Questions with MESA 2/90 poweramp repair...any help is greatly appreciated!!!

[Djentleman]

I apologize prematurely if my questions and struggles highlight a lack of experience...but i'm a quick learner I picked up this faulty MESA 2/90 poweramp for deal too good to pass up and needed a new project to occupy my downtime. The seller claimed he had smelt burning plastic after a gig, shut it down, and opened it up to find a burnt connection from the power trany to the pcb. It then sat in the closet for ~2 years before he decided to sell it. I'm somewhat familiar with these amps and the plastic quick release connection from pt to pcb is notorious for failing, and is a straight forward repair by hardwiring those leads straight to the pcb. Sure enough thats the problem. Now i've exhausted many options in trying to lift the pcb to work on this thing, and just can't find a good way to do it that won't require desoldering an army of connections. Literally every other connection is hardwired except the quick release from the pt, and there's connections shooting all over the place. After some days of trial and error, i decided the easiest plan of attack would be to just keep the pbc where it was and work from there. Now I was able to remove the pins and connector from the board fairly cleanly. However the pin connection that failed in the plastic (P12 on the board) also burnt the pbc a bit. I understand that it is possible the pad underneath has been corroded as well and could need a touch up?? It doesn't seem chared to badly, but i'm not sure how to tell if the pad would need to be fixed by looking at the top?? If anyone has more experience with this and can give me their opinion from my pictures on whether or not the pad underneath would also need attention, id greatly appreciate it. After a three day headache trying to pull the board, I would be overly relieved if i can finish this repair without even having to do so. Although if anyone has worked on one of these 2/90's before or a similar layout and can shed some light on a good approach to remove the board, i would also greatly appreciate it. Also one of my pictures points to the plastic mounts holding the pbc to the chassis. Has anyone had any experience with these as well?? I thought these would be easy to disconnect but they've been surprisingly stubborn...not sure if there is a better tool/method to it other than pliers and cardboard spacers, although I may just be struggling with the cramped workspace in this amp. Anyhow sorry for dumping out a wall of text lacking any sort of direction, but would love to hear others' advice!!

[Djentleman]

more pics

[eleventeen]

Some pretty beefy iron there.


The plastic board standoffs..you seem to have the right idea. You have to compress them w/a needlenose, pull the board up just a little so that the tooth stays compressed, and work your way around the board getting them all compressed.


With regard to being able to lift up the board to see the underside; perhaps you can unscrew the back panel, lift it up and over the insides, and then hinge the board upwards. (The plastic standoffs need to have been released) You leave all wires connected, though you might have to take off a cable clamp or two. It depends upon how the sheet metal of the chassis is built up/assembled. If you can do something like this, you can leave all the wires connected to the output tubes and hinge up the back panel and the board.

[drew]

It looks to me like the last green wire at the burnt end of the connector must be linked by the PCB to the yellow heater wire that comes out of the PCB (although no trace is visible; maybe it's on the under side of the board?).  Since soldering the green wire to the messed-up looking hole might be problematic, is there some alternate, electrically sound, way of connecting the two wires?  I don't suppose there is any room to mount a little tag strip?  Is a splice ever acceptable? 

Thread which discusses this issue and has a schematic:  http://forum.grailtone.com/viewtopic.php?f=5&t=53917

Edit: if you do end up removing the PCB, you should replace all the electrolytic caps while you have it out.

[PRR]

> Some pretty beefy iron there.

It's a 180 Watt box.

The green wires go to the heater pins on the nearest 6L6. Just stretch/extend them.

You still have to heat the little tubes on the PCB, I guess. If you can't loosen the PCB enuff to work under, then identify the right pins on any 12AX7 socket (remember you are looking at the top) and butt-solder to the socket legs. Just need to get one; the rest will get power through the PCB.

Eight 0.9A loads plus some small-stuff was in-SANE for these type pins.

[Djentleman]

Thanks for all the replies and advice!! This thing is some seriously beefy iron haha. I don't have a car at the moment and had to meet the seller about 20 miles from my place. I ended up taking two buses and walking 3/4 mile holding this ~45lb brick in my arms the entire time to get back home..when i finally made it my arms wouldn't stop trembling for the rest of the day!

@ eleventeen   the back panel is stationary unfortuanately...i tried pulling all the sockets through the panel, but that too proved to be a mute option. I was able to compress all the standoffs and now the board has some movement but it seems to get wedged between the power tranny and the line from the power cord when i try to hinge it into a workable position. I'm working on getting the leads from the power cable into a workable spot though to solve this. Thanks for the input!

@ drew   I found a post confirming this that has been quite useful as well. http://www.ultimatemetal.com/forum/backline/679742-mesa-2-90-opinions.html
I don't think i have proper space for a tag strip although i'm still toying with that option. I'm gonna try and stay away from a splice connection, but last resort maybe..although even as PRR noted i'd still have to run a lead to the 12AX7 socket. Thanks for the input!

@PRR    Cool i'll think about trying this if all else fails in removing the board. I was able to get some shotty pics of the board underneath when moving it around and from what i can tell the pad from the failed pin looks to be intact. I'm going to clean all the old solder from the connections tonight and try to get a better picture of this. I'll upload a picture of what i have right now and another with the cleaned joints when i can get around to it. This was definitely a design flaw on mesa's side..especially considering this is the only connection of this type on the entire board. Thanks for the input!

[Djentleman]

Also if anyone has a good way to remove this plastic mount holding the power cable to the chassis, i'd love to hear it. So far its been another struggle..

[sluckey]

Squeeze it hard with a strong pair of needle nose pliers from outside the chassis and wiggle until it comes out. It looks like this...

[Djentleman]

This repair is nearing it's end thanks to all your help
@sluckey Thanks for the heads up, much appreciated!! I pried that damn sucker off but still couldn't budge the pcb into a worthy position..

I've got all leads hardwired except the initial problem lead. It was rough starting out, and i ended up dismounting my second solder joint (pt to fan)..although i picked up a smaller iron and luckily found an easy bypass for the newly made crap solder. I'm gonna do as PRR noted and essentially run a 3-way splice in shrinkwrap with heater lead from power tranny (green), lead from pin 7 of closest 6l6 (yellow), and added lead to pin 4/5 of closest 12ax7 (i'm still awaiting mesa supports confirmation that 4/5 is where i need to go apposed to 9 but i'm pretty sure i was able to follow the  under-path correctly).  Anyhow here's a pic after removing old solder and another of how the board sits currently. Imma try and get to ace or homedepot today to grab some shrinkerdinkers and hopefully mesa confirms my inquiry asap. Will keep post updated once i get my shit together!

[Djentleman]

Thall

[drew]

Since you don't necessarily know that the melted connector was the only problem this amp had, before you fire this thing up, make yourself a light bulb limiter, a/k/a dim bulb tester, if you don't already have one.  Instructions for simple or fancy version at the beginning of this PDF: http://home.comcast.net/~seluckey/amps/misc/Amp_Scrapbook.pdf . 

If you don't hear back from Mesa, you could just use your meter to check for continuity between the green wire that is soldered to the board and either 4/5 or 9 on the preamp sockets.

[Djentleman]

I had a rather busy week with school but have finally finished this repair!! Mesa's tech support went above and beyond in getting me some original printouts of the 2/90's pcb so that i was able to finish this project stress-free When cleaning everything up i found a hairline crack in one of the 12ax7's (PI for channel A) so just left it off the board for now. Also i noticed a pad leading from one of the caps is lifting itself from the pcb so i'll probably get around to stabilizing it within the next couple days. I tested it a bit in mono channel B with only a few effects in the front but the amp sounds amazing..I can't wait to match this box with a proper high gain pre!!

@drew  Thanks for the heads up! I got a little too anxious once i had it back together to put off testing it. Also this fried connection is a very common problem on these amps, and only under extremely rare cases will it be a byproduct of a more severe problem. The Dim bulb tester looks pretty handy and i'll hopefully have some free time tomorrow to head out and grab some parts to build one.

[Djentleman]

pic of the lifted pad..

[Djentleman]

Also to note, i was looking for a 3-way butt splice but came up empty handed. I ended up lightly soldering the two heater leads together and crimping them in a female bullet end (14-16 gauge), and then crimped the 6.3vac lead from pt in a male bullet end (18-22 gauge) for the connection. Everything fit nice and snug and i'm pretty happy with the way it turned out. Thanks again to everyone that has posted and helped me through this project, you guys rock!!!

[Djentleman]

Post repair has been nothing but smooth sailing. I dragged out my old mp1 to pair with the 2/90 and must have unlocked a bruatal tone cheat code or something, as I've never heard my mp1 sound this good!

[misterbrown]

I know it's over seven years....but you mentioned getting schematics, are they clearer than the ones floating around online?

[Rontone]

https://el34world.com/charts/Schematics/files/Mesa_boogie/Boogie_290.pdf

Is this one better for you? its a bit bigger than the one on google image search

[misterbrown]

Thank you Rontone, this reminded me to upload the legible half of the schematic that MB sent me, sadly I don't have the section for the output side, which is what's needed.

[Rontone]

https://www.audioservicemanuals.com/m/mesaboogie/mesaboogie-290/8596594-mesaboogie-290-stereo-simul-class-power-amp-schematics

This one is a little clearer on some of the values