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Old 10th April 2010, 03:23 AM   #1
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Default MTX 421D supply issue

Working on a supply that had blown the two 75339 fets. With the fets out of circuit and monitoring the output from the TL494 what I thought should be a nice square wave is not. Could the TL494 have taken a hit?
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Old 10th April 2010, 03:30 AM   #2
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On most of the MTX amps, they drive two PNP transistord from pins 8 and 11 (not 9 and 10) of the 494. Do you have pulses on 8 and 11?
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Old 10th April 2010, 03:24 PM   #3
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I do have pulses, offset a typical amount from one another. I can post a pic, but I don't think it would be needed at this point. Placed my order with fairchild for the 75339's already, should see them tuesday.

I wanted to try a different fet until the replacements arrived, but only having 70n06 didn't prove out very good. The signal on the gates was very ugly. Removed those before I could cause anymore damage. These would work at low gain, if I adjusted to gain up any amount the output would look like noise and the xfrmer would squeal.
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Old 10th April 2010, 05:31 PM   #4
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The PNP driver transistors and the 10 ohm resistors that are connected between the base and emitter of the driver transistors both fail. Check those also.
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Old 16th April 2010, 03:32 AM   #5
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Replaced the blown PS fets and I have the same result. Once the gain is adjusted up (very little) the xfrmer squeals and the signal becomes noisy. I also get 2.5Vdc on the output. Attached are the waveforms on pins 8 and 11of the 494. The first is with gain adjusted all the way down, the next is failure mode. It seems the amp doesn't turn on until gain is adjusted up. I have no output signal until then.

The LM393 voltages don't change in the different states.

Any ideas?
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Old 16th April 2010, 03:40 AM   #6
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The drive signals look close to what I'd expect.The following image is an MTX amp.
http://www.bcae1.com/temp/mtxamp01.swf

What happens if you leave the gain at the minimum but increase the input signal level to get the same overall output voltage?

Is it possible that you have an open shield ground on the RCA jacks?
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Old 16th April 2010, 03:53 AM   #7
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I am using a cd changer for my source, no adjustment. I would have to switch over to the ipuke. That could lead to other problems though.

The ground measures fine, I get roughly 1k to the supply gnd. The rca jacks are physically loose, the crimp portion.

I will give the ipuke a try.
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Old 16th April 2010, 04:13 AM   #8
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If the gain is adjusted or the input level it trips out at the same point. I hooked a load to the output and that made no difference either.
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Old 16th April 2010, 04:58 AM   #9
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I should mention with the outputs removed I hear no squeal. The outputs were swapped.
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Old 16th April 2010, 09:18 AM   #10
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What outputs did you use as replacements?
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