Got this JL Audio M400/4 mobile (Marine) 4-channel Class-D audio amplifier in non working condition, owner purchased a new one and passed this one on to me. Designed to work from a 12V (14.4V) supply, and specs are 75W RMS x4 @ 4-ohm. Supposedly pretty decent brand that has a reputation of good quality build...
https://www.jlaudio.com/products/m400-4-marine-audio-m-series-amplifiers-98271Problem with this one is no audio output at speaker terminals, and scoping those terminals shows a slight DC offset of +0.3V along with a 1.7Vpp about 462KHz signal which I would think indicates that the amplifiers are oscillating. An input of a 1KHz test signal and/or changing any of the input switch/level settings has absolutely no effect on the output.
Found considerable ripple on several of the DC rails (+/-5V, +/-15V) and so far found and replaced a couple of dried out (85-C !!) dead caps -go figure using those inside a hot mobile amp. The main inverter output (based on TL494/KIA494) which is +/-25V seems clean, and I tested one of the caps in the filter bank just to get a sense of their condition, and it was ok for both value and ESR. At least those are 105-C, but brand is "SAMXON", and they seem to be getting very slightly warm to the touch (around 30-35C) so must be struggling a bit with the switcher output although for now they appear to be getting the job done.
I am looking for schematics (or the service manual). Manufacturer website does not provide anything further than the owners manual, and web searches also don't come up with anything else but same manual or sales brochures. Does anybody know where I could get the schematics of this amplifier, or even of a similar model of the brand which I could at least use as a general guide. All circuit boards are 3-layer (with heavy ground plane in the center layer) with plenty of jump connections between layers, and most components are SMD (except filter caps among others), so its not easy to try figuring out the circuit by looking at the board.
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