ZX Spectrum Diagnostic Board

After buying a few faulty ZX Spectrums on eBay which were having memory issues I decided to build a diagnostic cartridge to help troubleshoot the RAM issues.

After looking through various internet sites I eventually found a schematic and diagnostic ROM by Dylan Smith.

The original site however is no longer online but I did manage to find the schematic in waybackmachine. After creating this in KiCAD I created then created a PCB:

There are 2 ROM’s available which work with the Diagnostic board, Dylan Smith’s ROM and Brendan Alford’s ROM

Brendan Alford’s Diagnostic ROM
Dylan Smith’s Diagnostic ROM

The Diagnostic board has been configured as follows:

  • Page 0 – Dylan’s diagnostic 48k ROM v0.1
  • Page 4 – Flash utility
  • Page 5 – Brendon’s diagnostic ROM v0.38
  • Page 6 – Dylan’s diagnostic 48k ROM v0.1

When page jumper 1-2 is selected the board will boot from page 0

To change to boot ROM:

  • Set page jumper to 2-3
  • Plug in cartridge
  • Power on Spectrum
  • Enter command: CLEAR 32767
  • Enter command: OUT 31,36
  • The flash utility will now load
  • Press enter for other options
  • Press x for erase
  • Select 0 (Erase page 0)
  • When Done, press any key to exit
  • Press enter for other options
  • Press c for copy
  • Select 5 for Brendan ‘s ROM, or 6 for Dylan’s 48k ROM 
  • When Done, press any key to exit
  • Press enter for other options
  • Press p for program
  • Press 0 (ROM is written to page 0)
  • When Done, press any key to exit
  • Power off Spectrum. 
  • Change page jumper to 1-2
  • Power on Spectrum and ROM tests will begin.

For further information on using the diagnostic board with Dylan Smith’s ROM:


    • Alan on September 23, 2021 at 01:56
    • Reply

    Hi, enjoyed reading that. I had a 48K issue 4 back in the 80s and (like you) wanting to relive my childhood, recently acquired grey 128K +2. Still need to make a few tweaks to it (TR4 wrong way round etc), but after reading your post, I’m now keen to build one of these diagnostic boards too. Do you know if the KiCAD files are available anywhere or do we only have the schematics? (I haven’t checked ebay yet, but maybe there’s even bare PCBs if I look hard, or maybe that’s wishful thinking!)


      • Greg on September 23, 2021 at 06:48
      • Reply

      Hi Alan,

      I based my design off this schematic:

      If you like you can also buy the assembled PCB’s. I have a few listed on eBay or you can purchase from my shop:

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