way in the future. I have never really played through one before this (with the exception of my friends, but his was preset to crazy crazy distortion) and it was a lot of fun. Tonally overwhelming from clean to paint peeling. I think it will be a nice contrast to the JTM-45 1x12 combo I play through now. We shall see. I have been on the fence for over a year, it may be time to jump off and grab one.

SonicProvocateur · Nov 22, 2011

My Mark III is dead quiet in my wall outlet and makes just the slightest hum when plugged into my Furman power conditioner. At first I thought it was the amp, but until I took it to a friends house and it hummed like it was about to explode. Just something to do with the grounding . Surprisingly, the Furmans power regulator system makes the Mesa generate like ~1db of hum, but my wall outlet is perfectly quiet. Weird, but the added protection of the Furman is worth it (it's a older unit, not a newer one. Kinda wish I had splurded on the Monster one now lol)