Mackie 408m half loud

[Sowhat]

Schematic: [ATTACH]46097[/ATTACH]

Hey, thanks Jazz P Bass for thinking of me... Unfortunately, this one is not the one either... bummer! This is the Class H 808 series amp board, I've already taken a look at this document before. I got the Class AB 408 board. It probably has the same front end stages as the class H one but the reference designators don't match and maybe other things as well, especially the further towards the back end you go, but, once again thanks.

[Sowhat]

Dude! Great idea and yes, this amp has the crowbar circuitry on the output board. Here's the bad news: the crowbars are perfectly OK, no problem there... I really wish that was the problem, but it's not. I just simply unplugged the terminals going to the board from the amp and read the signal right from amp board directly bypassing the output board entirely... the problem persists, but elsewhere.

I took a reading from the first stage op amp U3A's output (pin 1) and I got signal, nice and strong amplified signal, exactly the same as on the good amplifier.



I next checked what I think is the driver transistors and here I definitely have signal attenuation as compared to the good amp.



So... the problem lies between these two points, just about 2 inches of tightly packed SMD stuff... it could be a leaking chip cap although I don't think those leak too much, more like go dead short or open usually but what's their value? who knows, they got no markings. Maybe it's a bad diode but is it a regular diode or a zener diode or whatnot, once again who knows. A resistor could have opened, shorted or drifted and those do have value codes, I guess I could tediously check through all those. Then there's all the silicon, at times hard to check in circuit without a road map, once again what's the part number, who knows, no markings. There's no way I'm going to lift components from the good amp to measure values... I would probably wind up with two broken amps, not just one.

This unit is store stock so I informed the boss that he got a lemon on his hands and he may benefit from leaning on his Mackie business contacts for resolution. Either get me a schematic or have Mackie fix it.

[Jazz P Bass]

Maybe this will help: sch_204aamp.zip

[nickb]

Thanks for the scope shots

I don't see any of the aforementioned horrible crossover or clipping distortion. This looks like a simple gain error. The first thing I'd check is the feedback network and in particular the capacitor to ground. For example on the 408 this would be C60 47uF on channel one. I'm looking at page 1 of the pdf that was in post #15 from JPB.