Mesa Boogie 395 power amp reverse engineering

[Phil smith]

Hey guys!
Long time no see.. Happy New year!

After several years of having this amp I finally pulled it out the rack, cause it suddenly became silent. Short tests showed that was a 5v rail related issue and the problem was simply a 1n4007 diode, that rectifiers 6.3 VAC for 7805 regulator. Without 5 volts input LDRs are open, thus no signal goes thru. After replacing it, the amp came back to life! But I thought it would be great to reverse-engineer this puppy, cause I still can't find its schematic whatsoever.. So, what the heck?!
And after 3 full days and 4 evenings I've finally done drawing. Here it is:

https://drive.google.com/file/d/1bzIvRelnxfL81XCXtqnjrrWVkSMnSQSL/
There are:
1) Different photos of the amp's guts.
2) PCB file, drawn in Sprint layout 6.0.
3) DWG file with schematic.
4) PDF file with schematic.

Also I'm attaching the circuit diagram directly here for the easy of reference.

Later I'll measure all voltages and include em onto schematic as well

If you find something obvious - please let me know.

PS. Lemme tell ya, this amp is a mess... As any Boogie amp - getting PCB out is a HUGE pain in the a... neck - desoldering is mandatory - I guess connectors are not a solution for Mesa.. LOL
One thing, that caught my eye, is channel A shielded cable is tided up together with AC mains input line.. Hmm.. is it strange? Also no AC wires are twisted at all. Of course, there is no instrument level signals, but why, Mesa?

PS.PS. I've been still figuring out the ac mains portion of the pcb layout - so take it with a grain of salt.

Cheers, Phil.

 

[Phil smith]

Hey guys,
I've corrected minor errors in the schematic. The file has been updated.
https://drive.google.com/file/d/1bzIvRelnxfL81XCXtqnjrrWVkSMnSQSL

 

[12afael]

Mesa amps are a pain in the butt to repair but Engl are even worst. I have done a lot of Reverse engineering even in some mesa amps but this is an incredible work. I can imagine someone at gibson using this as reference.

congratulations!

 

[Phil smith]

Thank you, man!
I got one word for MESA Engineering - HEADERS!

 

[john12ax7]

Why are so many PCB amps so ridiculously unfriendly to work on? I have a VHT that is the same way. Who is designing these things? It's like they don't consider serviceability, at all.

Rant aside, very nice job with the schematics.

 

[Rusan]

Wow, I'll second what these others said! You did a fantastic job of reverse engineering, drafting the schematics and labeling the chassis photos. Superb work, very impressive!

I guess connectors are not a solution for Mesa.. LOL.........I got one word for MESA Engineering - HEADERS!

I could be mistaken, but I always figured that was because soldered connections are bulletproof. The parts and build quality of every Mesa Boogie I've seen inside of (all have been older ones) was definitely nice, and obviously meant to survive multiple 100 meter falls onto concrete.

Having said that, I had a rather unpleasant experience with the only Boogie I've ever owned, a 50/50 power amp. I bought it brand new, and was most unhappy when it was motorboating badly the first time I turned it on. It makes me wonder if they even tested it for function.

I called Mesa to let them know they allowed an amp with a defective filter cap leave the factory, and that I could replace it myself. They were quick to tell me that if the amp were to ever be returned for service, the warranty would be void if they found any non-OEM components. The electrolytics were off-the-shelf Sprague TVA Atom and the coupling caps were Sprague 716P, but every single one had MESA printed on it, so they could easily identify if any had been replaced.

I started thinking, "What if a transformer smoked at a future date, and I have to pay their OEM parts prices for a replacement?" So, I bit the bullet and paid shipping costs to the factory, did without the amp for three weeks, AND had to pay the return shipping costs because they sent it freight collect! Having to pay the shipping to the factory was bad enough, but them not covering the return shipping was adding insult to injury. Remember, this was a brand new, unused amplifier.

Not exactly the best customer service I've ever seen.....

I don't know why I didn't think of it at the time, but what I should've done is replace the filter cap and keep the old one. If a transformer ever fried, I could just put the old cap back in and return the amp to the factory. Oh well. It was all an unfortunate situation, because the amplifier sounded superb, once the cap was replaced.

 

[Khron]

I could be mistaken, but I always figured that was because soldered connections are bulletproof. The parts and build quality of every Mesa Boogie I've seen inside of (all have been older ones) was definitely nice, and obviously meant to survive multiple 100 meter falls onto concrete.

Coincidence or not, soldered connections also eliminate the need for several (if not dozens of) BOM items, not to mention cost, so... I'd argue it's first of all cheaper; if it happens to also be more reliable, that's just a cherry on top.

 

[Rusan]

Why are so many PCB amps so ridiculously unfriendly to work on? I have a VHT that is the same way. Who is designing these things? It's like they don't consider serviceability, at all.

My thought is that it's all about whatever is the fastest, cheapest way to manufacture it, not the fastest, cheapest way to service it. Sure seems that way, anyway.

It seems to have gotten much worse (in many different ways) since all the big brand names were bought out by investment bankers. Even simple analog instrument amplifiers just seem to be designed much differently, not to mention the widespread transition from through-hole to SMT. I understand circuit design progresses and evolves, but it seems to be someting more than that.

Additionally, years ago you could call several of the U.S. companies' factory service departments to request a schematic, and they'd cheerfully e-mail you one. Not any more. I clearly remember the first time it happened to me: I had a Crate bass amp on the bench, right after LOUD Technologies (an investment group) had bought Crate's parent company, St. Louis Music.

I called SLM, expecting just another routine experience. But this time, the person asked me a million questions to determine my level of electronics competency. When they were finally satisfied, they first sent me a digital waiver form to e-sign. It stated that neither I nor my family could hold LOUD Technologies liable for injury, death or damages resulting from them supplying me with the schematic. Typical "Big Corporate" legalese paranoia.

That was over a dozen years ago, not long before I got out of music electronics repair. I haven't missed it one bit.

 

[Khron]

That's interesting - i believe LOUD also owns Mackie, don't they? But Mackie have loads of schematics available for download right on their main website, not even some hidden domain that you have to jump through hoops to find

 

[Matador]

Beautiful!

How closely did you trace out the feedback path? In most of the Boogie power amps I've worked on in the past (like the 2:90) this seems to be the area with the most strange networks meant to tailor the frequency response.

This is almost my favorite phase splitter of all time.

 

[Bobby Baird]

I added bias to my strategy 500. It's the beast!

 

[Bobby Baird]

38mA of 6l6 and 6550 destruction!

 

[john12ax7]

My thought is that it's all about whatever is the fastest, cheapest way to manufacture it, not the fastest, cheapest way to service it. Sure seems that way, anyway.

That's most likely it. Super annoying though. It's weird though because then their own service department has to deal with it, but maybe that's an added bonus if charging by the hour.

I try to buy and build things with serviceability in mind. Rather spend a little more upfront to make things easier down the road.

 

[Phil smith]

Thank y'all!

Yeah, i guess Mesa doesn't give a damn about serviceability, especially back in the day (what expect from company, which deliberately has removed bias pots and forced you to buy mesa "special selected" tubes? lol )
They just assembled pcb with pots, switches, etc, installed em in chassis and soldered the rest in place. Anyway, nowadays it's quite a lot of manual labor, especially for being "MADE IN USA", so even mesa started using headers and flat flex's here and there in their new designs.

Beautiful!

How closely did you trace out the feedback path? In most of the Boogie power amps I've worked on in the past (like the 2:90) this seems to be the area with the most strange networks meant to tailor the frequency response.

This is almost my favorite phase splitter of all time.

Thanks!
Oh boy, I've spent 2 days tracing feedback and switching circuits! I'm pretty much confident now

I added bias to my strategy 500. It's the beast!

Strategy 500 is a BEAST! Right on! \m/

 

[Rusan]

That's interesting - i believe LOUD also owns Mackie, don't they? But Mackie have loads of schematics available for download right on their main website, not even some hidden domain that you have to jump through hoops to find

This happened a long time ago, and each of the 3 or 4 times I requested schematics after LOUD Technologies bought SLM it was the same. I could see things being different by now, because if I'm not mistaken it's an entirely different company that presently owns it.

I think they were bought by another investment group a few years ago, that changed the name to LOUD Audio. The Mackie HM-400 I bought two years ago says LOUD Audio in the paperwork, not LOUD Technologies. I think they sold Ampeg to Yahama, Crate and Alvarez to somebody else, and now have Mackie as their only brand.

Oh boy, I've spent 2 days tracing feedback and switching circuits! I'm pretty much confident now

In most of the Boogie power amps I've worked on in the past (like the 2:90) this seems to be the area with the most strange networks meant to tailor the frequency response.

My 50/50 (which of course is a 2/90 with fewer power tubes) had that same crazy NFB network, with more RC filters than a rack full of 1/3 octave EQs. I'm glad I'm not the poor guy who had to do all the math in designing that....

This is almost my favorite phase splitter of all time.

I agree! That 12AX7 PI has no problem driving parallel pairs of 6L6GCs (in the case of the 2/90) through .1uF coupling caps without grid blocking, which is impressive. Marshall avoided the grid blocking problem by using smaller value coupling caps, which also affected the bass extension. The silverface Fender solved the problem with drastically different PI resistor values from the blackface, which also made the amp sound muffled. Mesa solved the problem with some impressive engineering and did it right, as evidenced by the nice high frequency sparkle and tight, extended bass without muddiness.

 

[Erik Bik]

Hi guys,

Still around on this page? I’m wondering if any of you guys figured out the mains part of the 395? I own two wich are both running at 110v but would really like to run them on 230v.. i know there are two jumpers but i don’t know exactly what to do and i really don’t want to ruin them… so since Mesa service centers won’t help i really hope you guys have some more info on this.. thanks big time!!
Best regards Erik Bik (Holland)