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Old 25th August 2008, 05:05 PM   #1
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Default power ground resistance

Hopefully someone can answer my question. I have recently repaired my brothers amp after it blew out the mosfets on the power side. After replacing them, I checked terminals of ever fet against every other (16 in all) to make sure all were properly paralleled and no terminals crossed somewhere and eveything seemed fine. However, after testing the resistance between power and ground, the resistance starts at zero and then grows slowly to a few MegaOhms. Before replacing parts, it didn't grow from zero, but I'm still not convinced starting at zero is too normal, it seems dangerous to me.

Any suggestions as to whats going on? Thanks in advance. If it helps, it's an older model Ultimate T4 amp.
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Old 25th August 2008, 05:10 PM   #2
ppia600 is offline ppia600  United States
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The 12vdc filter capacitors are giving a false reading. Your meter is charging them as it reads the circuit, lol. Normal
Don't worry... you can always turn the gain down!
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Old 25th August 2008, 09:32 PM   #3
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Thanks. I kind of thought that the meter was charging something up, since to test resistance it needs to put a slight current through the circuit.

Everything should be fine if I hook it up then, correct?
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Old 26th August 2008, 02:30 AM   #4
Eva is offline Eva  Spain
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If the power supply MOSFET blew, you should also check the transistors and IC that drive them, otherwise you risk getting another set of cooked MOSFET. Follow the gate terminals... (there are probably many threads on that subject)
I use to feel like the small child in The Emperor's New Clothes tale
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Old 26th August 2008, 03:15 AM   #5
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When I replaced the fets I also replaced the transistors that drive the gates on the fets. There were a total of four pnp's driving the system. Two sets of paralleled fets and two pnp's driving each section. Every gate had a resistor into the driver IC chip as well, but I didn't replace the resistors.
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Old 27th August 2008, 05:01 PM   #6
jol50 is offline jol50  United States
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Power it up with a resistor inline to limit power. I hook a cheap dvom on power line set to ampere draw. I put a little 10a or less fuse on the 12v supply and put power to it very briefly, then a little more. Most amps do not pull over ~2A to get fired up, for say <500w amp. I watch the amps and if they go over 3a I stop.

edit: a larger amp may take more amperage to power up.

Make sure it is all clamped to the sink when you do this or transistors can pop instantly. Another way is to put one working transistor in each bank and power it up, then it would only kill two if it is still messed up...usually only do that if I am waiting for new parts.

There are better ways to check the PS IC, above is quick and dirty I often do on amps I will junk if they need extensive repair, or I've tossed a couple of used transistors in just to test it and don't care if they blow.

I just did this with an amp and it would start sucking big power in a few seconds. Ended up finding I had missed a test on one output transistor and it was shorting out the PS. It tested good except one leg combination. Replaced that channel and its good to go. You can can isolate the PS for testing if the amp is not working. The smaller amps I have been doing lately the PS starts slow enough to see the amp draw go up if the amplifier is shorted, if PS is shorted it will blow the little fuse you put in it.
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