In post #1 you have a link to a site that has schematics and photos. I noticed in the photos that their amp has a dark (burnt) area in the same location as your amp. This would suggest that this is a common problem. I hope you bought extra parts.

 

Good eye. In that case, increase R17 to 1/2W or 1W.
R17 must be getting very hot. You can even try increasing the value from 220Ω to 330Ω and see how this affects the performance at max output.

 

As stated before, I'm a bit surprised to see 0Ω at R18 and R19.

I agree. Obviously the original design had emitter resistors on the output stage. Some production person probably complained the amp was 50 cents over budget and the engineer was forced to cut costs.

 

On second viewing, I think the board you are seeing here is the sub-woofer amp which is a different circuit:

 

Are you referring to the High Frequency board I've posted above, or another one? The one I've posted was burned from the original R17 going up but otherwise I didn't see much other discoloration. Mine has a lot of darkness this time from the smoke rising when the Q9 and R17 went up at the same time.
As for the rest of the board, the Low Frequency amplifier does have a *lot* of discoloration and all pictures I've seen of these boards online all show the same thing. Here's an angled shot of mine:

Everything seems to check out on it. These subs are known for running very hot and some have even modified them to put a small cooling fan into the case. I originally thought my problem would be found here before I dug deeper and found the smoked out R17…

 

The brown PCB is darker around the components indicating elevated temperatures. This could lead to an issue on reliability, and in a worse case scenario perhaps thermal runaway.

 

Good eye back at you.

 

Hello,

When you look at the schematic, R15, R16 and Q6 make a "rubber diode".
These parts are used to set the quiescent current in the power transistors.
Read this page for more info:
http://www.st-andrews.ac.uk/~www_pa/Scots_Guide/audio/part2/page3.html

Bertus

 

I agree. Obviously the original design had emitter resistors on the output stage. Some production person probably complained the amp was 50 cents over budget and the engineer was forced to cut costs.

That was exactly my thought when it was pointed out to me that they're just jumpers on the final board.

Honestly, if I had the board in my shop, I'd probably add those resistors. In addition to being a very convenient place to check the actual running bias of the output devices, they also aid current sharing between the TIP-41 and the TIP-42...it's easier to get both transistors biased correctly at the same time if they're there.

 

Hello everyone! I finally got my parts in and got time to go through everything. A huge thanks on the suggestion to take the parts out of circuit to test them! I found that Q6 was definitely no good anymore. It didn't have continuity but it also didn't have anything left between the emitter and the base. That hadn't shown up at all when it was in circuit. Fortunately that's one of the few components in existence that you can still buy at a local RadioShack, a 2N3904. I double checked everything again on Sunday night and spent last night reassembling. I finally got the nerve to plug it in and….

Only thing I heard was the relay kick on. No loud pops. No smoke. Yay!
I plugged up the speakers and such and tried it out and the amp seems to be working. Only problem is, now that everything's working, I found out that it did take out my left channel woofer. It has a tweeter that's still working but the woofer's dead as a doornail. I swapped channels out and confirmed it with the right woofer which is still working. I seem to remember the other post from years back had the same issue, and a quick check on google seems to back up that it's not uncommon for this system to take out the speakers with it when it goes. Fortunately I found a replacement on ebay for cheap, so I'm going that route since I like the sound of these suckers so much. I just wanted to close this topic out and not leave another orphaned thread on the internets incase some other person comes googling this behind me someday!

So the parts I ended up replacing were:
C3 - 47uf, 25v
R14 - (was close to R17 when it burned up, replaced just in case even though it tested good)
R15 - 8.2k Resistor though it's listed as 6.2k on the schematic. Cross check it with what's on the other board as earlier versions may have a 6.2k.
R17 - 220ohm 1/8watt
Q6 - 2N3904
Q7 - 2SC2235 <--
Q8 - 2SA965 <-- Q7 and Q8 I had to get on eBay, they're obsolete now.
Q9 - TIP41C
Q10 - TIP42C

Thanks to everyone who contributed here! I appreciate it. I don't know enough about the "why" of circuitry to attempt to improve this circuit, I'm merely trying to get it back to factory condition. You all clearly know your stuff though. I've got a tv power supply board that I've tried to fix once or twice on and off for about a year that I'll be posting about in due time but I'll make another topic for that. You guys rock!

 

I just wanted to close this topic out and not leave another orphaned thread on the internets incase some other person comes googling this behind me someday!

Wish everyone was a socially responsible.
I'm sure I'm not the only one who has enjoyed this thread.

Happy listening.