Help with Subwoofer amp

[Broovwa]

Hello all!
i'm having a bit of a stab in the dark with this one. I have an 18 month old Krix subwoofer that slowly died around xmas time. The symptoms of this death are as follows:
- when i switch the amp on it outputs a loud hum that is unresponsive to the preamp volume control
- the amp makes the same loud hum whether an input is present or not
- auto turn on seems to work properly - when i set it to auto-turnon, it switches on without a hum, and when the volume of the input gets loud enough, the amp switches over to the active state (green light for a split second), then the hum starts (back to red light to indicate clipping).

The subwoofer seemed to sound a little flat (or just different) for a few weeks prior to it not working. When i turned it on one day (after it was unplugged for a few days), it did the hum, then i switched it off and back on, and it worked. This happened a few times, then it just stopped working altogether... i just get the loud hum everytime. Made me think that a component or components had failed, slowly.

Anyway, i did a bit of research to find out what components can fail to cause these symptoms. I found a few forum posts that discuss how blown or dry capacitor(s) can cause this to happen in a power amplifier. I proceeded to pull the amplifier apart and locate the electrolytic capacitors (the big ones that were in pairs anyway). I have just replaced them (3 pairs) with new, better components, and the problem is still present.

Can any of you guys give me any other ideas as to what might be the problem? I have contacted the manufacturer (krix) to see if i can get a schematic, but they completely ignored me (no reply).

Thanks for any help guys.

Brad.

[jfitz57]

Stab in the dark is right Try this:

1, Put the two boards in the pictures back together exactly how they were with all of the screws, washers, insulators and whatever.

2, Connect all of the connectors, wires and whatever so that you can test it.

3, Does it still power up and act like before? If so power it down (unplug the power cord!) and remove the white wire from the 7 contact connector and test again.

As far as I can see looking at the pictures the white wire is the center conductor of a coax cable and disconnecting it will split the input part from the power amp part.

Jim

[Broovwa]

Thanks jfitz for the idea. I'll have a crack tonight when i get home and see what happens.

Through a little more research i've kind of come to the conclusion that the next possible culprit is the 2 output transistors under the grey hoods. Toshiba K2698. I've ordered another pair to try in there and see if that helps (only cheap so i dont mind trial error approach). If no good, ill probably just send it to the dump.

[mastodon]

If those transistors are shot, then replacing them will most likely destroy the new ones as well, as the cause is usually elsewhere in the circuit.

[GoranB]

Hi Broovwa,

If I was on your place i would try to resolder all punkts on PC boards

[jfitz57]

Originally Posted by Broovwa

Thanks jfitz for the idea. I'll have a crack tonight when i get home and see what happens.

Through a little more research i've kind of come to the conclusion that the next possible culprit is the 2 output transistors under the grey hoods. Toshiba K2698. I've ordered another pair to try in there and see if that helps (only cheap so i dont mind trial error approach). If no good, ill probably just send it to the dump.

Those are (I am quite sure) not output transistors. It is obviously a SMPS and most likely a class D amp.
I thought about telling you to post this also to "Class D". You would probably get some help there.
As far as resoldering, that was a common fix for single sideded PC boards, This looks like double sided plated through, so it's not likely solder connections are the problem. Don't send it to the dump. If you don't have a junk box just find a box!

Jim

[jfitz57]

Originally Posted by mastodon

If those transistors are shot, then replacing them will most likely destroy the new ones as well, as the cause is usually elsewhere in the circuit.

Yup! First thing I learned about repairing SMPS's was that if you don't replace ALL of the bad components the instant you turn it on it needs to be repaired again.

[Broovwa]

Yeah, its a double sided board. I dont think solder is the problem... however its hard to see under all the gobs of glue that these monkeys used. I've gone over the board under good light and nothing LOOKS like it has burnt out or damaged (although i understand that things dont have to LOOK burnt for them to be busted).

And yeah, it is an SMPS and these transistors are DC to DC converters. I'm in the position where i dont know enough about electronics in general to troubleshoot the board. I'm merely using others experiences that i've found on varying forums as a means to come up with a lead of what i can do to get it working again.

I understand that if i put these transistors in, there may be another problem on the board which will blow them again (if they are blown to start with). There is a chance that it was just a dud transistor and once the one goes, the other goes shortly after.

ANyway, they are cheap, so i guess its worth it. I'll be back with the results.

Cheers Guys.

[jfitz57]

Originally Posted by Broovwa

however its hard to see under all the gobs of glue that these monkeys used.

Glue used to be a big problem. They used to use this brown S#!T that Mitsubishi called Sony bond. Don't know the story behind that but there must be one. It would corrode copper PCB's and also become conductive. Looks like white silicon RTV (not a problem) and that red (epoxy?) stuff. Seen many times where removing it fixed problems with connectors. Not sure why but guess it has to do with not allowing the connector to move. Cut off the glue, unplug connector, plug in, problem solved.

Jim

[Broovwa]

Quick question. What is that big yellow thing behind the two big caps?

and the blue box in front of the big caps?