Editing Amstrad CPC 464 Troubleshooting Guide (section)

=== Expansion port problems ===

* The expansion port is a 50-pin PCB edge connector that exposes the full Z80 bus plus the AY-3-8912 SOUND output, the cursor and light-pen lines from the CRTC, and the +5 V/GND rails.
* Symptom: peripheral inserted, system fails to start. Likely cause: short between adjacent contacts on the edge connector due to oxidation. Clean the gold-plated fingers on the main PCB and the peripheral.
* Symptom: peripheral inserted, system starts but the peripheral fails. Check the /EXP signal on pin 48 (active-low strobe asserted whenever an expansion peripheral is selected) and the ROMDIS / RAMDIS signals.