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Old 13th November 2007, 01:37 AM   #1
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Default MTX 500D help

I just got this amp with a problem. It has mega DC out put on the speaker out puts. I saw that the pre-amp stage fails. If someone could give me the common parts that go wrong?
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Old 13th November 2007, 06:18 AM   #2
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Does the DC on the output vary with the gain setting?

Do you have plus and minus 18 volts on the op-amps (pins 4 and 8)? Use the chassis ground for the reference point (black meter lead).
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Old 13th November 2007, 11:49 AM   #3
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I will check that later when im home, but which chips are the op-amps?
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Old 13th November 2007, 07:10 PM   #4
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The op-amps will be either TL072s or NE5532s. They are the square, 8 legged ICs.

Remember to have the transistors clamped to the sink and have a 10 amp fuse in the B+ line while testing.
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Old 13th November 2007, 07:53 PM   #5
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Perry I have both of them in their there is 3 TL072s and 2 NE5532N I will check those now.
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Old 13th November 2007, 08:56 PM   #6
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Did not get that, Here are my readings. Also I notice alot of hack job resistors and missing one's. Why did the previous repair shop do this?

The 3 TL072CP's were
pin1: -1.6
pin2: -91.7mv
pin3: -1.6
pin4: -2.59
pin5: -1.5
pin6: -1.5
pin7: -1.7
pin8: -2.4

One NR5532N was
pin1: -1.1
pin2: -0.88
pin3: -1.4
pin4: -2.58
pin5: -3.11
pin6: -3.42
pin7: -1.4
pin8: -2.37

Other one was
pin1: -1.41
pin2: -1.41
pin3: -1.66
pin4: -2.59
pin5: -1.66
pin6: -1.41
pin7: -1.41
pin8: -2.36
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Old 13th November 2007, 09:48 PM   #8
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Some of the components were left out by MTX. I've seen alot of amps like these (including the 250Ds) that had resistors jumped from point to point. I believe it was done by MTX.

Assuming that the power supply is producing rail voltage (you said you had DC on the outputs), it appears that the regulators have failed. Generally, when the regulators fail, they destroy virtually all of the op-amps. It's often easier to replace all of them than it is to try to find the defective ones. Sometimes, defective op-amps work but produce slightly distorted output. This type of problem is difficult to find without a scope.

If you want to confirm that the power supply is operating and producing rail voltage, measure the DC voltage on the legs of the output FETs (near the small green toroidal inductor). You should find both positive and negative voltage (~50v?).

If the regs failed, this amp may not be worth repairing. You can buy the 500D used for very little on eBay. If you decide to repair it, it probably won't be easy.
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Old 13th November 2007, 10:20 PM   #9
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(2) were -46.4 -59.4 -59.4 thats across legs left to right
(2) were -58.8 +59.5 -59.5
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Old 13th November 2007, 10:40 PM   #10
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The power supply is operating. The DC is being driven to the outputs because the low-side FETs are being driven on (or so it appears from the voltages you posted).

Near the center of the audio section of the board, you'll see an area that's been blackened. There are two 150 ohm resistors, two MPSW42s and two MPSW92s. The transistors will likely be a slightly different shade due to overheating. These four transistors need to be replaced. You can check them but they're almost certainly defective. Even if they're not, they've been run hot for a long time and should be replaced.

There are two orange/black zeners that are likely defective. They will be connected to the center leg of the transistors.

The filter capacitors connected to the transistors are likely defective and often vent or swell up.

The 150 ohm resistors often open.

Check these components.
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