Quest Amplifier/Crossover Module Heat Buildup

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Soundtechguy

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I recently performed some extensive repair work on a pair of Quest speakers and found a few noteworthy issues common with this product. I felt it was worth mentioning to help inform current owners (or someone looking to by a used pair) to be on the lookout for these inherited problems.

One thing I found was a pair of ceramic resisters that can generate a lot of heat and potentially damage the crossover circuit board. If you ever remove the module from your speakers, these two guys are easy to find as they are the only two mounted "underneath" the circuit board (all other surface mount parts are soldered to the top side). I assume Martin Logan opted to mount these two resistors to the underside of the board to keep them away from the acoustical batting used with the subwoofer cavity shared by the PSU/xover module (as these two resisters can generate some heat). The problem with that idea is that sandwiching these resisters between the printed circuit board (PCB) and the steel rear panel of the module allows for serious heat buildup which can potentially destroy the PCB. Here is a good photo of what can occur:

Resistor.jpg

Good idea to take a look if or when you remove the rear panel to see if you have an issue. Generally, this sort of thing is only a problem for those who like their music loud or have overdriven the product. If you are a conservative listener... probably wont be a problem. In the case pictured here, the damage was already done. I merely jumpered the damaged circuit board tracer with a wire designed to withstand high temperature. The PCB is "though hole" soldered so these two resisters could essentially be moved to the upper side of the board if desired. The advantage there is that a technician could make some standoffs and raise the resister well above the PCB for better ventilation. The only thing you would have to ensure is that the acoustical batting stays away from this area so it would not melt or burn.
 
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Afaik there is no amplifier in any ML before 2005, so I'm not sure what you're looking at. Perhaps it's the ~1 ohm dummy load that got overloaded by over-driving an ESL with bad PS/no HV?
 
Sorry... I referred to an amp / crossover module when maybe the proper term is power supply / crossover module. I said amplifier only because of the step up transformer power supply design of this unit which controls the electrostatic panel. Call it what you may... this photo is of the passive crossover network. I believe the excessive heat was caused by the previous owner elevating the volume to compensate for the loss of efficiency of the ESL due to the failure of the interconnects to the panel (as described in another post). Martin Logan is aware of these given situations and my post is here to aware everyone of what can or may have happened if buying used Quest speakers.
 
Gotcha. It's a good reminder for everybody to conduct some basic checkups on the inside of their ESL. Stuff will loosen under gravity and vibrations.
 

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