Alpine MRV-100M Z48 substitute

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Got this amp in needing all (2) new power supply fets. This amp uses two IRFZ48s.

I think I can use IRFZ44 for testing as I have some of those, but I also have a bunch of matching IRF3205 fets. I know the Z44s are ~15A less powerfull than the Z48s, but the IRF3205s are ~15A MORE than the Z48s, which tells me that IRF3205s might be the better choice. This amp has 4 larger output transistors and might benefit from the added power if it will work.

Would IRF3205s work with or without modification?

This amp is strikingly similar to an Alpine MRVT303 like whats on the dvd. I can tell one or two things are different, but its generally the same layout. Biggest thing is my 494 is surface mounted on the BACK of the board where the T303 is on the front and through-hole.

Since I had quite a few 3205s I went ahead and put a pair in. The amp now powers up but still has issues.

The amp will play a clean sin wave... Unfortunately after about 4 seconds the rail voltage drops from 30v down to ~10 and the sound then stops. Its almost like the amp is going into protection or the 494 is being told to power the amp off. Im thinking the 3205s are not going to make the cut here.
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To be honest, Im not even sure how this amp is working for even 4 seconds. I measured the 494's irregular activities :)


Pin 1: 0.001 (Blips to 0.250 for ~1 second then back to 0.001)
Pin 2: 0.88 & drops to 0.734 (after 4 seconds)
Pin 3: 0.085
Pin 4: 4v & Drops to .2v
Pin 5: 1.66 & Drops to 1.275
Pin 6: 3.7 & Drops to 2.8
Pin 7: 0.002 & Drops to 0.000
Pin 8: 8.89 & Drops to 5.10
Pin 9: Rises up to 1.335 after ~4 seconds
Pin 10: Rises up to 1.335 after ~4 seconds
Pin 11: 8.89 & Drops to 5.10
Pin 12: 8.89 & Drops to 5.10
Pin 13: 5 & drops to 4.10
Pin 14: 5 & drops to 4.10
Pin 15: 5 & drops to 4.10
Pin 16: 0.002 & Drops to 0.000
I think I figured this amp out. Its not the 3205s. I think someone tried to raise the rail voltage via the VR601 screw. Thats probably what burned out the power supply fets in the first place, and also what was shutting the amp off after 4 seconds.

The screw was all the way clockwise, so I turned the screw all the way counter clockwise. The amp stayed on, powered up with stable audio output. I'm presuming this MRV-100M is pretty close to an MRV-T303. The service manual of the 303 says to set the voltage to 53v +-1v so i did; the VR601 pot is turned only about 20% where as before it was all the way. This amp seems stable and hopfully fixed now.
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Actually it wasnt the rail voltage after all. I must have stepped on a few things while assembling/disasembling the amp. Seems now if I have the front cover LED plugged into the PCB; then the amp powers off after ~4 seconds. If I leave the LED unplugged the amp stays online. It looks like Q901 may have slightly burnt the PCB so going back in to check it out.
Were the 3205s getting hot?

This is the way that an amp will act if the power supply FETs aren't being driven properly. Having the pot fully clockwise would make the pulse with greater. At a lower setting, the pulse width would be less and drive problems would be masked.

If your power supply doesn't have variable voltage, you should set the pot fully clockwise and measure the DC voltage across the emitter resistors. If the DC voltage doesn't increase with the higher rail voltage, the problem is in the power supply. You can also remove the rectifiers to confirm that the power supply FETs are being driven properly.
There might be a combination of issues here. The amp shuts off if the rail voltage is higher than 38v, and thats about 2/3 the way turned clockwise on VR601. If I leave the rail voltage at say 30v, as soon as I plug in the cover's LED the amp also shuts down.

I let the amp run awhile with and without test speakers and the 3205s dont get hotter than about 90F. The rectifiers get hotter than the 3205s.
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That would indicate that the FETs aren't being driven off quickly enough. To confirm, remove the rectifiers. That should make the pulse width go to 50%. If the current draw increases or the 3205s begin to heat up, they're not likely going to work in this amp without modifying the drive circuit.

What are they using for a pulldown resistor between pins 9/10 and ground?

F10P20F in D601
F10P25F in D602

On the scope I see a clean ~28v square wave, middle leg of the 3205s. The 3205s dont get hot at all. The transformer coil got to 84F after ~10 minutes while the 3205s are at 77F. My work area is about 75F.

Still; if I insert the REM/power LED connector CB901 the amp powers off. I verified the LED is good and lights up with the meter.
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