Fixing a Lexicon MPX-1

The MPX-1 is a good quality stereo multi-effect from Lexicon. I was chasing one at a descent price, and was lucky to find one cosmetically perfect for half the usual price for such a used item.

The reason: the unit shows a constant hi-level noise on channel R (channel L is clean). Is this easily fixable?

Step 1: Is the noise coming from input, output or audio processing?

In Audio Menu, select audio source: left channel, right channel or stereo.

No change -> input stage is clean on both channel

Then, with a scope, measured output of both DAC: clean -> processing stage is OK

Going down the audio chain, we'll find 2 opamps: one as amplifier, the other for muting. Scoped both, no noise.

The last stage of audio is a bipolar 10 uF capacitor (C92) that removes the DC componant from the signal. Scoped both sides and bingo! Noise was clearly visible on one side of R channel.

I changed the capacitors on both channel (C92 and C101). Make sure to get bipolar capacitors from Nichicon, high quality and cheap (30 cents/cap).

Summary: a now perfectly working MPX-1 for less than 1$

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