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Dr Zeus 24th December 2010 07:45 PM

Alpine MRP-M450
This amp has IRFZ44N and IRF5305 transistors for outputs. one of the IRF5305s was reading shorted and now it is off of the PCB. Nothing much else seems to be degraded.

With the transistor gone the amp powers up and is producing a high ammount of distortion at the outputs. Voltage accross the outputs are ~0.040v.

Does this amp require all output transistors to be fully functioning and clean?

Perry Babin 24th December 2010 08:06 PM

What's the part number on the driver IC (square IC with ~60 legs) near the outputs?

Dr Zeus 27th December 2010 07:16 PM



I'll read on about it and see what I can tell from power, inputs/outputs and operation.

Edit: The IC has +25vDC on pin 12, and -25vDC on Pins 26, 27, 58, & 59.

Perry Babin 27th December 2010 08:10 PM

All of the outputs have to be in the circuit for it to produce clean audio. Replace the outputs. If it still doesn't produce clean audio and all of the gate and current sense resistors are within tolerance, the driver IC has likely failed which makes it uneconomical to repair.

Dr Zeus 27th December 2010 08:34 PM

I dont think I have a substitute for IRF5305 outputs. Anything common I can use for testing here? Do you think I'll have enough need for IRF5305 transistors to order a small batch?

Maybe not for this amp...

If I remove all the outputs perhaps I can get a better status on the driver IC...

Perry Babin 27th December 2010 08:47 PM

I now of nothing else that uses the 5305s.

Class D amps can be finicky. Using a sub may work. Try an IRF9640 if you have one.

if you remove all of the outputs and drive a strong signal into the amp, you 'may' see a 10v p-p drive signal on the gates. If all of the gate resistors for the outputs have a clean square wave, the IC may have survived. I don't know if this will work with this amp but it works on various other class D amps.

Dr Zeus 27th December 2010 09:30 PM

I removed all outputs and did what you said. The signal picked up on the scope is very weak and I had to max out the intensity on the scope just to make it show anything. Its a 10v square wave... Found on all of the output's gate pads.


I might try all new outputs in this amp.

Perry Babin 27th December 2010 09:49 PM

It's not that the signal is weak. A weak signal would be a signal with virtually no amplitude.

It's normal to have to increase the intensity as you go towards the maximum sweep rate but you shouldn't have to max it out. The scope's CRT or the internal drive circuit may be a bit weak.

The drive signal looks OK but I'd expect the 'corners' to be a bit more square. Install the new FETs to see if it the driver IC survived.

Dr Zeus 27th December 2010 11:04 PM

You are right, it was my scope. The above image had the scope set to ~2uS which I gather was too fast or my scope is a little off, or old. When I set it to 2mS the image looks better, a little slanted though.

AC trigger, 5v/Div, 2mS

I'll give new outputs a whirl.

Dr Zeus 1st January 2011 04:52 PM

This amp is fixed! It just needed new IRFZ44N and IRF5305 output transistors (2 of each)

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