Samson 300dB Active system repair

 This is a pair of 2-way 12" active loudspeakers built by Samson. Each speaker contains two power amplifiers that deliver about 300W rms to its woofer and tweeter.
http://www.samsontech.com/samson/products/pa-cabinets/db-series/db300a/

Circuit Protection system fail
 The on-board power amps are well designed, with very large heat sinks, enormous toroidal transformers, discrete through-hole components and there's plenty of power available. Nevertheless, both amps had a small issue... After years of studio use, the power amp protection circuit was acting up. The protection circuit is a common circuit for many high power amplifiers, especially the class D ones. A protection circuit checks the amp for short circuits, overheating, feedback, over-current and any dangerous condition that could harm the speakers or the board itself. The protection system consists of a power relay and a low-power controller board. Once the controller senses something dangerous, it shuts off the power relay to avoid damage.


Enough with these cheaply made components...
 This is definitely a frustrating situation because the amp could possibly work okay if the protection circuit allowed it to... After some research, we spotted that the 1st speaker had a no-name Chinese 10 Amperes (2 X 5A) relay switch on it. This relay could not be found outside China. The relay was worn out internally so it couldn't make contact. I replaced this relay with a brand new Matsushita (Panasonic) one and it worked okay!
 The 2nd speaker had the same issue but replacing the relay couldn't fix the problem. Then, I spotted a burnt pcb trace and found out that the LM7815 and LM7915 +/-15V regulator ICs were worn out too. I replaced these regulators, their mica pads plus a power resistor. Once everything was soldered and bolted in place, I turned the amp on and voila! Problem solved!
 Both speakers turn on every time and they work fine! It was a matter of poor quality electronic components. The next step would be to change all electrolytic capacitors with new quality ones to make sure the amps won't let us down in a future gig.

Here are a few photos of the speakers:

Cheers,
Thanos