MTX BT 754, blown B+ caps - diyAudio
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Old 28th May 2008, 04:01 AM   #1
jol50 is offline jol50  United States
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Default MTX BT 754, blown B+ caps

This 754 supposedly had channels out and all tests ok, all I find is 3 of the 4 B+ 1,000uf 35v 85c caps are blown and leaked out the top. None of it got on the board. I figure it should play if they are not shorted, find no short, so I fire it up and get 1.6A idle draw at about 13v input. My 2.7 ohm resistor was too large for it to run on, hardly ever have that. It shows .01A when not running so that could be leaking caps. Amp dated 1997. Larger 35v caps on rails appear fine and other voltages good.

Is this normal idle for this? DC offset is <10mv all channels except one bridged is little more. If it only does it when on then the B+ caps are not leaking it as I think they are hot all the time. I have seen few mtx amps so not familiar with them.

So I run the amp for a short time full 13v and all channels and bridged play music ok. Could not crank it up due to late hour but not sure what the deal is here. Unless the person tested it on a small supply and it cut out, the 2.7 ohm resistor pulled down to just over 8v and it would cut out trying to play any music....or the bad caps are affecting it.
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Old 28th May 2008, 05:04 AM   #2
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1.6a seems just a bit high but if each of the 4 channels is biased slightly higher than it needs to be, it could account for the idle current.

Sometimes, excessive ripple causes an amp to draw more current than normal but if you have one good cap across B+ and ground, that's not likely to be a problem here.

The 0.01 amp when off sounds normal. The LM339 and the 494 are probably operating even with no remote voltage (common on newer MTX amps).
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Old 28th May 2008, 04:39 PM   #3
jol50 is offline jol50  United States
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I tested capacitance and had 3300 I think for B+, so three caps may be working. The diodes tested good on 12v side there. Maybe they hooked up power backwards?

Today I will crank it up and see what it does, also running it on a battery so I am not adding any ripple except a 1A charger. I could set the bias just assumed it was good with that lower offset, and I don't know what to set it at. I found no heat in it but did not run for long.

Most people say they are great amps, it does say made in US on it. Everything on the sink is near the same as a JL and a dragster 4ch amps I have handy. One pair of TO220 per ch do not look very hefty, but I have not looked their ratings up and it seems common.
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Old 28th May 2008, 07:52 PM   #4
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The ripple would have been from the switching power supply.

If it's producing clean output and isn't running hot at idle, I doubt that there's anything wrong with it.
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Old 2nd June 2008, 04:57 PM   #5
jol50 is offline jol50  United States
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I tested it on speakers and a sub, it seems to work fine. Sorry I guess I should not believe people when they say an amp is blown Strange I tested a bolt 4ch rated about the same (300wrms) and the MTX might go a little louder on the sub, but it has a deeper more powerful sound for sure. Wonder if the bolt has a built in subsonic, there was *quite* a difference at say 30hz area and this a sealed 12. Even at full range/no boost on each amp it was very apparent. Unless rail capacitance is different I will look. I'll get some new caps and it should be good to go.
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