ZX Diag Cart – DIY kit



ZX Diag Cart – DIY kit

Update Jun’24: added green board option
Update Mar ’22: added link to community 3D printable frame for the ZX Diag Cart

The ZX Diag Cart will visually show all kinds of details of your ZX Spectrum, which is both looking cool and useful at the same time!

It will make diagnosing defects a lot easier:

  • Quickly check if voltages are present
  • If there is a dataline ‘stuck’
  • If the M1 line is working…

If you are thinking of repairing your own ZX Spectrum (any model, including many clones), this is the tool you must have!

DIY kit

The kit contains:

  • The circuit board with SMD parts already assembled
  • All parts in separately labeled bags with their part numbers for easy matching
  • IC’s and sockets on anti-static foam
  • The manual and DIY build guide

Hardware features

Autodetection of ZX Spectrum model

The ZX Diag Cart automatically detects to which ZX Spectrum model it is connected, so it can control the ROM signals on ZX Spectrum 16/48K, 128K+, +2, +2A or +3, and many clones.


The ZX Diag Cart has 20 LEDs showing these groups of information:
– Control signals status, indicated by the blue LEDs
– Data lines status, indicated by the red LEDs
– Power lines status, indicated by the green LEDs

The speed of change of the LEDs is limited by a clever timing circuit, so it’s easier to see what signals are acting and which are stable

Slider switch

Slide it to the right to have the blue LEDs show the status of the data lines present on the edge connector.
Slide it to the left to have the blue LEDs show stages of the diagnostic software.

ROM1/OE jumper

This jumper is for compatibility with the Brazilian clones that use this line for audio.
When using a Brazilian ZX Spectrum clone (TK90X and TK95), disable the jumper (store it on one of the pins).
Enable this jumper for original ZX Spectrum models and other clones.

J1 jumper option – FLASH/SPECTRUM

When a jumper is present on pins 1-2, it will enable the integrated diagnostics software.
When a jumper is present on pins 2-3, it will have the ZX Spectrum boot as normal.

J2 jumper option – M1 Ignore / Normal

When a jumper is present on pins 1-2, it will ignore the M1 line in case it’s not working properly.
When a jumper is present on pins 2-3, it will use the M1 line.

Of course, when the M1 line of the Z80 CPU is failing, the Z80 CPU must be replaced!

Integrated Diagnostic Tools

The ZX Diag Cart comes with the ZX Spectrum Diagnostics software, developed by Dylan Smith and Brendan Alford:

If lower ram fails, the software also shows border effects to indicate bad chips:

The manual containst much more information about how to debug such a situation.

The basic diagnostic tool tests:

  • ZX Spectrum Internal ROM
  • ZX Spectrum lower RAM
  • ZX Spectrum upper RAM
  • ZX Spectrum RAM paging*
  • Interrupts

*With 128K models

There are more features that you can launch with a key-press directly after powering up – this is shown on screen.

Needed to assemble the kit

If you ordered the kit, you additionally need these tools to assemble it:

  • Solder wire
  • Soldering iron
  • Side cutter to cut the legs of parts
  • Coffee or tea – and a beer for when you’re finished

SMD parts

The SMD parts have already been soldered on with the DIY kit.

Firmware upgrade instructions available

Follow the intructions on this page to upgrade your Diag Cart to the latest firmware:

ZX Diag Cart upgrade instructions

3D print design for frame

Richard shared this wonderful frame, to print yourself, if you prefer this instead of the rubber feet: https://www.prusaprinters.org/de/prints/138684-zx-diag-cart-case



What is included with the DIY kit:

The board after assembly (green version):

Additional information

Weight 0.2 kg

Black, Green

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4 thoughts on “ZX Diag Cart – DIY kit

  1. The diagnostics cart was easy to assemble, especially given the great step-by-step instructions. All parts are of high quality and fit together perfectly. I’ve used it a few times now, and it has been great in diagnosing basic problems, or confirming stable machines. Even used it with a Harlequin 48K!

  2. Wow… I was really impressed by the completeness of the kit! All parts are sorted and labeled, Very clear and detailed instructions: Even a beginner should be able to assemble it. If you have some soldering skills and a soldering iron with a fine tip, there should be no problem getting this kit to work.
    Highly recommended!

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