Alpine MRV-1000 yet again

spooney

Member
2006-12-01 4:41 am
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.
 
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

Member
2006-12-01 4:41 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?
 
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

Member
2006-12-01 4:41 am
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
 
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

Member
2006-12-01 4:41 am
still playing around with this old dog.I finally found some original toshiba outputs and installed them and the amp still appears to idle very high and rise slowly.I haven't had a chance to recheck the bias current.If it is indeed still rising would replacing the bias transistor solve the problem?
 
It's possible that replacing the bias transistor could solve the problem but there's no guarantee. It's unlikely that the bias transistor is defective. If the new one has a lower threshold voltage, it would probably solve your problem but there's no way to know if it will be lower.

It would be easier to change the resistors that determine the biasing.

What are the circuit board designations for the output transistors you replaced?
 
The biasing transistor is inbetween the output transistors. There are two resistors directly in front of it. The closest one is R230. The next one is R232. R232 should be marked red, violet, red gold. Change that resistor to 3.3k ohms. That should reduce the idle current. If it reduces it but not enough (particularly at high temps), change it to a 4.7k.
 

spooney

Member
2006-12-01 4:41 am
I know this thread is forever old but I did finally get around to swapping those resistors.I first tried the 3.3 k resistor and although it did greatly improve the current draw it still was up over 1 amp and the voltage measured from the emitters of the output transistors was still about .015 volts. I removed the 3.3k and soldered in the 4.7 k resistor and all appears to be well now.It idles very low and there is virtually no voltage from emitter to emitter.I've been playing the amp for about 20 minutes now at very low volume and so far all seems well.nothing overly hot or drawing excessive current.What further testing can I do to insure that this amp is truly good to go this time?
 
You need to run it up to thermal cutout (or until it is FAR too hot to touch for more than a second) and confirm that the idle current is holding. Check it every 2-3 minutes as it heats up so you can catch it if the idle current begins to increase.

If you don't want to wait for it to heat up from the audio, use a heat gun to heat the outside of the heatsink. Use both audio and external heat. Don't use the heat gun alone.
 

spooney

Member
2006-12-01 4:41 am
I was running some sine waves through it and checking the output with my oscilloscope and I noticed that at low levels the output from the repaired channel looks "fuzzy" compared to the original channel.This was at well below 1 volt of output it was actually more like .2 volts if that.The unrepaired channel has a nice thin line on the display but the repaired channel has a thick hazy looking line when probed.As the volume/output is increased the line on the display starts to look just like the waveform produced from the original unrepaired channel.Does this indicate any problems?I did not notice any differences in sound between the two channels
 

spooney

Member
2006-12-01 4:41 am
what size pot should I use?Would a 10 K pot work?Does this low level oscillation cause any issues?Is this what makes the amp unstable?If I remember correctly a small trimmer pot has three terminals.How would I integrate this into a spot with only two through holes?