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hakentt 16th October 2008 03:33 AM

Help me fix MTX 152 and MTX 500D
I bought two burned amps MTX 152 and MTX 500D, these amps are great and I would like to fix them. First is MTX 152 I opened it up and found this burned:

Looks like one of the big resistors was not soldered correctly and came lose making too much power go to other components burning up few resistors. What components I need to test and what I need to but to fix it.

Next is MTX 500D, I fixed one before changing power and output FETs, and transistors Q61,Q2,Q3,Q62, op amps, few resistors and few diodes.

Perry Babin 16th October 2008 08:47 AM

For the regulator, you're going to have to do the same as you did for your other amp. Don't forget to check the Zener diodes behind the regulators.

I'm not sure of the values of the components near the LM311. They appear to be 1k ohms and 10k ohms but there's not enough voltage in the amp to burn a 10k ohm resistor. One of the resistors is connected to Q1 which senses voltage on the speaker level inputs. It's very strange that it would burn. You may have to pull the covers off of your other amp to measure the values of those two resistors.

I have no information on the 152. Send me a set of high resolution photos if possible.

The large resistors are likely source resistors. If the outputs shorted, the excessive current flow probably produced enough heat to cause the solder to melt and the resistor fell out.

Are the resistors near the orange diodes burned? If they are, the diodes may have shorted.

hakentt 16th October 2008 07:15 PM

hero of the day perry babin.

Here is my first 500D, the fix was easy.

The resistor that is connected to the Q1 is not burned but it is charred because resistor next to it burned that is connected to the Q101.
So I should replace the same components and OP amps and all the FETs even if they are not bad because they ran so hot for so long?

The 152, not all resistors that are dark burned, only 2 but because they got burned so bad the char went across the board darkening the other resistors, I will take high resolution photos and send it to you or I can post link here.

Also I have MTX 302 that works fine but at some time it cuts off the same way as if you would connect and disconnect RCAs, it goes "pop" then audio cuts out but the power stays on, it is not protection, the amp does not get hot, then audio comes back. I see nothing burned in it, I think I should replace OP amps and FETs as well, what do you think?

Perry Babin 16th October 2008 07:31 PM

Only replace the outputs if they're defective. You'll have to replace all of the regulator parts like you did in the other 500D. If there are defective op-amps, you'll have to replace those also. If you can easily replace them (I think you said you could), it would be easier to replace them all. After that, you'll power it up to see how many problems remain.

Q101 is part of the over-current protection. If the resistor connected to it is defective, you need to check the bank of resistors (2 rows standing) to determine if any are open or out of tolerance. If you read ~0 ohms across them with all in the board, they may have survived. You will have to desolder at least one leg of each resistor for definitive testing.

Send me the photos. Most of the photo sites reduce the resolution.

This amp may have a broken connection on the RCA jacks but that generally produces a whine. You may need to use a non-conductive probe to push on the various parts of the circuit board to determine if there is a broken connection somewhere.

hakentt 16th October 2008 08:28 PM

I will do those tests on 500D.

on 302, I bought the amp new in 1999, I pushed two 8 ohm PPI subs in parallel mode giving the amp bridged 4 ohm load. Amp performed well but it would ocasionaly overheat and shut off, protective mode, after while it started making that cut sound, then I conected it every speaker to its channel normal mode, and the amp did not overheat anymore but that cut sound only comes on after few minutes after the amp warms up a bit. I checked all the posibilities for lose components but they all look nicely soldered. I suspect bad output FETs, because they ran so hot for so long, what say you?

On 152, here are the hi def photos, to me looks like 3 resistors got cooked:

Perry Babin 16th October 2008 08:48 PM

I doubt it's bad outputs unless the outputs have a broken leg (common on some amps). Solder connections don't always look bad. You need to prod the board to see if it makes the amp shut off.

It appears that there may be 6 burned resistors (1 is one of the large source resistors). You need to clean the board with acetone and a toothbrush to remove all of the soot.

hakentt 17th October 2008 01:25 AM

302, I will do what you say.

152, I did not have any aceton but I do have alcohol and cleaned the board the best I can, I can see 5 resistors have been seeing major heat, when I trace the leads they all have to do something with IRF540 fets, so I asume they failed. I want to know what to order from digikey, I got 4 amps to fix and I will order anything you say

hakentt 17th October 2008 04:33 AM

I just pulled all the IRF 540 from the 152 and one of them is shorted between the legs.

Perry Babin 17th October 2008 06:05 AM

For the 500D, order the same parts that you ordered for the last repair (specifically, the op-amps and the parts for the regulators). If you need the resistors that are standing in two rows, order those also. Order the outputs or power supply FETs if you need them.

I don't know the value of the burned resistors in the 152. It's possible that the coating has burned off but the resistor is otherwise intact. Pull them and measure the resistance of each resistor. If the value is relatively low (less than 1K) and coincides with common values, the resistor has probably survived but should be replaced. If any opened/failed, you'll have to find the equivalent part in the other channel to get the value from it. For example, R2xy should be the same as 3xy.

hakentt 17th October 2008 08:11 AM

Am ordering everything now trough digikey, resistors are not a problem because I can get them at the local store. One of the burned resistors I cannot read so I will have to see a picture of one good 152 amp or wait and buy another one and see what resistor it is.

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