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Old 22nd November 2009, 11:53 PM   #1
ajkustra is offline ajkustra
diyAudio Member
Join Date: Nov 2009
Default New, need some guidance

I am new to repairing my own equipment, but have experience installing and also a good knowledge of electrical pricipals. I have a few questions about three amps I'm working on.

The first is a Directed 1100d that is older. I replaced eight irf9640 transistors of which four were found to be bad, and six fb31n20d transistors of which three were bad. This amp works great, until it sees a heavy load for a little while at which point it gets very hot and protects on me. But when it cools its fine. Does anyone know why?

The second is a Kicker 500.2 which was getting hot and going into thermal protect. I checked the fan circuit and found no power at the fan, so I powered it up otherwise (remote teminal and ground). I know this is quick and dirty but the amp now works fine. Is there anywhere I can look for a schematic to further diagnose this?

The third is an MTX TA7801, This amp stoped working out of the blue. No popped fuses, or anything abnormal. I opened it up and checked mosfets and checked for power here and there but don't really know what I'm looking for. While I had this amp powered up to check for voltage at mosfets, I did something really stupid and shorted a mosfet with my dvom leads. Here comes the wierd part...The amp works now! I'm sure there is still a problem though. Now the prism efx even work, which were not working before.

I read about the 7801 in another post, and I found out about your cd version of the website. What all is included with this as far as schematics and brand specific info?

Thanks for any help in advance,
this beginner needs it!

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Old 23rd November 2009, 03:33 AM   #2
Perry Babin is offline Perry Babin  United States
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
The directed may have defective drivers. Power it up when it's cold (hasn't been run for several hours) and drive it hard enough to make it begin to heat up. Does the heat come from one particular location on the heatsink? If so, that will help lead us to the part of the amp causing the problem.

This amp probably controls the fan with a comparator (likely an LM324). You'd need to post the voltage on all 14 pins of the IC. Place the black meter probe on chassis ground. Place the red meter probe on the point where you need to measure the voltage. Page 1 of the following datasheet shows the pin numbering configuration.


Pin 1:
Pin 2:
Pin 3:
Pin 4:
Pin 5:
Pin 6:
Pin 7:
Pin 8:
Pin 9:
Pin 10:
Pin 11:
Pin 12:
Pin 13:
Pin 14:

I don't have a schematic for it. If it's working, you should probably leave it as it is. If you do attempt to troubleshoot the amp do NOT take the clamps off of the transistors. The transistors must be tightly clamped to the heatsink when the amp is powered up. The regulators are the most likely to fail if not clamped down and can fail within seconds causing extensive damage.

I can't discuss items that I sell on this forum (the posts will be removed) so if you want information on the tutorial, you'll have to email me. One note, the full version of the site and the tutorial are not the same.


In the future, if you need help with multiple amps, it's probably best if you start a different thread for each one.
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Old 23rd November 2009, 10:15 PM   #3
ajkustra is offline ajkustra
diyAudio Member
Join Date: Nov 2009
I will check out that comparator soon, as well as the Directed 1100. Back to the MTX, I was reading a thread on this amp (link below) and in your first reply you refer to notes on this amp. Where can I find them?

MTX TA7801 repair
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Old 24th November 2009, 04:30 AM   #4
Perry Babin is offline Perry Babin  United States
diyAudio Member
Join Date: Nov 2003
Location: Louisiana
He purchased the tutorial I sell. I was directing him to the file/folder where he could find the notes.
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