Power amplifier SEEA 200W - SMA300

I got this device from my friend. It was laying in his garage and he had no concept what to do with it. He even wasn't sure if its still working. It took about a year when finally I had time to look into it. I wanted to check its condition, but I hadn't speaker with appropriate jack, so I took of metal shield and soldered loose wires directly to output PCB. I was amazed when it turned out that all input channels are working. Only in three of them, potentiometers from higher frequency sounds were blocked and I was not able to turn them. Generally I was positively supprised and I thought it will be enought to replaced broken potentiometras and it will be it. Unfortunately, I had to spend a bit more time on it.

I removed broken PCB and checked fuses on it. They were burned. In most of cases, it means that too big current flew through power transistors. I checked them, and there was permament short circuit between emiter and colector. I could simply replaced them, but when you don't know real cause of demage, you can replace them over and over again. Power ending consist from three pair of transistor. When I soldered them out, it turned out that some of them are still good, so I left them for future experiments.

For fixing such devices you can use your intuition or schematic. Second option is better, because final result is not a matter of coincidence. Unfortunatly it is impossible, to find drawings for this device in the Internet. I had to spent two evenings doing the reverse engineering. Below you can see schematic.

Power amplifier 200W SEEA SMA-300

As one can see, transistors are connected in mixed way. Two of them are in paralel and connected with third they make Darlington circuit. After removing broken transistor I decreased minimal current. When fuses didn't burned out after switching on power supply, I thought, I'm on a good way. Unfortunatly, after connecting mp3 player, I didn't hear and sound from a speaker. Minimal current should have influence only on distortions on output. It shouldn't block output signal. I downloaded signal sample of sine wave with frequency of 500Hz. Using an oscilloscope I could track down place where singal was lost. It turned out, that transistor from class A amplifier doesn't get it on its base. After replacing it, signal flew from input to output. I only thing had to set up again was minimum current for transistor from class B amplifier. At the end I changed also very small 4,7uF capacitors to 47uF, isolators between transistors and heatsink. I also arranged cables inside and replaced broken potentiometers. Some pictures below:

PCB after removing broken parts
Bottom side
During diagnosing I found it very helpful to attached signal directly to PCB
Very early version of fixed PCB
Fully fixed with all parts soldered
One of a broken potentiometers. It looked like rusty.
Inside I found a big mess in cables.
Signal and ground wire were a single/separate cables.
LED didn't have isolations on legs
Front panel also required some refreshment
Volume potentiometers looked like as if the were cut with some heavy tool.
DREMEL did his job

I have isolated LED
Clean up mess grouping cables according to their destination

Signal cables changed to shielded one
Inside after all
Front panel after washing all knobs.

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