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Multivox MX 3000 repair

Posted: Tue Jan 03, 2017 7:44 pm
by miaomiao
I got a Multivox MX 3000 from ebay and it seems to have a short circuit between the positive supply and GND. After disconnecting some cables the short circuit was gone and I couldn't measure it on any of the boards, so I guess it must be a wiring issue. I've got a service manual from, but it only has schematics of the boards.

Does anyone have a same/similar issue with this synth? Could someone, who has a working MX 3000 share a picture of the wiring connection under the front panel and behind the main boards and also on the jacks.

Re: Multivox MX 3000 repair

Posted: Mon Jan 16, 2017 10:33 pm
by miaomiao
It works!

I checked all the connections according to schematics, disassembled it and put together again. Except cleaning some dirt off the PCBs in the bottom and using plastic washers to avoid contact between PCBs and screws I didn't do anything. Now only the VCF of the poly section is silent. I guess it is a faulty ca3046 transistor array IC. The keyboard contacts may need some cleaning too.

All I read about Multivox' build quality seems to be true. PCBs have soldering tin spills everywhere and the soldering quality is quite poor. It looks like the PCB layout was done before any prototype was built, so during the assembly process they needed to scratch off some leads, use wires or solder some components directly to each other. Luckily the hand drawn schematics I got seem to correspond with the the assembly very well.

It was a good experience to study this synth: all discrete components, every section of the analog synth (VCO, LFO, VCF, EG) is build on a separate PCB. It seems the synth I got has never been serviced before: all the screws still had their original sealings. The condition is surprisingly good for its age. Even the sliders don't show any signs of wear.

Re: Multivox MX 3000 repair

Posted: Sat Sep 22, 2018 5:58 pm
by jigsaw
I've had one of these in my attic for 15 years. Just pulled it out to test. it definitely needs some work. At first it didn't make any sound, and the presets lights were hard. I gave it a firm smack on the top and it came back to life. Only output 2 is working. there's some crackling and such when turning the rotor on and off and a few other anomalies and instabilities. At times it can be fairly stable, I'm not sure if it's worth paying someone to work on it, but i'm evaluating