Walkthrough of a AudioBahn A8000T Repair


AudioBahn A8000T Mono Amplifier, Class AB



This one came in from the Internet. Had some blown power supply FET's.

Most Mono amplifiers these days are Class D. This one is a Class AB, but it is a mono channel. Just 1 channel, made for subs for the most part. While not as efficient as Class D, these AB sub amps can still be powerful, and have a good sound to them. And it costs less to make these AB amps than it does to make a Class D amp. The AB amps are much simpler in design and implementation.


Click image to Enlarge the image



Someone has already taken apart this amp and put it back together, it had very loose screws on the clamps and was missing 4 heatsink insulators/pads. If you send an amp in with missing pads, we charge $1 each pad. So try not to tear up or lose the pads, they are essential for operation and reassembly of the amplifier.

In this shot, you can see where someone must have fired up the amp with the heatsink insulator missing. The tab of this transistor is "hot" and the heatsink is grounded. Therefore the insulator is needed to keep the tab from shorting to the heatsink, without the insulator in place, and the clamps not installed it will arc voltage from the tab of the transistor to the heatsink. Therefore the black "soot" is visible where this occurred.

You can see the FET on the far right is cracked and the legs are discolored. Sure sign that at least this one FET is blown. However after testing, we find there were several FET's that were blown. Even though they don't outwardly appear to be blown. If you would have just installed 1 new FET, it would have went up in smoke again the first time you turned it on. Even if you replace all the FETs, if the FET drivers are bad, or the IC is bad, when you turn on the amp for the first time it will draw lots of current and fry your new FETs. We discourage trying to repair an amp yourself unless you know what you are doing. Just send it to us, in the long run, it will cost you less money.

A full shot of the A8000T's insides.

Here we are removing the amp from the heatsink to begin repairs on the main board.

A closeup of the cracked FET.

A closeup of the power supply IC and FET drivers. Also, notice the FETs on this side of the amp have no discoloration on the legs, and in fact they were all in good shape. However we replaced ALL 8 of them anyway. No need to mixmatch FETs, they are not that expensive. Matched set is always the best way to go.

A closeup of the Audio Amp and Input section of the A8000T. Very simple, because it is a 1 channel amplifier or "mono block" as they call it.

Closeup of the very simple drive circuity for this 1 channel amplifier.

All 8 power supply FETs removed. The 2 remaining parts are not FETs, they are rectifiers, and are not in need of replacement.

A shot of the botom of the mainboard with the FETs removed.

New power supply FETs installed!

Bottom view, new FETs installed, the soldering should look factory original except for the little bit of Rosin-Core residue around the new solder joints.

Pre-Test. Amp fired up, with new FET's installed, all lit up and fan running, no excessive current draw. Next waveforms are checked with the scope to make sure everything is uniform and in proper working order.

A shot of my scope display while testing. Power supply waveform taken at one of the legs of one of the rectifiers shown here. All looks good.

My scope probe under one of the legs of the rectifiers. This can be dangerous (no-hands on probe) so don't try this. I only did it so I could have both hands free to operate the camera. They make clip on adapters for scope probes, but I couldn't find mine. It's around here somewhere, I usually have my hands on the probe at all times when testing.

A shot of the Heatsink. It's a little nasty with the old thermal grease (heatsink compound) all over it so we will clean it up with a rag and apply new compound. We don't mind taking a little extra time to make your amp repair as clean as possible.

All cleaned up, ready for new thermal grease. Isn't that much nicer looking? And it will help the heat transfer even more to have fresh, clean surface and compound on the sink.

The transistors are all cleaned as well, and new thermal grease is applied. The kind I use is mostly clear, so you can't see it very well without clicking and enlarging the picture, but it is there.

Heatsink Pads/Insulators are re-installed and a light coat of thermal grease applied to them as well.

Amp is all buttoned back up and ready for load testing! Almost done!

Load test complete, cover re-installed, all crossovers and switches and adjustments are verified working as well. Wiped down, and ready to pack-up to send back to customer.

Another shot of the amp, reassembled ready to go.

While Chrome is "pretty" on amplifiers, it holds in a lot of heat. A satin finish is best for heat dissipation. Amplifiers used to all be flat black for a reason. With people buying amplifiers these days, based more on looks than performance, chrome has become popular on even the better of the amplifiers availible today. Despite the fact that the amp would be better off without the chrome. That's marketing for you. Amplifier companys are out there to make money, they don't care if chrome shortens the life of the amplifier. They want you to buy a new one when your old one blows up.



Hope this Gives you a good idea of the work we do at dB-r Electronics.
We do it right!