M&K V-90 subwoofer amp repair - diyAudio
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Old 10th April 2015, 03:11 PM   #1
Joekewl is offline Joekewl  United States
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Location: Columbia, SC
Default M&K V-90 subwoofer amp repair

Hey all, long time reader, haven't posted in a while but wanted to see if anyone could help.


The situation: My grandmother bought a house that came with a home theater, while messing around with fixing some issues with it, I found an old dysfunctional sub she was throwing away. Figuring I could make use of whatever still worked, I snagged it. It was a m&k v-90.

Initially, it didn't cut on at all, so I opened it up, and there were two resistors (500ohm/5w R-72 and R-74 located on either side of the fuse in the pic) and two diodes (1n4-742A D-17 and D16 located directly under the fuse next to the gold caps) that had noticible heating damage. So, I desoldered them, cleaned the circuit board as best I could, and replaced them with new components (The resistors I changed with 2x series 250 ohm/5W to help with some of the heat dissipation).

Now, plugging it in, it cuts on just fine. I haven't tested the sound from it yet, but there's still a problem. The D-16 (located under the lower gold cap in the picture) diode gets extremely hot very quickly (~5 secs) so I unplugged it. The other one doesn't seem to have any issues atm. The overloaded diodes bridge the negative power/signal terminal and the other sides go to all over. There doesn't seem to be any other places that have noticeable damage/overheating. The voltage over the components when powered on are .8V on R74(left), 34 on R72(right). On diodes, for the D16(lower that overheats), it jumps to 12.4 then continues to climb slowly, for the D17 it goes to .8V and hangs there.

So, I'm wondering if anyone has any hints as to where the problem could be? And/or is it probably fixable (for a decent price)? Obviously, I could just get a new plate amp and tack it on, because the driver tests out to be in good shape. However, I'd like to fix this because it's fun, and I've got access to all the basic components I could need (caps, res, diodes, some chips). Most of my audio experience is in speaker cabinet/crossover design, and would like to work on the amp. So, if it is a matter replacing basic components, all it costs is elbow grease. Anyone got any ideas to help me get started?

Thanks in advance,
JoeKewl
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Old 11th April 2015, 02:10 PM   #2
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Agreed, it's much more fun to fix than to replace

That's a really good close up image, but could you upload one that includes the whole board and another of the backside of it?
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