Alesis Multimix Hum Repair

I bought this little Alesis firewire mixer for home use and recording multiple tracks to a Macbook. It has served me well over the years. The sound quality is decent for its price point.
Over the years, the overall level of hum has increased to a point where it became too noisy at idle. There was also an annoying 1Khz whine coming from the unit.

It is worth mentioning that this little mixer gets quite hot when powered on. The power supply regulators are attached to a heat sink that is bolted against the chassis. All the power supply capacitors are very close to this source of heat, which is not a good thing as electrolytic capacitors degrade much faster when the ambient temperature rises.  The entire unit has no ventilation, whether is be active (fan) or passive (convection). Everything is sealed, a mistake that Alesis has fixed on later models.

 

Alesis power board

 

Its pretty clear that the surrounding heat did these caps in. They are low end caps (Lelon and Pamicon, rated @ 85C). The main 2200uf Pamicon cap is definitely out of spec with a dissipation factor of 0.6 and capacitance of 1’435uf (35% off spec):

Alesis Pamicon main 2'200uf capacitor

Alesis Pamicon main 2’200uf capacitor

The solution is to replace all the supply caps at once. The caps have had a tough life in there and are all nearing their end of life. I replaced all of these with caps rated @ 105C, given the design of this power supply.  I went with Panasonic electrolytics from the FR and UHE, rated for a life @ max operating conditions of  between 5’000 to 10’000 hours.

Alesis old caps

 

 

The new caps all installed and secured with a spot of hot glue:

Alesis power fixed

 

The 1 Khz whine that I could hear, even with nothing connected to the unit, was caused by a weak connection between the power supply board and the board next to it, that houses the DSP chip. I cleaned the connector contacts and re-flowed the solder.

 

 

 

Re-assemble and test. This thing is back up and running as it did when I first got it. To make sure you get the most life out of your Alesis Multimix, make sure you turn it off when not using it.

Alesis MM 8

 

Posted in Electronics Repair, Repair Work