Alpine MRV-1507

[Dr Zeus]

Got this amp in for repair. It has part of one channel fried, as well as all the power supply fets. i removed the power half of the board from the heat sync and began removing all the fets. I'm able to apply power and rem to the amp... Regerencing amp ground, Leg 1 of the power supply fets is registering 3.5vDC via the DVM. On the scope the wave form is a perfect square wave from which rises from 0 to +8v. Is that normal for this amp? I've usually seen only +5v square waves on this type of amplifier.

Next, this amp blew through 12x IRFZ48 transistors. Can I use IRFZ44 as a sub? How about just for further testing if no?

Thanks and best regards

[Dr Zeus]

Actually, this amp was using IRFZ48N which appears to be ~130w max transistor. Can I use either IRFZ44N or FQP50n06 as a sub? The FQP50n06 is ~120w max. I put one in the amp and its holding ~74v rail.

[Perry Babin]

A square wave that only swings from 0 to 5v isn't generally a good drive signal. Most swing from about 0 to 10v.

The Z44s would be OK for testing.

[Perry Babin]

The rail voltage is supposed to be set to ±49v.

[Dr Zeus]

I have 4x FQP50n06 transistors; one in each cornor basically.

This amp has rail voltage adjustments for both the positive and negative areas of rail voltage. Its a little hard for me to understand why, but referencing amplifier general ground I see positive rail voltage on center leg of D606 rectifier, and negative rail voltage on D609. D608 and D607 are showing 2.9vDC on their center legs. Does that seem right? I would have thought I'd see rail voltage on all 4 large rectifiers.

I still have the power supply board removed/disconected from the audio section.

[Dr Zeus]

Looking at this just a little bit closer, Here are the denomenations for the rectifiers:

D606 and D609 are C25P20FR
D607 and D608 are KCF25A20

On one side of the board, theres D606 and D608. Legs 1 & 3 of these rectifiers are getting positive regulated voltage from the fets.

On the other side of the board, theres D607 and D609. legs 1 & 3 of these rectifiers are getting negative regulated voltage fro mthe fets.

I think this means its OK to only see rail voltage only on D606 and D609. What are D607 and D608 doing for this amplifier?

[Perry Babin]

One power supply produces positive voltage. The other produces negative voltage. The rail voltage can be measured for each supply across the outer two terminals of the 6-pin connector.

The center leg of one rectifier on each side is connected to the secondary ground. The 4 diodes form a bridge rectifier and allow output from both secondary windings to produce power supply voltage (positive from one supply and negative from the other supply). The 2.9v that you read is either from using a bad reference or from the loss in the diodes (or possibly both).

[Dr Zeus]

Referencing amplifier ground, here are the voltages on the 6-pin conenctor:

Pin 1: +49
Pin 2: +2.9
Pin 3: +86
Pin 4: -68
Pin 5: +2.9
Pin 6: -49

I dont believe I am able to find the seconary ground for this amplifier. None the less, the voltage across center legs of D606 to D609 is 98v, and the voltage across center legs of D607 and D608 is 0.00v. Its like the banks are only getting half the width of the power supply.

[Perry Babin]

Pins 2 and 5 of the 6 pin connector are the secondary ground. The two large screw terminals next to the connector are also sec. ground.

[Dr Zeus]

OK I understand now and I just misunderstood when you said the center leg of two rects are "connected" to secondary ground. I mistook that as measurement should "reference" secondary ground.

I also now set the rail voltage (Pin 1 and 6) correctly as I referenced secondary ground rather than amp general ground.

If thats the case, then I believe the power supply second of this amp is rebuilt and working (With FQP50n06 fets). I'll continue this journey with the rest of the amp.