Mesa Boogie Nomad 45 tube amp repair question - Page 1
Forum section: electronics / repair

author: Planobilly
author_rank: Regular Contributor
timestamp: March 03, 2016, 01:25:35 am
content: Hi Guys,A friend gave me a Mesa Boogie Nomad 45 chassis. The amp is very clean, almost new looking. The idea would be to build a case and use it as a head. The amp is not working so I would have to repair it. It needs a set of tubes...4 EL84's and 5 12AX7's. The main filter caps test OK. The bloody thing has 23 pot controls...3 channels and I have no idea at this point what is not working on the amp.It looks like it is a fairly complex amp and the schematics I have are not easy to read. The PCB does not have component numbers as such...almost less trouble to not even use the schematic...lolhttp://www.thetubestore.com/lib/thetubestore/schematics/Mesa%20Boogie/Mesa-Boogie-Nomad-45-Schematic.pdfTrying to make up my mind if I want to tackle this projectAnyone ever worked on one of these amps?Cheers,Billy

author: Planobilly
author_rank: Regular Contributor
timestamp: March 03, 2016, 02:36:51 am
content: UpdateWell, after screwing around with the amp for a bit I have it up and running on at least one channel. The output sounds OK. I don't like the way it sounds much but I have not checked the bias yet or really dug into the amp. Perhaps this will not be as big a deal as it first looked. Mesa amps don't have a bias control and use tubes with certain values. I may mod the amp and create bias control...not sure yet how I want to deal with that issue.Hope someone here has some experience with this model of amp and can give me some ideas.Cheers,Billy

author: OilsFan
author_rank: Regular Contributor
timestamp: March 03, 2016, 05:22:26 am
content: I can't offer much in the way of help but that complexity is what made me quit using Mesa's. Give me a good old Mk III any day. You can tackle it though because it still is just a tube amp when you get right down to it. All the requisite warnings on high voltage etc of course.

author: Anks
author_rank: Frequent Contributor
timestamp: March 03, 2016, 07:01:40 am
content: I've got some experience repairing these. Even made a custom foot switch as the original is hard to come by and expensive in the uk. I was going to add midi control and even added the footprints for the opto's on the pcb.As tube amps go yes it's a bit of a headache getting your head around the switching. You just have to try and look at it as separate preamps.As for the bias the ones that I retubed all got a bias pot added. So if you need help with that let me know.Keep us updated and I'll try and help in anyway I can.

author: Planobilly
author_rank: Regular Contributor
timestamp: March 03, 2016, 04:23:24 pm
content: Thanks Anks,I messed with the amp for a few minuets this morning and have it up and running on all three channels. It had a small amount of hum and I fixed that issue. The EL84 are running at 35.2 ma, 35.6 ma, 33.4 ma, and 38.0 on the last tube. They should be running at around 28 to 30 ma. These are new JJ's but I did not buy them as a quad set so I will buy a set of better matched tubes. JJ did some R&D into El84's that had lower plate dissipation which I may like to try. The normal El84 has plate dissipation of 12 watts and the new tubes they are making are at 9 watts. The idea being that they will break up at lower volume. I had in mind to use this amp at home and don't want or need a loud amp.I am running it into a Marshall stack with four Celestion speakers at 8 ohms at the moment . Perhaps not the best choice of speakers for this amp...don't know..I have no idea what the amp sounded in it's original combo case or even what speakers Mesa normally used.Got some research to do...lolFor sure I would like to know how you went about installing bias pots.The tubes are running at around 260 F or 126 C which seems a bit hot. I have seen some other EL84 based amps that run hot as hell and having no experience with this one I have no idea what is normal. The JJ data sheet does not give a max temp for the El84's. I have not measured the plate voltages yet. The chassis is not grounded to the PCB so I need to find a good grounding point. Input jack I guess.I need to read the owners manual really well as the controls and their interactions are a bit complex for sure.Are you using the original United Chemi-Con 220 uf 300V filter caps? I have not checked the ripple or how much AC they are leaking if any yet.Update: I have everything working on the amp now...sounds OK...300V on the plate of V3 which seems a little high.Looks like a good tune up and build a foot switch and find a reverb tank and this beast will be ok..Well... there is the issue of building a cabinet to put it in....oh wellBilly

author: Anks
author_rank: Frequent Contributor
timestamp: March 03, 2016, 10:33:05 pm
content: Hi yes these run the el84's hard and on the edge with the plate voltage. To be fair your tubes don't seem to far out of balance.I'll have a look through my things tomorrow and let you know how I installed the bias pot. If your interested I'll send you the files I have for a custom foot switch but it requires programing a pic. I never got round to implementing midi but is shouldn't be to hard if your into that.Anyway have fun with your new toy.

author: Planobilly
author_rank: Regular Contributor
timestamp: May 13, 2016, 11:33:35 pm
content: UpdateI have rebuilt all the power supplies. Replaced all the electrolytic caps. Replaced the two 15V zener diodes for the low voltage supply. After that the record out and headphones out started working.Replaced the two op-amps and the IC that controls the LED'sReplaced all the relays except RY2...Screwed up and only ordered 6All the channel switching seems to work both with the new foot switch and without. All three channels play and sound different.No EQ control on channel 2 but the gain control works, also vintage/modern switch does nothing.I could use some help trying to figure this mess out....lolCheers,Billy