Good Day All
I have a Phonic SEM712A Powered speaker that I am in the process of repairing. It has 2 Power Amp Boards (Identical, One for LF, and One for HF). The details on the amp boards are : 94v0 pan ks 14, SEM712A/715A AMP(H), PCB Rev_1, 290-37D90-200-0.
The LF Board developed a fault, where it was crackling, and later smoking. When I opened up the unit, I found that R312,R317,R318,R319,R320,R326,R327 Are all burned that you cannot identify the bands. They all are 1/4w 5band resistors, which are on the emitters and other of the Q304 and Q307 transistors.
When on the bench, the 2SA1943 Power transistor got so hot that it actually melted its solder. After a short bit the SMPS was shorted and ticking....
When I removed the 2SA1943 and powered up again (i think that was a bad idea) The SMPS did start up fine, but then there was big sparking, flames and crackling from those burned resistors.
The HF board is identical, so I swapped positions on the PSU/Preamp board, and the LF worked fine. So It is definately only on the 1 amp board.
I can get the values from the other board, to replace the resistors and do comparisons, but perhaps somebody can help me out, as Phonic does not want to give me a schematic.
Thanks Peter