PowerBass ASA 2000.10

[Dr Zeus]

I got this amp in for repair from a collector who was hesitant in letting me work on it. I've never worked on a Class-D amp before but it really doesnt look all that much different thatn what I've learned thus far. This amp is big, one of the largest I've worked on. On the cover it says 4000w which makes it potentially the most powerfull and/or most dangerous.

The amp powers up with a 10a fuse and plays, but its got an absorbant ammount of distortion. Theres not much the built in corssovers do for tuning the signal. If I adjust the qBass control it almost looks like the amp is about to correct but by the time I stop turning the qBass knob the signal is well distorted again.

I checked the JRC5034G opamps and all have +/- 13.5vdc. The large coild in the center measure up about 110v, and the rail voltage is +- 70v.

When I remove power from the amp, the test speaker POPs. I think it might be a Class-D thing, as if I simply remove REM the amp powers off and the speaker wont pop.


Here is what the signal always looks like on the scope while the test speaker is distorting badly.

 

[Perry Babin]

If you remove main power, the mute circuit can't do its job and the amp may pop. This can happen with all types of amps.

Post a photo (best quality you can get) of the component side of the driver board. I think I know what this is but I want to be sure.

 

[Dr Zeus]

Heres the pics. In these, youll notice I removed and moved some of the rail caps. The ones I pulled off tops were puffed out, and now that they are off the board I can hear sometime moving inside when I shake them. They measure OK with the meter though. To balance the caps, I left 2 on the 'bottom' row and moved two to the top row. I'll likely need to buy probably all 8 for this area after the faults are solved. The amp still powers up and makes the distortion.

I shuld have mentioned because I just love this forum. A previous repair shop quoted $120 to fix this amp

 

[Perry Babin]

That's the edge of the driver board. I need to see the component side of the driver board (preferably with the circuit board designations legible). Set the camera to macro mode if necessary.

 

[Dr Zeus]

Sorry bout that, I thought you meant the whole PCB. Here are a couple of the driver board. If you'd like sharper pictures I've got them on my PC. I think photobucket reduces quality.

 

[Dr Zeus]

Here are the best pictures I've got. Very legible. Any better I'll have to pull the driver module off the PCB, which isnt such a big deal anyway...

 

[Perry Babin]

If Q124 and Q125 are MPSA42s, I'd suspect them.

 

[Dr Zeus]

They are indeed. Not sure if you can see, but Q120 and Q121 are also MPSA42s. Suspects as well? The rest in that line of transistors are MPSA56s.

 

[Perry Babin]

Those are the only two that commonly fail. They overheat. I replace them with MPSW42s.

When you check them, check them for leakage and open junctions as well as shorts (they rarely short).

 

[Dr Zeus]

Just pulled Q124 and Q125 off. I forget which one came out of where, but one of these boys isnt measuring anyting at all on the meter. its completely OL across all leg combinations. Looks like both are going to be replaced!

BTW, Has anyone ever told you how much AWSOME you are?

 

[807tubenut]

Perry knows how awesome he is....

 

[Dr Zeus]

This amp is FIXED!

Two new MPSA42s and 8 new rail caps.

Thanks Perry!