MTX 4320 wants a second chance

[Dr Zeus]

I got this amp from a car audio place after it had been collecting dust in the back for many years. Its a MTX 4320 with the year 1997 stamped on the PCB. A 4 channel amp rated for 40x4 @4 ohms or 80x4 @2 ohms. The amp looks a bit more powerful than that by the way it is fused and the number of output transistors; however I guess powersupplies were probably not built strong enough to withstand the output's requirements. would personally like to see at least one power fet per output transistor in an amp; ratings aside. This amp has twice as many output transistors as there are power transistors. Not to say that was the cause of the issue but it could have only helped. The amp is fused for 70A.

First things first; opened her up and found several transistors shorted so I went ahead and clipped them off the PCB. FET5 and FET6 out of the 4 power FETs stamped P60N06 are shorted. FET9 and FET10 out of the 8 output transitors stamped 50N06V are shorted. I also clipped RECT2 because I thought it was shorted (FEN16CT) but it turns out it might be alright. i thought it was shorted but it checks out with diode test - I can resolder it on when I need to.

At this point I pretty much know that all the large transistors are going to need replacing, but would like to work with what is left on the board first.

I cant seem to get this amp powered on safely; the amp pops 10A fuses on the workbench when connected even with REM disconnected. Resistance across +/- quickly rises such as a capacitor being tested.

[Perry Babin]

Remove the other two power supply FETs and apply power. Do you see a waveform on the gate pads for the power supply transistors? It should swing at least plus and minus 10v (referenced to ground).

[Dr Zeus]

Pad for pin 1 has this output with Volts/div set to 10:



Pad for pin 2 is getting 12v +DC.

Pad for pin 3 shows nothing - reference from ground is .9 ohms.

Before running these tests I removed RECT1. I guess the way the circuit is laid out make it look like pin1 and pin3 were shorted on both RECT1 and RECT2. I'm going to put them back in.

[Perry Babin]

That looks OK.

The rectifier was being shorted by the transformer windings.

Replace the power supply FETs (with new ones) and leave the rectifiers out. Does the amp power up without blowing the fuse?

[Dr Zeus]

We're playing a bit of tag here. RECT1 and RECT2 are back in and the amp powers up without the powersupply fets. The RECTs have 0vDC reading from general ground. I think the amp is getting power but the red LED is not lighting up. I think it might be blown, cause I get .35vDC across the LED.

Will IRFZ44NPBF-ND work in place of the P60N06 power supply fets? I have a dozen matching spare from the PPI amp rebuilds.

[Perry Babin]

You can use them for testing but they're unlikely to be as reliable as the originals.

[Dr Zeus]

I put 4x IRFZ44NPBF-ND in FET5 through FET8 and the amp powered up! LED and all. No fuses blown. I'll run it through some tests to see how far down the rabit hole I can get. At the very least the LF channel is totally blown because the transistors were shorted; clipped off yesterday.

I see where I cant really use IRFZ44NPBF-ND in place of the P60N06.

IRFZ44NPBF-ND
-rated for 55v and 49A

P60N06
-rated for 60v and 60A

At least I can now finish troubleshooting.

[Dr Zeus]

While I'm playing search and destroy, I noticed P60N06 runs about $5 each from Mouser.

Would this be a good alternative? They are $1.80 and rated for 150w - 30w more than the P60N06 which this amp should enjoy.

ST: P60NF06

STP60NF06 STMicroelectronics MOSFET Power

[Dr Zeus]

Looks like this amp is going to be fixed. Three out of the 4 channels sound good enought for me to leave alone. I just need 4 power and 2 output transistors (LF channel blew).

Parts list:
4 x P60N06 / P60NF06 (Need confirmation that NF06 can replace N06)
2 x 50N06V

Does this power supply output look correct for this type of amplifier? Measures the same shape at pins 1 and 3 on the power FETs. One has slightly more amplitude than the other.

[Dr Zeus]

Actually, it looks like the P60NF06 is on backorder for 5 months.

What about the P60NF06L?

STP60NF06L STMicroelectronics MOSFET Power