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Sneaky ZX Spectrum Fault Hiding the TRUTH

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Special thanks to PCBWay for sponsoring this video. Check out their community projects page for open-source mods and fixes for your own retro hardware.

https://www.pcbway.com/

In this episode, I'm repairing a ZX Spectrum Issue 3B that came from a batch of computers previously worked on by another technician. While it initially seemed like a straightforward fix, it turned into a deep dive into the data bus.

After ruling out the ULA and CPU, I discovered that the signals on data lines 5 and 6 weren't reaching a logic high. The culprit? Two partially failed RAM chips (IC20 and IC21) that were dragging the bus down. It’s a great example of why modern chip testers can’t always be trustedβ€”both chips passed several external tests but failed immediately when put to work in the actual machine.

Key moments in this repair:

  • Initial Triage: Identifying previous "bodges" and a poor composite mod.
  • Power Rail Checks: Testing for shorts and verifying the voltage regulator.
  • Diagnostics: Using the Steam Card Pro to pin down IC11 as a failure point.
  • Advanced Troubleshooting: Using an oscilloscope to identify weak data line signals.
  • The Solution: Testing pull-up resistor values and replacing the faulty lower RAM.

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