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Author Topic: MOSES mpx838 powered mixer output issue  (Read 313 times)

ilyaa

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MOSES mpx838 powered mixer output issue
« on: January 11, 2017, 03:13:57 PM »

hey everyone

working on this solid state powered mixer - main issue is i cant find a schematic anywhere so its a bit hard to troubleshoot it. its having a strange problem:

the output is nice and powerful, but once it gets to a certain volume, it starts to experience a periodic drop - every 1.5 seconds or so, the volume will dip between full power and 1/8 power - almost like a very slow tremolo. ive traced the problem to the power amp - it does not appear in the main preamp out, but does appear throughout the power amp. it only happens with a load plugged in - if i scope throughout the amp, all the way to the output transistors, when its unloaded, it works perfectly. but once a load is plugged in, the power amp seems unable to maintain full power.

now whats hard is that without a schematic i cant quite trace it properly - i was just wondering if anyone has a thought what might cause such a strange symptom?

its a long shot but anyone seen a schematic for this thing around??
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Enzo

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Re: MOSES mpx838 powered mixer output issue
« Reply #1 on: January 11, 2017, 03:57:24 PM »

What load are you using?   What rating is on the amp?  My Mackie subwoofers will motorboat on a 4 ohm load, but not the specified 8 or 12 ohm load.
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ilyaa

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Re: MOSES mpx838 powered mixer output issue
« Reply #2 on: January 12, 2017, 03:19:33 PM »

whoa interesting result -

it says 4 ohms on the back - but it motorboats into a 4 ohm load. into an 8 ohm load its all good!

it says 250W into 4 ohms on the back, but there are two jacks per side. i was plugging one 4 ohm load into just one of the jacks on just one of the sides.
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