Faulty MTX 500D

[Chuppster]

I hate to hijack this thread 4 years after it started, but I have one of these MTX 500D's and I need to repair it. There are a few burned out MOSFETS and two resistors that are as black as coal, making it difficult for me to identify what there are so that I can replace them. Does anybody have a diagram for this board?

 

[Mooly]

New thread for you.

 

[Perry Babin]

You should know, before you begin, that these amps have a lot of problems.

Which FETs and resistors are burned (give the circuit board designation).

IF you haven't done so already, read the basic amp repair page (in its entirety, even the parts that you think are irrelevant).

 

[Chuppster]

Thank you for the link, it's very informative. FET203, 202, and 204 are blown to bits. FET201, REC202, and REC201 appear to be shorted. The four FET's on the other side of the board seem to be operational. Are the blown transistors the power transistors?
As for the resisters, I can't make out the board designation, but here are some pics, I hope they help.

 

[Perry Babin]

Those resistors are 150 ohms.

When these fail, it's likely that many other components have failed. Are you repairing this amp because you need a working amp or because you're interested in learning to do this type of work?

 

[Chuppster]

Actually, it's a bit of both. I got the thing at a garage sale for $10, I plugged it in to my subs and it worked, but I didn't use it much. I then played with it a bit too much and shorted something. I am going to Purdue in a few days to study Computer Engineering, so most of my interest lies in the knowledge I could gain from fixing such a thing. Electronics fascinate me, and I am constantly learning new things on the internet. I won't mind replacing things, but if it gets too expensive I better leave it for now. If the resistors and transistors don't fix it, what do you think it will cost to get it working?

 

[Perry Babin]

Parts will likely cost about $20 (not including shipping). There is a chance that it won't be repairable so bear that in mind. Expect to spend several hours working on it.

There are several 500D threads in this forum. Do a search and read all of the older posts.

 

[Chuppster]

The regulators failed regularly in these amps because they ran too hot. When they failed, they often destroyed all of the op-amps in the amp. If you don't have any shorted outputs or power supply FETs, the regulator damage may be the only thing you need to repair but expect at least several hours work and expect to replace at least about 20 parts. Refer to the following:
http://www.bcae1.com/temp/mtx500dregulatordamage01.swf

Should I start out with following these directions?

 

[Perry Babin]

Read all of the 500D threads. The information there is only part of what you need to know.

I don't know if this was stated earlier but the rectifiers are likely OK.

 

[Chuppster]

Okay. I read all of the other threads and none of them pertain exactly to this problem. I'm going to start with these resistors. 150 ohms, but how many watts do they have to handle? Also, you recommend using 390 ohm resistors. Does this decrease the performance of the amp? Won't the resistors heat up too much?

 

[Perry Babin]

The increase in value won't cause the amp to produce less power. There is a thread where it shows the resistors under the board and against the heatsink.

 

[Chuppster]

Okay, I finally got my multimeter back, and now I can resume work. REC201 and REC202 have 2 different kinds of transistors. Are they supposed to be the same?

 

[Perry Babin]

Those aren't transistors. They're rectifiers. One is the positive rectifier. The other is the negative rectifier.

 

[Chuppster]

I see. If I'm testing these correctly, they seem to be okay. It only shorts 1 way, and that way is opposite between each side of the center pin. There is no apparent damage. Should I order parts anyway?

 

[Perry Babin]

Rectifiers (in the board) generally read shorted between legs 1 and 3. They should not read as shorted any other way.

 

[Chuppster]

These rectifiers read a short between 1 and 3, then they short when and only when neg=1 pos=2 and pos=2 neg=3.

 

[Perry Babin]

Shorted is 0 ohms. Are you saying that the meter reads 0 ohms 1 way and not the other (lead placement)?

If so, your meter is likely giving you false readings.

 

[Chuppster]

The meter is in diode mode, which, if i'm not mistaken, means it acts as a continuity tester. The multimeter could be giving false readings, it is a joke. I'm ordering an new one.

 

[Perry Babin]

You may not need to buy a new one. If you do buy a new one, buy a Fluke (preferably a model 10, 11 or 12).

If you pull the rectifiers, they should read as OK.

'Continuity' can mean a lot of different things.

 

[Chuppster]

Haha, I see. Alright, I'll leave them in.