New Guy - Mesa Subway Blues... Hum

[RayG]

Hello All from Ray... this is my 1st post.

I have a 1st edition Mesa Subway Blues amp that sounds very good but once it warms up it gets a very audible hum. I took it to a tech many years ago and all he did was change the tubes BUT the problem still persists. I have done searches on the net and understand that the fix is as simple as swapping a .1uf cap for a .68uf cap. I was hoping that somebody could explain the cap change or point me towards a schematic for it.

I would like to take care of the noise problem and give the amp to my son. Any info is greatly appreciated.

Thanks in advance,
Ray
NJ, USA

[nickb]

Welcome!

I think this might be what you mean -- see page 5 http://bmamps.com/Schematics/Mesa_Bo...Schematics.pdf

[RayG]

Thanks so much, nickb!

[Chuck H]

I've seen that circulated. I sure hope it works. I have related experience. It doesn't address the problem directly but it may help, ultimately...

Mesa stopped using those potted boards soon after. I had one of those potted amps. It hummed. This was about 17 years ago. I had bought the amp already used some 3 years earlier so there was no warranty. I called Mesa about it and they said "Ship it to us and we'll fix it free." So I did. They replaced the entire board with a new, non gooped board (fresh filter caps are a bonus) and even added the "fat" switch as a courtesy. Now THAT'S customer service

My point is, call them (707-778-6565). I doubt they're still doing free board replacements on 21 year old amps, but they may have a factory mod solution to share. They've always been very accommodating to me with phone time, faxing/emailing schematics (depending on era), etc. There's nothing to lose and you don't get anything if you don't ask. So ALWAYS ask

[nickb]

Any know what is inside the potted bit?

[Chuck H]

Any know what is inside the potted bit?

They use to be very secretive about their effects loop design. I'm pretty sure it's just the effects loop and maybe some peripheral stuff that happened to be near by.

[Tom Phillips]

Any know what is inside the potted bit?

I had one of these amps pass through my shop in 2003. There was no problem with anything in the potted modules so I didn't attempt to figure out what was in them but I did take photos. A couple are attached so that readers of this thread can see exactly what the guts look like. Notice that there are three individual sections of encapsulated circuitry.
They aren't really "modules" because they are not removable. It appears that the potting compound was poured into rectangular dams placed around the sections of components that Mesa wanted to hide.
Cheers,
Tom

[nickb]

I was curious only to understand what the thermal hum inducing mechanism is. Based on the location of the block in question makes me think it's closely related to the power supply. Increasing the value of a smallish cap suggest low pass filter and so conjures images of MOSFET type series voltage regulation, except the the 250V rating isn't enough. It will remain a mystery, I guess.

[Chuck H]

I might be able to gain access to the amp I had. When/if I open it up again I'll check it out. No goop on the new board, so no more secrets. Randall must have gotten the patent approved so no more goop was needed