Why the MZC Dies

The Good:

Over-designed amplifier

​The SpeakerCraft MZC series amplifier/zone controllers are overall robust.  The amplifiers, especially, are over-designed, with component selection erring on the side of over-design.  The IC chosen for the MZC-66, for example, is rated by the supplier at 100 watts/channel (Phillips TDA8920), where SpeakerCraft has limited it to 30 watts/channel.  The amplifiers also have short circuit and power supply protection that automatically shuts the amp down instead of burning it up.  




Solid power transformer design

The power supplies use heavy, and expensive toroidal transformers, which are regarded in the industry as a superior design because they introduce very little noise to adjacent circuitry.  This helps to assure clean audio.








Proven voltage regulator design (mostly)

The power supply also uses linear voltage regulation for the pre-amp/matrix board, FM tuners (MZC-88), etc.  These are isolated to reduce noise being introduced to the audio from the 'noisy' CPU and other digital components of the MZC.  Linear voltage regulators may be 'old school', but they are reliable and have a number of built-in safeguards to prevent failure...as long as they are not allowed to get too hot, or are not heavily overcurrented or overvoltaged.





The Bad:

Poor heat elimination from case

The weaknesses in the MZC's frequently center around design choices that produce excess heat. This occurs both in the power supply and in the zone amplifiers, primarily.  Excess heat damages electrolytic capacitors and sensitive IC's adjacent to the heat source, especially over a period of time.  Electrolytic capacitors contain an electrolyte, which evaporates over time.  As long as they are not overheated or operated at the upper end of their temperature ratings, they can last upwards to 20, even 30 years for high reliability versions.  If they are operated at the upper limits of their ratings, they can lose up to 20% of their rated capacitance in just a few months (values from Nichicon VX general grade)


Components placed too closely to heat sources

The first place this heat problem shows up is in virtually all MZC power supplies.  A resistor which drops the supply voltage for the 5v linear regulator is planted adjacent to the output capacitors of all regulators.  It drops the voltage from about 17v to 9v.  This puts about 8 watts of heat energy into a closely confined space, baking the regulators, electrolytics AND the circuit board.  After a period of years, while the electrolytic capacitors degrade to a fraction of their design values, the power supply becomes unstable, lights flash, displays don't display, amps don't turn on, and a number of important components (like the main CPU) can get fried if the unit is left on with the power supply freewheeling.  Beware any MZC that has a flashing power LED, or with tuner that displays blue with no displayed data! 

Zone amplifier electrolytics inadequate circulation

The other place this problem shows up is in the zone amplifiers, particularly in older units that had no circulation fan.  In the MZC-88, the big output capacitors are hidden under a metal shield, which prevents them from getting cooling circulation.  To make matters worse, they are situated close to the amplifier heat sink just as they are in the MZC-66.  In earlier MZC-66's, SpeakerCraft just left all the amps running, and just turned off the preamps.  These units ran HOT.

Other less common issues

Less frequently, there are a number of other issues we run into during our repair/rebuild process that are failure modes less heat related and probably more related to voltage surges or simply component failure for unknown causes.  We have to replace the main CPU in about 10% of the units because the original CPU won't accept programming. 

The pre-amp/matrix board occasionally has a problem with turning on the amps.  This is most often caused by a failure of the shift register IC which takes instructions from the onboard controller, enabling each amp when it is called for.   These are a problem in about 10% of the units we see.

We occasionally see a failure in the pre-amp microcontroller, which is a deathknell for the pre-amp board.  The chip is a common microcontroller chip, but the programming is proprietary SpeakerCraft, and they won't release it.

There are a number of other issues we see with the CPU board, tuners, and pre-amp/matrix board that need good knowledge of how the MZC's work, in concert with good troubleshooting technique can be found and resolved.







Amp IC compared to massive heat sink

Robust toroidal power transformer

Linear voltage regulators in a rebuilt MZC-66

Drop-down resistor for 5v linear regulator...notice scorch marks!

MZC-88 electrolytics bulging from long term heat.  Bulging is a visual indication that they are shot, but bulging is not always present with overtemped or spent capacitors.

What a lightning strike does to chips on the processor board!

Beware what you buy used from somebody that knows little about these units.  This is an example of the power supply failure issue on a recently listed unit from eBay.  The fact that the tuner display is blank is a highly probable indication of the loss of at least one power supply regulator.  If things flash and pulsate like this one, it is an indication of a failure in one of the 12v supplies and the 5v supply as well.  

Click on the image for a short video demonstrating the symptoms of a typical MZC-88 power supply regulator failure.