MTX MTA 225 Repair

[l7runner]

Hey guys,

After browsing around a bit this forum seems to be a pretty useful site regarding amplifier repairs.

I recently picked up an old MTA 225 amp locally, It was working great for a few weeks and really surprised me. However, the other day it let out it's magic smoke and I discovered a toasty FET. Crappy Pic attached

The blown part is a power supply fet (MTP25N05E) which seems to be discontinued by Motorolla. Looking through these forums I found a direct replacement FET (IRFZ34N) as well as other commonly recomended replacement parts (IRFZ44/IRFZ48) It there a real difference with the different suffixs (N,Z,V) on these FETs?

I was wondering if there were any real advantages to each part choice. I believe I will need to change out the gate resistors if one of the latter 2 parts are used. The current resistors are 470 ohms which is quite a high value for most FETs.

I plan to intially replace all 8 power supply FETs if necessary (they seem to be in 2 groups of 4) along with the gate resistors.


Any insight into parts choices/recomendations is greatly appreciated.

Thanks

[Perry Babin]

That appears to be a clone of an older PPI amp. The 470 ohm resistors should be replaced with 100 ohm resistors if you're going to use the IRFZ44s.

The suffix generally only means that there are minor differences. For power supply applications, it's typically insignificant.

You need to replace all 8 FETs in the supply. You also need to check the output transistors to see if any have failed.

[l7runner]

Originally Posted by Perry Babin

That appears to be a clone of an older PPI amp. The 470 ohm resistors should be replaced with 100 ohm resistors if you're going to use the IRFZ44s.

The suffix generally only means that there are minor differences. For power supply applications, it's typically insignificant.

You need to replace all 8 FETs in the supply. You also need to check the output transistors to see if any have failed.

Yeah I think these amps are similar to the older PPI Pro-mos or something.

I assume I should take the outputs out of the circuit to check them properly?

Thanks for the help, Ive read over your tutorial and it has a lot of great information

[l7runner]

OK I pulled all of the PS FETs and their gate resistors, and am waiting for the parts.

So in the mean time I hooked the amp up to a limited power source to check the IC. DMM neg probe on amp ground, with pos lead touching the IC pins. Below is a beautiful MS Paint drawing of my results. I hope these recorded values are what is to be expected. Any 2nd opinions would be appreciated.

IC is a SG3525A

[Perry Babin]

I didn't see your post from the 8th.

The outputs can generally be tested in the circuit. When they fail they will most commonly be shorted between legs 2 and 3.

The voltages on this IC are approximately what you'd expect to see.

Do pins 11 (4.66v) and 14, (4.84v) go directly (~0 ohms) to the gate resistors?

[l7runner]

Originally Posted by Perry Babin

I didn't see your post from the 8th.

The outputs can generally be tested in the circuit. When they fail they will most commonly be shorted between legs 2 and 3.

The voltages on this IC are approximately what you'd expect to see.

Do pins 11 (4.66v) and 14, (4.84v) go directly (~0 ohms) to the gate resistors?

Yeah I tested the outputs for shorts in circuit and they all seemed OK.

And yes pins 11 and 14 lead to the two different sets of PS FETs (within ~0.2 ohms) could be within the tolerance of my crappy meter. I also noted ~4.6 volts at the gate resistor pad of the PS when power was applied.

Thanks for all your help

[Perry Babin]

It's likely to be OK when you power it up after replacing the resistors and the FETs but you should have all transistors clamped tightly to the heatsink and have a 10 amp fuse or a current limiting resistor in the B+ line when you apply power.

[l7runner]

Yeah I have been testing it out with a 15amp resistor thus far. I will use that or a bulb in series with the B+ when testing it out.

I will put the new parts in next week. Thanks for all your input

[l7runner]

I replaced the PS fets with the IRFZ44 and changed out the gate resistors to 100ohms. I clamped it into the heatsink and powered it up. It blew the 15 amp fuse quickly. It doesn't blow a 30 amp fuse though. The power light does not come on, It did flash for a second at one point but doesn't stay on.

The PS fets get warm quickly when powered up. ~0.5 Volts is still read at the the pads for the gate resistors.

Any ideas? Thanks

[Perry Babin]

Are there any shorted output transistors?

Can you post a photo of the entire board?