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Old 22nd September 2007, 01:59 PM   #1
dpaton is offline dpaton  United States
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Default Another dead MTX 500D

Yet another dead MTX500D on the forum....

Symptoms: +60ish VDC on the speaker outputs ($5 meter reading, I'll be checking with better things on Monday), no response to audio, and the same burned Q61/Q2 and Q62/Q3 pair as some other folks have had. Oh yes, C265, one of the 220uF/25V (preamp power supply filter?) caps blew it's insides all over the inside of the case. It's unhappy. There's also some collatoral damage to a few resistors. I expect these are common failures as well, since everything seems to be designed to collapse together when a failure occurs.

I just tore into this one last night, so I haven't gotten very far on the troubleshooting yet, but what I was really hoping for was that someone would point me toward a schematic for the amp. MTX was (as expected) a negative amount of help, and Google let me down as well.

My business card says "Senior Hardware Engineer", so I'm not afraid of this thing, but troubleshooting blind annoys me more than slightly.

Pictures of most of the damage below.

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Old 22nd September 2007, 07:53 PM   #2
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Join Date: Nov 2003
Location: Louisiana
This amplifier may be VERY time consuming to repair. When the regulators fail, they often damage multiple op-amps in the audio section. Sometimes, it's easier to replace all of them.

It's likely that the 150 ohm resistors between the regulator transistors have opened. I'd suggest replacing them with 330 ohm 1 watt resistors. Phoenix passives (digikey) makes miniturized resistors that will fit. The PPC330W-1CT may fit. Check the dimensions on the datasheet before ordering. I install them on the other side of the board. The higher value resistor drops more voltage before the regulator transistors which helps them run cooler. Installing them on the other side of the board helps by removing one source of heat from the area of the regulators. The regulator transistors still run hot but they will be less prone to failure.

When you get the regs repaired, it's likely that at least one or two op-amps will have failed. Sometimes they fail shorted and destroy the new transistors if you don't monitor the reg transistor's temperature when you initially power it up. Sometimes the op-amps produce no audio but don't draw excessive current and sometimes, they'll produce distorted output.

After you get the audio through the op-amps, you'll need to troubleshoot the audio output section.
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