New Amp Problem...

[tdonoughue]

Ok, so now that I have the new Polks in and have had it out for a number of times, the forward starboard Polk is acting funky. Suddenly gets loud and all muffled sounding, then crackles and goes back to normal. I think the port one is in on the act, too (harder to tell on that one from the helm). I have basically ignored it for a couple of trips (it will go away, right?).

Well, Saturday we had visitors out and it started acting up again. So I popped open the hatch to the amp and gave it a look-see. It is a JL M6600. I thought maybe the input sensitivity rheostat had some dirt or water in it or something? Anyway, I thought moving it a touch one direction or the other would be a good idea to try. As soon as I touched it there was an immediate reaction from the speaker (crackle, become normal, crackle, super loud). Wiggled the input back and forth; it crackled a bunch, but didn't stop its nonsense. Turned it down; turned it up. No relief.

So, I touched the filter mode switch. More reaction from the speaker. Switched it off; no improvement. Switched it to LP and the speaker cut out totally (probably by design or something). Switched it back to HP and no improvement.

The fact that I got reaction from both the rheostat and the switch is leading me to believe that I have a subboard loose in there or that there is some solder breaking in the assembly that holds the two. But I have never had the amp apart yet. And, before you ask, bought it used--long out of warranty.

Anyone have any experience popping one of these apart? Or any input? I am comfortable soldering, etc. And, given how cheaply I picked up the amp, if I break it it didn't really owe me anything...

TIA,

 

[David Analog]

Generally the daughter board that contains the input circuitry is very well strain-relieved. But the connections to the main board may be on a plug for easy serviceability.
At some point the amplifier may have gotten wet and the circuitry, pots and switches you described are typically the most vulnerable.
When you open up the amplifier have a temperature-controlled pencil tip solder iron, a can of cleaner duster, a can of tuner wash and a fine & VERY soft bristle tooth brush. Look for anything that is a white residue indicating exposure to water.

 

[tdonoughue]

Thanks, @David Analog . Got all of that stuff. Looks like we will be doing some surgery...

 

[tdonoughue]

Well... I pulled the amp and brought it home tonight. Popped it open. Here is the back, pre-opening. The screws are all hex.

So, first I took off the back plate (maybe it is the front?) where the inputs are and where the suspect set of switch/pot is:

I found one (1) spider body and some webbing around the lower right pot. Of course, the one giving problems is the pot shown in the middle on the top. I could also see from this vantage that the motherboard is on the top and the daughterboard is on the bottom. So much for my theory. Anyway, couldn't see much from this angle, so I removed the back plate:


So the pots are at the top of this picture. You can see the daughterboard. You can also see no evidence of water on the daughterboard from this angle. Also, no corrosion, bad solder joints or anything. Completely clean. Ah! But it could be a problem on the other side, so I removed the daughterboard:

So there you have it. The full Monty. Every pot, top and bottom, looks absolutely perfect. A couple of little specs of dirt here and there. And one other web (well away from anything). I cleared those out with a toothpick. But distressingly little to comment upon. I considered yanking the motherboard, but the transistors around the perimeter are set to the heat sink/case with that goo they use (the nice thermally conductive paste--I forget the name). Really didn't want to mess with that. Plus, with a flashlight I could see the solders on the back side of the subject switch and pot. All solid. No corrosion. They don't wiggle.

Of course, I went to shoot some contact cleaner on the switch and pot, only to find that the propellant in the contact cleaner went out (probably 10 years ago). So I have a new one of those on order. I will shoot it up and put it back together and see if it works. If not, I will be in the market for a new one... I am not yet optimistic.

End of report. <sigh>

 

[David Analog]

If you remove the main board you must have more silicon conductive grease on hand. But besides a good tuner wash you have done what you can.
Those pots are quality and well sealed. The switches might be more vulnerable though. Maybe get in behind the felt gasket.
That is a great quality amplifier.
Keep in mind that JL Audio has a flat rate to repair the amplifier....as long as there is no water damage. It's a bargain.

 

[tdonoughue]

Yes, I can slip the felt gasket off and get right to the switch. So no problems there. Just waiting for supplies to arrive to do the final cleaning.

On the flat rate repair--ooh! That is new information! Thanks for that. I will be checking that out if this attempt fails.

 

[tdonoughue]

Ahem.

SUCCESS!!!

Installed the freshly-cleaned amp this evening. Didn't get to tune it, so just had the sensitivities a touch over minimum. Sounds great. No crackling. I win! Thanks everyone!

 

[Bill D]

So in the end it was just dirty @tdonoughue ??? Good thing you didn't toss it and buy a new one!

 

[Glassman]

Ahem.

SUCCESS!!!

Installed the freshly-cleaned amp this evening. Didn't get to tune it, so just had the sensitivities a touch over minimum. Sounds great. No crackling. I win! Thanks everyone!

EXCELLENT !

 

[MrMoose]

Ahem.

SUCCESS!!!

Installed the freshly-cleaned amp this evening. Didn't get to tune it, so just had the sensitivities a touch over minimum. Sounds great. No crackling. I win! Thanks everyone!

Your persistence and results are impressive. Now I know who to send my amp to when it doesn't work.

 

[tdonoughue]

Of course, it is hard to tell exactly what did the trick. Was it the spider body? Or the web? Or cleaning the switch? Or maybe the daughter board that was not well-connected? Who knows. There was nothing that was very evident. I just pulled it apart and put it back together after cleaning the switches.

Oh, and I rarely toss anything. Always try to fix it first. One of two things happens: either you wind up fixing it or you wind up really breaking it. At least that way it is clear whether you need to replace it or not...

 

[David Analog]

I think you willed it to work again.