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Sams Computerfacts CSCS26 Amiga 500

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Sams Computerfacts CSCS26 is Howard W. Sams & Co.'s technical service data for the Commodore Amiga 500, copyright 1988. Its cover lists complete schematics, preliminary service checks, troubleshooting tips, waveforms, replacement parts lists and a semiconductor cross-reference. The hosted scan has 73 PDF pages and no text layer. A previous owner wrote a note on the cover about a power supply fault.

Contents

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The folder's own index gives these printed page numbers.

  • Safety precautions, p. 11
  • IC pin-outs and terminal guides, p. 16; logic charts, pp. 12-16
  • General operating instructions, p. 17
  • Disassembly instructions, chassis overall view and miscellaneous adjustments, p. 18
  • Parts list, pp. 19-22
  • Line definitions, p. 23; block diagram, p. 24
  • Test equipment and troubleshooting, pp. 25-29
  • Schematics of the system board, the keyboard, the power supply board and the power supply regulator board, with grid-trace location guides and photographs

What it settles

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  • The power supply's internal circuit, which Commodore's A500 service manual does not give. The troubleshooting section works through fuses F1 and F2, the bridge rectifier BD1 (about 156 V DC at its positive output on 120 V mains), the switching transistors Q1 and Q2, the shutdown transistor Q3A and the overvoltage shutdown SCR IC1, and the regulators for the +12 V and −12 V outputs.
  • Two adjustments in the supply: the shutdown level control, set for 2.1 V between the base and emitter of Q3A, and the 5 V adjust control.
  • Its service method for the supply: disconnect it from the system board so that high voltages during servicing cannot reach the board, never run it without a load (a #1129 lamp will load the 5 V output), and power it through an isolation transformer with step-down and current limiting, starting at a low AC voltage. It warns that defeating the shutdown circuits can cause further damage if full 120 V AC is applied.
  • A reset check: pin 18 of the 68000 should stay low for about half a second after switch-on and then go high.

What it does not contain

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  • Commodore part numbers for most parts, or anything about board revisions other than the one Sams examined.
  • The A501 RAM and clock expansion.
  • The model or part number of the supply that was examined, in any part legible on the scan. Several makers built A500 supplies, so check the circuit against the unit on the bench.

Pages that cite this document

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Download: Sams Computerfacts CSCS26 Commodore Amiga A500.pdf (7.4 MB)

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