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Amstrad CPC 464 Troubleshooting Guide
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== Diagnosing the CRTC (IC108, HD6845SP) == The 6845 CRTC generates the horizontal and vertical sync timing, the row address counters for the video DRAM, and the display enable signal. Failure modes: * '''No video sync''' (monitor displays "no signal"): CRTC clock missing, or CRTC output stuck. Probe CRTC pin 38 (DE, Display Enable) and pin 40 (HSYNC). * '''Rolling display''': VSYNC missing. Probe CRTC pin 39 (VSYNC). * '''Wrong colours / wrong character height''': CRTC registers not being initialised by firmware. Suspect Z80 / OS ROM bus, not the CRTC itself. The CPC was sold with multiple CRTC types (CRTC Type 0 = HD6845SP, Type 1 = UM6845R, Type 2 = MC6845, Type 3 = Pre-ASIC AMSTRAD 40226, Type 4 = ASIC CPC Plus). Software that relies on undocumented CRTC behaviour will behave differently between types. The CPC 464 ships with Type 0 (HD6845SP) on Revisions 1 and 2, and with the Type 3 / 40226 ASIC on Revision 3 cost-down boards.
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