ZX Spectrum 128 (toastrack) / grey +2 RGB Scart BLANKING signal

This document only relates to the ZX Spectrum 128K ‘toastrack’ and the grey model ZX Spectrum +2, NOT the black ZX Spectrum +2 or +3!

I once started writing a document with several video output solutions, fixes and improvements for ZX Spectrum 128K machines.

Old BLANKING circuit for toastrack and grey +2

One of the circuits described in there was the way to get a BLANKING signal for a SCART connector (only used in Europe).
This BLANKING signal is needed to let the TV know that RGB signals should be used from the SCART connector, instead of composite video.

When I started writing those paragraphs, people that made these RGB cables used the SYNC signal from the RGB socket of the toastrack or grey +2.
So I put that solution in the document.

Avoiding ULA damage

After a while I found out that the solution of getting Scart BLANKING from the SYNC pin of the RGB socket, will actually stress the ULA chip inside the ZX Spectrum, causing it to overheat and in time damage the chip.
A replacement ZX Spectrum 128K ULA chip which costs over £30 nowadays..

So I deleted the parts in my document about that BLANKING circuit and instead wrote that a separate power supply must be used for that signal.
Here is the current version of that ‘ZX Spectrum video solutions’ document (click).

New method of getting the proper signal from the RGB socket

Today I found out that there is a way of getting the signal from the RGB socket on a toastrack or grey +2.

It requires an RGB cable that gets BLANKING from the COMPOSITE VIDEO pin on the RGB socket, and..
It requires a modification inside the toastrack or grey +2.

+2 modification

The modification inside the grey +2 is this:

  1. Set the wire for LK2 instead of LK4
  2. Fit a 680R resistor at R9 (there is nothing soldered there yet)

WARNING! After this mod, instead of Composite Video there will be a voltage on pin 1 of the RGB socket (just like the ZX Spectrum +2A/+2B/+3 have by default).
Do not connect this to a Composite Video input of a TV or monitor!

Before:

After:

Again: WARNING! After this mod, instead of Composite Video there will be a voltage on pin 1 of the RGB socket (just like the ZX Spectrum +2A/+2B/+3 have by default).
Do not connect this to a Composite Video input of a TV or monitor!

If you still want to use composite video, do NOT use pin 1 but use a wire from the right side of LK4 to the RF output connector like on the next photo.
The resistor that is soldered to the RF connector should be detached first.

Toastrack modification

(too follow)

Where to find a correct cable?

The correct RGB cables, that get BLANKING from the Composite Video pin of the RGB socket, are the current ones from Retro Computer Shack (click) and Cool Novelties (click).

How to modify a current (bad) cable?

If your cable gets BLANKING from the SYNC pin on the RGB socket, it really should be modified.
Most probably you will have to swap pins 16 (BLANKING) and 20 (Composite Video, or SYNC for this cable) on the SCART connector.

You will also need to do the internal modification as described above.

What happens if you have the correct cable but not yet have the internal modification

In that case the BLANKING pin is connected to the composite video signal of the RGB socket.
That signal works out at roughly the right voltage overall, but whenever a ‘sync’ happens, the voltage goes to 0V, which is why the display jumps or goes wrong on unmodded Spectrums.

Fitting LK2 and R9 (as described above) gives a proper voltage on the composite video pin.

Credits

Credits go to Ian Gledhill for helping me out with this solution.