MTX Thunder 81001 fets

fragmental

Member
2010-09-25 6:41 am
Hi guys, new to the forum and looking for some help with a MTX Thunder 81001. The output filter caps were blown and one of the output fets is bad. The other ones seem good in circuit but I`ll pull them to double check.

Can i replace one of the fets or do i have to replace them all to get them to match? Also 4 resistors got hot if someone could confirm the value of r490-494.

Do these work of a split +- rail voltage or a single supply. This is the first class D I`ve seen.

Any idea what causes a failure like this?
 
Installed my parts today, 6 fets (same date code) 4 10k resistors and the 8 output filter caps. The amp is running and the short lite is out:).

How hot should this be running with no input. I have an 8 ohm speaker connected but no input signal and after about 40min the fans are on and its very hot. Measured input current 4.6A at idle. Also got 180khz 1vpp on the output. Is this normal or do I have other faults?
 

fragmental

Member
2010-09-25 6:41 am
When I first power it up there is some varying dc on the output but it settles down after about 10 secs and ends up at 30mv dc. The heat is coming from the bank of 4 to220's behind the rca inputs. I pulled the bar off them and the IRF540 seems to be generating the most heat when i touched it directly.

Thanks for the help with this btw.
 
The inductors operate at relatively high temperatures. Even at idle, they'll get too hot to hold your hand on continuously.

The two regulators will get hot very quickly. Do not operate the amp without having them clamped. If they fail, they can cause extensive damage. They can fail within seconds if not clamped tightly to the heatsink.

When the fans were running, were they running at full speed? You can determine if they were at full speed by measuring the voltage at the white 2-pin connector for each one. If they were not running at full speed, it's likely that the temperature wasn't a problem.
 
Yes. There's a soft thermal interface pad that needs to be placed between the thermistor and the heatsink. It's possible that it fell out of place when the heatsink was separated from the board. I think it's on the corner of the heatsink that's nearest the HCPL3120 opto-couplers. There are likely two diagonal traces that exit from that corner. I think that's where the thermistor is.

The attached photo shows the thermistor and the thermal pad (moved off of the thermistor so the thermistor would be visible). This is on a 7801 which should be similar to the amp you have.
 

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It's probably OK.

A better indicator may be the rate at which the heatsink behind the various groups of transistors heats up. If you drive the amp hard and no section of the heatsink heats up quickly, it's probably OK. If one section (behind the outputs, behind the power supply FETs...) heats up very quickly when driven hard, that could indicate that there's a problem.