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homertooties 12th November 2006 09:13 PM

can irf540 +irf9540 be tested with dmm
Hi all in have been working on a pioneer in my previous posts.

I was advisedabout the psu caps hey presto the amp stayed on with output stage pair of psu caps i found in a box were buggered also,then found 2 elnas and the amp stayed alive.

i soldered the output stage back in and the amp went back to its on ways stayed on for 4 seconds and tripped,i noticed the temp on the right channels heatsink got very hot,indicating short to me.

Took the output stage out again and run the amp tested drivers

readings -trannies -39vdc b and e 0.6 on collectors.

same on + trannies except positive volts
When the amp has the direct button out the bias pot voltages fluctuate from -2vdc up to 12vdc and back down again repeating itself.

When the direct button is pressed in the bias pots stabilze at about -5vdc

Is there a simple test to do on the hexfet output stage to determin whether they are dead?

Also when the hexfets were in the amp powered up i got a reading of about 3vdc on the S terminal + and - is this normal as i am use to working with bipolar out puts .

Appreciate any help:) :)

ilimzn 13th November 2006 09:17 AM

Based on what you wrote I am not sure you are actually getting into this with any sort of systematic approach. Just removing and refitting parts will not get you far because the operation of the amp depends on ALL parts working at the same time, so without knowing the circuit and being very careful what you take out or put in and in what order, watching just one variable (stays on or does not stay on) is no good. Heaven knows what other effects you are causing, and basically, all those strange and weird effects is what you are going to measure - without much hope of figuring out the one or more common denominators, i.e. components you need to change to repair it. You are basically changing some of the many variable of the circuit (are you sure which?) trying to solve your 'equation' for all variables at once. Due to all possible combinations you have, it is likely you can't make enough measurements in your lifetime to solve the puzzle, so obviously, you need to use a different approach.

As for your original question, you can test IRF parts out of circuit with a DMM, providing it uses at least 2.5V or so open circuit voltage on it's probes, when in resistance mode. Being able to recognise the measured voltages as OK or wrong in a circuit, depends on knowing the circuit. Given that 99% of all amplifiers with complementary outputs (BJT of MOS) share the same basic topology elements, I would recomend taking a pencil and paper, and drawing out at least the output stage schematic from the PCB you have. It really should not be that difficult, and it will help you understand what you are seeing.

jerluwoo 13th November 2006 06:19 PM

Mosfets can be tested with dmm. Use the diode test position. Attach the negative lead to the drain. Apply positive lead to the source lead (try to keep from touching the gate lead with fingers or probes), check that there is little or no conduction. Apply positive probe to gate lead for a few seconds. Apply positive probe to source lead again and see that there should be conduction. While keeping the positive probe on the source, place your finger across gate and source. Conduction should go from high to none as the gate discharges through your finger.

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