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Multivox MX 880 monosynth issues

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Multivox MX 880 monosynth issues

Postby open ocean » Sun Nov 15, 2020 12:18 am

Hi all

My Multivox MX880 duosonic has some issues. At first half of the keyboard triggering the same note, the left side. The right side plays normally. Also 2 presets "fuzz guitar 1/2" have lost their attack portion of the sound so the do work and engage but it takes 1-2 seconds for the sound to be heard after playing the keys. Any ideas about what do i look in order to solve these ?
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open ocean
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Re: Multivox MX 880 monosynth issues

Postby Adam Inglis » Mon Nov 30, 2020 5:30 am

Wow, that's a rare synth.
It appears to be chiefly based on the similar Roland model (like so many Multivox products), in this case the SH-2000.

Now, the good news is, this will almost certainly be a fairly cheap and easy fix.
The bad news is, it could take you a long time to find the culprit components you need to replace.
If you have some technical skills and know how not to kill yourself when testing live electronics, you might first want to check that the power supply is providing correct and stable voltages. If that checks out, then you will need a schematic, an oscilloscope, and patience... or you will need a good tech!

The first problem could be the key CV sample and hold op-amp having failed.
The second is a bit trickier. Each parameter in the synth is controlled by a matrix of diodes, different lines being switched in and out by FETs, which are themselves switched from the front panel paddle switches. If one of these FETs has failed, or is "leaky", then the wrong set of parameters can be switched in for a preset - e.g. the "Fuzz Guitar" gets the envelope characteristics of, say, a slow string preset. This why you'll need a schematic - there are a LOT of lines to test!

Sorry I can't be of more help.
Your tech/ you might start by perusing the service manual for the Roland to get an idea of what I'm talking about, which is available online.
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