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Old 16th March 2012, 11:08 AM   #1
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Default memphis mch1300 repair help.

Ok, here is the story..

I have been running this amp for a couple months now with no issues. yesterday I noticed that my tweeters were sounding funny, so i checked them and they read lower than 1 ohm, 3 of the 4 on each front channel were blown. I replaced them, as well as relocated the amp in my truck. turned on the radio, and it shortly cut out. 1 of the new tweeters sounded like crap, so i checked it at the amp, it read .2 ohms, there was a short, I fixed that, and turned the radio on again, same thing, it cut out. so this time i checked all of the speakers, and found another shorted speaker wire, this time on a rear channel. So I fixed that short, checked everything again to make sure everything was ok, turned it on once again. It played for a few seconds, then cut out again.
Now the amp stays in protect mode. It trys to power up, I hear it click, and notice a power draw when it does, but it does not power up, just stays in protect and keeps clicking, and got pretty hot to the touch.

I pulled the cover off of the amp to check for any obvious signs of a prooblem, and have found none. nothing appears burned, or smells funny.

Im confident I can fix this amp myself with a little guidance, which is why I am here.

Im thinking output transistors would be a good place to start, as that seems to be what was getting hot, plus a bit of research last night about amp repair, lead me to believe they are usually a pretty common problem.

Could somebody point me in the right direction ? Thanks in advance.
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Old 16th March 2012, 12:08 PM   #2
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It likely has shorted output transistors. If you disconnect all speaker wires and the RCA cables from the amp, does it still go into protect?

If so, post a photo of the board in the amp.
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Old 16th March 2012, 12:12 PM   #3
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Hi
Check your voltage supply connection. Most automotive stereo products have a diode to protect against reverse polarity. Typically they are designed to hold up under most circumstances, To test this you need a multimeter, with the power disconnected at the battery check ohms of resistance between V+ and Gnd reading should be high, if its low or worse very low, there is a short indicating condition for high current flow, and further damage to occur.

The complex problems of the speakers may also be explained by a reversal of polarity if Gnd is wired common with battery ground.. ie there may have been 12v+ across them.

Try to strictly follow color coding, and keep any exposed wires away from possible shorts.
In the bigger world you have discovered conductivity vs insulation, its a matter of getting your wiring right the first time.

Cheers / Chris
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Old 16th March 2012, 04:12 PM   #4
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Quote:
Originally Posted by Perry Babin View Post
It likely has shorted output transistors. If you disconnect all speaker wires and the RCA cables from the amp, does it still go into protect?

If so, post a photo of the board in the amp.
Yes, I have disconnected everything. speaker wires, and RCA's, and the amp still does the same thing.

I will take pictures and post them in a minute.
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Old 16th March 2012, 04:44 PM   #5
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Ok, here are some pics, if you would like something particular, I will get it.

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Also, I checked the pos and ground at the amp like stated above with the batteries completely disconnected.
I checked it on all of the ohms settings shown on my multimeter, and get the same reading "OL" . Im no wizz with multimeters, but my experience with this one is that if it reads "OL", then the ohms are TOO LOW for it to read them.

what would be the next step here? backtrack all of my power wires and look for some sort of short?
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Old 16th March 2012, 04:51 PM   #6
ultra is offline ultra  Greece
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If you reverse the multimeter probes and see the resistance increasing, its likely ok.
This is because of the capacitors getting charged/discharged.
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Old 16th March 2012, 05:45 PM   #7
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ok, did the tests again with my mm. hooked it up, and let it sit to give it a few minutes to settle.

meter set on 20k, I get 7.16
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Old 16th March 2012, 06:16 PM   #8
ultra is offline ultra  Greece
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Remove from the amp all speaker wires and rca connectors as Perry suggested.
Does the amp power up properly?
If so, let the amp on, and measure all speaker outputs with your mm set to DC.
What is the readind ?
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Old 16th March 2012, 06:33 PM   #9
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Quote:
Originally Posted by ultra View Post
Remove from the amp all speaker wires and rca connectors as Perry suggested.
Does the amp power up properly?
If so, let the amp on, and measure all speaker outputs with your mm set to DC.
What is the readind ?
nope, amp still does the same thing.

I have the amp out, and am currently checking the output transistors, so far it seems that most all of them are bad.

At this point, I think I should start looking at ordering parts. ?
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Old 16th March 2012, 06:49 PM   #10
ultra is offline ultra  Greece
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Yes, but its better to gather all the parts you need, in one order.

Can you lift the center leg of the rectifiers?
Normally, there should be 4 twin rectifiers mounted between power supply fets and output transistors.

Last edited by ultra; 16th March 2012 at 06:59 PM. Reason: correct spelling
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