Help me repair my MTX 500D please! - diyAudio
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Old 1st February 2008, 03:51 AM   #1
hakentt is offline hakentt  United States
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Join Date: Feb 2008
Default Help me repair my MTX 500D please!

I have been reading this forum and I've found few posts of MTX 500D amps having design flaws. I got this burned amp for free and when I open it it has visibly burned components. I don't know how bad it is but it is not a problem sense I know how to solder and I have a nice electronics shop near by.

I got few questions also. I will post pictures explaining my problem and what I need to do to fix this amp.

Click the image to open in full size.

this capacitor was ripped off and I don't know what value it is nor what is is called. If anybody knows please post it here.

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this capacitor has numbers 22J100 on it, what value is it so I can replace it.

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These resistors appear burned and will probably have to replace them, with what values?

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now common problem in 500D the transistors q62, q3,q61 q2 appear burned or overheated.

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I know that there is few people on this board who fixed these amps before and know what components are recommended to be replaced, I will replace them all just tell me which ones.

Thanks a lot

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Old 1st February 2008, 04:32 AM   #2
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If you're doing this to learn, it may be worth it but if you're repairing this because you don't have the money to buy an amp, stop now. It's likely to cost more in parts and shipping than a used, working amp.

The cap near the green inductor is probably marked .22j100 which would be a 0.22uf 100v capacitor.

The 2 burned resistors are 47.5 ohms. These only burn when the power supply FETs fail.

I don't know the value of the cap under the terminal block. If it's across the B+ and ground terminals, a 0.1uf mylar should be close enough.

The regs fail often and usually destroy op-amps in the process. If you decide to repair it, be prepared to replace all of the op-amps and the VCA (2155?). You need to check the two Zener diodes connected to the center legs of the regulator transistors. You should also change the two electrolytic capacitors connected to the first leg of the regulator transistors. At least one of the 150 ohm resistors between the regulator transistors generally fails. Check both of them.

Did the output transistors fail?
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Old 1st February 2008, 07:26 AM   #3
hakentt is offline hakentt  United States
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Am doing this for, for hobby. I do have money, I just bought Hifonics brutus 1206D for $200 brand new and bought two speaker boxes with phoenix gold dual 10" pro subwoofers. I do have a 300zx with about $2K invested in Alpine sound system. I worked in precision power here in Phoenix until they got sold to MTX in 1999. I used to build those flat speakers. I know everything when it comes to speakers but am not that good with amps.

I don't know what failed, I got basic knowledge in electronics and I don't have osciloscope. Am willing to replace everything nesesery to get it to work but I do not whant to exed $50 in parts. I don't know how much the op-amps cost but the rest of the stuff you mentioned should not be much.

If you can help me I can test and replace everything you suggest. I got good digital volt/resistance meter
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Old 1st February 2008, 09:32 PM   #4
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The op-amps are relatively inexpensive. I doubt that the parts would cost more than $50 (including shipping).

You'll need to check the output transistors near the green inductor. If you read something near 0 ohms between any of the legs of any individual transistor, the transistor has probably failed. You'd need to remove it from the board and recheck them to confirm. If the outputs have failed, the damage may be extensive and the easiest way to repair it would be to replace ~10 inexpensive transistors instead of trying to troubleshoot it.

When you replace the regulator transistors, you're going to need to repair any broken traces. It looks like the board is in pretty bad shape there. Clean it with acetone and a toothbrush (use one NOT made of clear plastic) so you can see how bad it is. Remove all of the regulator transistors (3 remain) to make it easier to clean.

Remember to check the zener diodes and the 150 ohm resistors.
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Old 2nd February 2008, 04:03 AM   #5
hakentt is offline hakentt  United States
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Thanks for that. I posted the photo of the whole board up there you can use it to circle the components that I need to replace and post it here. The board is in pritty good shape actualy, a little heated up but all traces are good.

What are the op amps? the mini chips?
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Old 2nd February 2008, 04:03 AM   #6
hakentt is offline hakentt  United States
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Quote:
I don't know the value of the cap under the terminal block. If it's across the B+ and ground terminals, a 0.1uf mylar should be close enough.
Yes it is across B+ and ground. Si I go to store and asl for 0.1 UF mylar capacitor and that should do it? you shure that will work? What kind of transistors are the one burned? they look the same but got diferent markings on them MPSW42140?
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Old 2nd February 2008, 04:48 AM   #7
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Quote:
Yes it is across B+ and ground. Si I go to store and asl for 0.1 UF mylar capacitor and that should do it? you shure that will work?
It's not a critical component. The amp would work without it.

Quote:
What kind of transistors are the one burned? they look the same but got diferent markings on them MPSW42140?
I think 2 of the regulator transistors are MPSW42 and the other 2 are MPSW92.


The op-amps vary depending on when it was produced. Some used five NE5532s. Other versions had two NE5532s near the RCA jacks but the other 3 were TL072s. The 072s draw less current and may have been used to take some of the load off of the regulators.

Mini-chip?
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Old 2nd February 2008, 05:54 PM   #8
hakentt is offline hakentt  United States
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mini-chip I ment the op amps look like small eproms. 2 closest to the RCA jacks are NE5532N and they got 8 legs. and there is 3 closer to the adjustment pots and they are TL072CP and they got 8 legs. I should replace all of those? What else?
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Old 2nd February 2008, 08:40 PM   #9
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I would suggest that you order them but I wouldn't replace them unless they're defective.
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Old 3rd February 2008, 12:53 AM   #10
hakentt is offline hakentt  United States
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so replace the transistors first, and see if that fixes the problem?
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