MTX 1805 repair and resistor ID - diyAudio
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Old 3rd June 2012, 04:32 PM   #1
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Default MTX 1805 repair and resistor ID

I believe that there was a mechanical problem that caused my amplifier to plop. As one can see, one of the FET's literally broke in half, and I believe this was caused by the heatsink being screwed in too tightly by the factory, and when a sufficient force (i.e. boat moving) it caused it to crack and short the system. These two transistors and R1013 and R1014 are the only components visibly damaged. I have ordered all new transistors, but I cannot figure out where to buy the resistors. I believe that they are 47.5 Ohm resistors, and if anyone could point me in the direction of where I can buy some and how to remove the old ones / apply the new ones (if I just de-solder them / how I solder the new ones on, I'm only used to wire resistors,) it would be much appreciated. I am competent with an iron, and I hope to fix this amp instead of paying the surely-ridiculous price to get it repaired by MTX. Also, there is some sort of paste around the transistors, and I was wondering how I should go about replacing that (i.e. what to get and how to apply it) or if I need to at all?

Thanks,
amdz


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Old 3rd June 2012, 06:30 PM   #2
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Join Date: Dec 2005
Location: Northern California
You should go to Perry's web-site and read up on his tutorial it will tell you much of what your asking about and you can do it at your own pace. I have cut and pasted his links below:
Hope this is OK Perry ?
Links >> Basic Car Audio Amp Repair --- Basic Car Audio Electronics --- Basic Transistor Testing --- Basic Switching Power Supply Design --- Basic Computer Skills << Links


Your fets are blown and they failed electronically not by excessive mechanical clamp pressure reasons. They can explode and fracture just like what you have pictured when they short out inside due to extreme gas pressures when they short internally. What you have pictured is typical failure mode, BUT power supplies don't fail by themselves and or alone so you likely have one or more damaged channels which caused this condition to manifest itself like you see it. You may be looking at a symptom of whats wrong and not the whole failure based on my many years of doing this.
I suggest Perry's site as a quick reference training guide to help you gain a perspective on your gear failure. His site is the best I know of and if your into electronics it will teach you loads about this type of gear.

Please replace any silicon heat sink compound when you replace those blown fets without it the voids between the mating surfaces will allow for very poor heat transmission and the devise will fail again from heat issues. That is what that white stuff is and why it is there, and this amp uses Sil-pads and they require somewhat stiff clamping pressures to work properly so the clamps were supposed to be tightly screwed down....hope this is helpful...


PS:Oh any parts and materials like heat sink compound SMD resistors etc.. can be found at any major electronics distributors and even at your local radio shack in some cases...

Last edited by 1moreamp; 3rd June 2012 at 06:37 PM.
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Old 4th June 2012, 02:41 AM   #3
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Location: Louisiana
If you need the value of the resistors, contact MTX. They will tell you what they are.
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