Alpine MRV-T301 with a problem I don’t understand

I have an Alpine MRV-t301 that I purchased with a fault - the seller described it as ‘no sound, it just stopped working’. Powering it up with music input and speakers, the amp plays music but kicks in and out steadily a second or two on and then off absolutely rhythmically.

Opening it up, I notice that there’s a large filter capacitor near the small transformer by the 12v input that is way too hot to touch within 30 seconds of turning the amp on. Desoldering it from the board it measures correctly using the Capacitor test on my DM. It’s 3300uF 16v. I’ve also checked all the diodes around it and all seem to be good.

None of the transistors on the heat sinks have shorted and there is no visible damage anywhere on the board. I don‘t have a scope and I’m very new to this but I have read an awful lot on the internet and watched a tonne of videos. I have managed to download a schematic but the resolution is so bad I can’t read many of the details.

Could someone which more knowledge than me perhaps give me some hints as to where to look. Also perhaps answer the following if you can from what I’ve provided so far:

1. Could the hot capacitor be the root cause/problem/fault even though it reads correctly on the DM when out of circuit?
2. Does anyone have a service manual for the MRV-T301?
3. Is absolutely steady cutting in/out of the music indicative of a common fault?

Probably worth saying that it looks like this amp has never had the cover off and not been worked on before. The inside is clean as a whistle.

If it seems I haven’t done enough homework, let me know also.

Thanks very much

Stephan
 
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Hi Perry, thanks for coming back to me so promptly.

I’ve soldered the capacitor back in. The designation on the board is E605.

I already have the schematic you linked to, but the resolution is very low and I can’t read half of it and because I’m not strong in this it’s not very helpful with half missing.

I followed the advice in your starter tutorial and replaced the 20A fuse with a 10A one. It blows immediately.

I can’t measure the output as requested without putting the 20A fuse in, which I’m hesitant to do.

From this I suspect that the reason the cap is so hot is because the PS goes high current. I now also wonder if the regular on/off it was doing with the 20A fuse is protection kicking in?

As I mentioned in my original request I don’t have a scope.

I think I may buy one because I’m super interested to learn about this stuff.

Is there a logical next thing to check without a scope and without putting the big fuse back in?

Thanks
Stephan
 
Did you check the output transistors to see if any were shorted?

Did you check the rectifiers?

If needed, can you easily desolder L603 and L604 (near the power transformer)?

Don't buy a scope blindly. Email me with any that you're considering buying.
babin_perry@yahoo.com

The attached diagram is a similar circuit that's much better quality.
 

Attachments

  • alpine - SM - MRV-T501 MRV-T505.pdf
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Thanks! I desoldered the two chokes L603 and L604. Both measure open circuit, which I think is correct for a coil.

I removed the output fets and they are all fine as far as I can tell with my DM on diode mode.

Will email you regarding my plan to buy a scope.

Thanks
S
 
Ok. Now have scope and a PSU other than a car battery and report as follows:

Using the car battery as a power source the unit powers up when the chokes are removed and does not blow the 10a fuse.

Something in one side of the output stage started smoking and smelling and I can’t tell what.

I then put in the bench psu on instead and set it to only 9v it draws about .5 amps.

Q515 and q517 get warm and d501 is not lit. On the other channel nothing gets warm and the diode glows red.

I did some random measurements with the scope and on the centre legs of what I think are the rectifiers in the power supply D605 and 606. One has +18v and the other -18v.

Thanks for you help so far
Stephan
 
Attached
IMG_5648.jpeg
 
Yep there’s a difference between the readings on the good side and the bad side on the transistors.

Not sure on the diodes. The mrv501 diagram is quite different.

I need to stop for the evening unfortunately.

Why did the fuse blow with the chokes in? Are there two problems or is it all related to whatever is happening in this amp channel?