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Looks like the middle PCB is the amplifier section. Although it is hard to tell from this angle, the middle PCB looks like it has 2 output transistors. Typically, each of these will have 3 leads that will be soldered to the PCB. They will also be securely mounted to the aluminum backplate (which acts as the heatsink). There will either be a heat conductive pad between the backplate and the transistor, or heat conductive grease (thermal grease). I recommend replacing both of these output transistors with the exact same transistors (as labeled on them) or finding a generic. I suspect the problem occurred due to poor heat sinking of the output transistors, which cause these IC's to fail -- typically due to internal solder joints breaking down, which is why the sub might work for a short period of time but after it gets warm, problems occur as the heat causes things to expand and conductive connections break down.
Depending on the transistors, they can be as inexpensive as $10 each (or less) and its worth trying to replace them.
Good luck and hope this helps!
Thank you David. Seems like I may have a solution. I will take your instructions to a qualified electronics repair person, then cross my fingers. I'll report back later.
Oh. I just remembered another question regarding the amp. Will changing the output transistors remove the very audible hum I've always heard through the sub? All of my other components are very quiet.
Last edited by moris4wildlife; 03-06-2014 at 07:47 PM. Reason: One more question
Well, just prior to taking the amp to the electronics shop, I took the speaker cone to a speaker repair shop to assess the subwoofer, just in case Dave was correct about the possibility of the speaker being fried (as he mentioned in an earlier post). She hooked up the speaker to her testing equipment and gave it some juice in a variety of frequencies, while I was holding the speaker. My hand was jumping and shaking but the speaker sounded clean; not a trace of a loose component. In fact, she was quite impressed with the clean sound and depth at which the little 8-incher could produce; she ended by saying that "it's definitely a keeper, the problem is not the speaker". So, I later dropped off the amp at an electronics shop, but got a call a few days later from the tech who said he didn't want to try and figure it out; that it would cost more to try and repair the amp than purchasing a similar one. So, this is where I stand now. I am looking to purchase a Dayton 250 plate amp that will drop right in; no modifying the box and the screw holes will match. Seems like a few HSU owners have used this amp with good success, so it's likely what I'll do. Just wanted to put some closure to this thread. Thanks to all for your advice.
September 4 Update- I purchased the Dayton SPA 250 from Parts Express and have used it for about 5 months. So far so good. The sub sounds tight, clean, musical and punchy. Now I hope it holds up. There seems to be a small percentage of amp failure with this model, based on reviews by other owners.
Last edited by moris4wildlife; 09-04-2014 at 11:31 PM. Reason: Update