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-   -   Alpine MRV-1000 yet again (http://www.diyaudio.com/forums/car-audio/118812-alpine-mrv-1000-yet-again.html)

spooney 6th March 2008 12:18 AM

Alpine MRV-1000 yet again
 
Well after seeing that the parts I had used for replacement before appeared to be fakes I decided to give this old dog one last try.I replaced the IRFZ44ns with FQP50N06's because fairchild had it listed as a direct replacement on their site.I also replaced the 2SA1265N's and 2SC3182N's with FJA4210's and FJA4310's.I used a ten amp fuse on each side for initial power up and although the amp powered on it idles very high (around 10A) and starts to slowly rise.Just wondering where I might look from here.There is rail voltage present on the outputs verified by a quick check before I shut the amp off.Do I need to change the gate resistors for the new power supply fets or is the FQP50N06 not a good choice for this amp?I'm admittedly lost on this amp.

Perry Babin 6th March 2008 12:29 AM

The problem may be the bias current in the audio amplifier section. In each channel, measure the voltage from the emitter of an NPN output transistor to the emitter of a PNP output transistor. The voltage should be very low (ideally 0.000v).

spooney 6th March 2008 02:24 AM

the original channel measured 0 volts.the newly repaired channel started at about .08 volts and then just keeps rising.

Perry Babin 6th March 2008 02:36 AM

The bias circuit and the outputs are not a good match (assuming that there are no other problems).

Try increasing the resistance of Rx31 (connected to Qx11). This will reduce the bias voltage on the drivers. Try a 3.3k initially. You want to use the value that just gets the emitter to emitter voltage to ~0v. If you go too far, you will have notch distortion. You will need to carefully recheck the biasing as was described in an earlier thread.

It would be better to use the original outputs but this should make it work with these outputs.

spooney 6th March 2008 11:03 AM

There are no part designations on this amp and I have no schematic.Do you know where I might find this resistor Perry?If its best that I uses originals I can just pull the outputs that I have in there now and get originals from MCM.Will the FQP50N06s work ok in this application or should those be original as well?

Perry Babin 6th March 2008 11:49 AM

The 50N06s should be OK.

I can't remember ever seeing an Alpine amp without circuit board designations. Email me and I'll send you the schematic.

The biasing transistors are mounted between the outputs. The resistors are connected between the outer legs of the biasing transistors.

Using the originals would be the best option unless you're willing to do extensive testing to confirm that the new parts are suitable replacements.

spooney 6th March 2008 09:11 PM

I'll probably just get the originals.I was hoping the fairchild parts would be a drop in replacement.I've had enough issues with this amp already so I'll go the safe route and get the 2SA1265's and 2SC3182's.I noticed that there is a 2SA1265N and a 2SA1265.Would either of these parts work or do I need the part with the N in the part number as that is what is in the amp now?I'd still like to see that schematic though so I will e-mail you when I get a minute.Thanks again Perry

spooney 10th March 2008 12:41 AM

would a difference in max hfe in the replacement transistor cause the problem I am having or is it totally unrelated to that?I just noticed that the FJA4310 has a max hfe of 180 and the 2SC3182N has a max hfe of 160.

Perry Babin 10th March 2008 08:28 AM

That's insignificant here. The threshold voltage is likely the problem. The bias voltage used for the OEM parts isn't sufficient to make them conduct. It's likely that the fairchild parts requre less voltage to make them conduct and that's why you have high current flow with them.

There's also a possibility that the amp is oscillating with the fairchild parts. That would cause excessive current flow through them. It would be easy to see on an oscilloscope.

spooney 13th March 2008 09:29 PM

i removed the fairchild outputs and the amp powers on now and idles as it should.I'll have to find somewhere to get the originals from.MCM seems to be out of them at the moment.


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