M&K V-90 subwoofer amp repair

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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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I admit to digging deep on an OLD thread to find any reference to the Miller & Kreisel V-90 sub, but here I am.

I picked up one of these recently, and got it primarily for the oak veneer box that matches pretty closely with the Hafler 300 speakers I also recently got. (And they all match the furniture in out guest room.) So my questions are:

Did this fix stick? The one I got works fine. I just don't like how low it doesn't hit compared to my smaller SVS SB2000. I don't believe it makes sense to replace the amp without replacing the short-excursion 12" driver.

Given the size of this sealed box, would anyone have suggestions on which driver and plate amp from PE might be the best choices to get at least close to the output of today's 12" sealed subs from ID sellers?
 
I'm not sure what you're trying to tell me. Is this discouragement? I'm not fixing a broken one, I'd be re-engineering it to be something it couldn't be when it was born. ;)

The sub as it is outputs only slightly lower than the two 6-1/2" woofers of the Hafler 300's, and with little force. I don't have any decent measurement tools (yet) but the spec for the low end on the Halfer 300's given in the manual is 35hz. They don't give a tolerance, but I'd be surprised if it was less the -6db at that frequency.

I can't even find a spec for the M&K V-90, but it's probably similar though with a little more SPL. The driver cone is barely moving. By comparison, the SVS SB2000 driver can be pretty active and level line on response to about 30hz. That's the performance I'd like to see. I am NOT expecting (nor wanting) 20hz output. I just want to faithfully augment what the 30 year old Hafler's can do.
 
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