The Marathon 32K aka Lambda 8300 is a ZX-81 clone, more or less. More info here.
2010-08-11: Voltages + 12V and + 5V are present and ok. The voltage regulator is ok, all diodes seem ok.
2010-07-19: I bought the machine. It came with a manual (in Danish), but without a power supply. It requires a 12V DC psu (with + on center). When powered on , the power LED (red) lights, I get a gray picture on the TV set (tried both UHF channel 36, and the AV (composite video) input), but nothing else happens. When I press the RESET button on the keyboard, the gray picture sometimes changes pattern. I tried re-seating all chips (Z80A cpu, 2364 PROM, 4 x 2114 RAM chips and the ULA (marked "C4005" and "2783"). It didn't help.