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Old 28th October 2011, 04:29 PM   #1
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Default Mtx Thunder 280 Problem

Hello all. This is my first post, so please forgive me if I post improperly.

I have an old mtx 280 that I still use to power a single 12. I've had a problem with it for a while now where it will introduce very loud feedback to the speaker if I up the gain or Thunder EQ over about 1/2. Also, the same thing happens if I use the HP/LP switch with a lower gain. Same thing happens when using a different speaker. I've taken the board out and checked the FETs, capacitors and resistors. All look ok with no obvious burn outs or leakage. I've also checked the RCA jacks as I've read that they can cause problems if they are loose.

The amp works ok as is, but I would like to be able to use the HP/LP switch.

Any suggestions? Thanks in advance!
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Old 28th October 2011, 04:46 PM   #2
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That sounds like a signal ground problem. Are you sure that the shield ground on the head unit is intact?

When you checked the shield ground on the RCAs, did you push back on the RCAs while checking for continuity between the RCA shield and the shield ground on the board?
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Old 28th October 2011, 04:56 PM   #3
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Thank you for the quick reply Perry. I did not check the shield grounds, but will do so tonight. However, my deck (JVC Kameleon) has 3 sets of RCA outs. I'm running 4 amps (3 mtx thunder, 1 MB Quart Onyx). The other 3 amps do not experience this problem. I've also swapped out the RCA cables and the problem still exists.

Thanks!
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Old 28th October 2011, 07:10 PM   #4
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Re-check the shield ground on the amp while pushing back on the RCAs. Sometimes you can miss a broken connection if the jacks are not pushed back.
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Old 3rd November 2011, 05:00 PM   #5
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Just wanted to update on this. The shield ground appears to be the problem. I'm going to attempt to repair it by grounding the RCAs directly to the shield with a fused line.
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Old 3rd November 2011, 05:32 PM   #6
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One other quick question. My jvc deck has a "Rear Ground Terminal" tab on the back. The instructions don't make much mention of this, so I never grounded it out. Would it be a good idea to ground this in addition to the black ground wire?

Thanks!

Last edited by jamesl75; 3rd November 2011 at 05:40 PM.
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Old 3rd November 2011, 05:54 PM   #7
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It's probably not necessary. If the head unit is mounted in an iso type bracket, it's already well grounded. Even if it's mounted in plastic, the ground wire should be good enough, especially if there is no recommendation to ground the tab in the owner's manual.
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Old 3rd November 2011, 06:08 PM   #8
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Would another symptom of this problem be an increase in Alternator whine and general noise in the system? I've seen that problem crop up in the past month or so.
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Old 3rd November 2011, 06:19 PM   #9
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The ground of the head unit isn't likely to be the problem but to eliminate it as a possible problem ground it and see if it helps.

An open shield ground on an MTX amp generally causes the amp to oscillate. This varies with the crossover settings so in low pass mode, it can present other problems.

With no RCAs plugged in, what's the resistance between the RCA shields on the amp and the chassis ground?

Is the problem constant or does it vary when you hit bumps in the road?
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Old 3rd November 2011, 07:15 PM   #10
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Thanks for the reply. I'll have to check the resistance tonight. The alternator whine is constant. The low frequency oscillation also seems to be constant, although I turn the amp gains down so as not to experience it when I am driving. I'm going to check all of my grounds tonight and also pop my hu out and redo the grounding there. If the problems persist I will direct ground the RCAs to see if that fixes the problem. Thanks!
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