RGB from grey +2 - no sync

[system11]

So I bought a +2 on ebay, it's the older grey type and I then bought a 128/grey +2 RGb cable from retrocomputershack. Testing on my Sony LCD or an old Panasonic CRT I keep for just this kind of problem, both of them exhibit what looks like rapid horizontal and vertical rolling - you can just about see that the colours are ok though.

I had a look inside the computer end of the cable - pin 4 is being used for sync, isn't that what the +2A uses?

I bought a second +2 which although it has 'issues', works enough to display some corruption on the screen - exact same issue.

I turned TR4/5/7 around on one of them as I've read these are fitted incorrectly - no change.

[system11]

Update - plugged it directly into the big LCD and that sync fine although the picture is a little dim. Doesn't work at all on an old Panasonic CRT still.

Is there any way to make these switcher friendly? Not sure why it's even upset given the wide variety of absolute garbage signals I put though here without issue, including arcade PCBs.

[Ast A. Moore]

I had a look inside the computer end of the cable - pin 4 is being used for sync, isn't that what the +2A uses?

The +2 can be factory configured to use the output similar to that of the +2A via a few onboard jumpers. You’ll have to look inside your machine to determine how it’s configured. However, Pin 4 is used for composite sync in both configurations.

Here’s the output of the Spectrum 128K (and some configurations of the +2):

1. Composite video
2. Ground
3. Bright signal from the ULA
4. Composite sync
5. Vertical sync
6. Green
7. Red
8. Blue

Here’s the output of the +2A (sometimes referred to as the Peritel configuration):

1. +12V (via a 1 kOhm resistor)
2. Ground
3. Audio out
4. Composite sync
5. +12V (via a 1 kOhm resistor)
6. Green
7. Red
8. Blue

If your +2 cable is advertised as compatible with the 128K (Toastrack), then it’s configured as in the first diagram I showed, and combined the brightness output with the color outputs via a simple logic gate (made out of three diodes and resistors) inside the male SCART connector. It will also have a flying lead with a 3.5-mm jack for audio.

If it doesn’t have the diodes anywhere inside, then it’s in the Peritel configuration, and you’ll have to check the position of the jumpers inside your computer to make sure they match.

[system11]

Ignore this - I feel like such an idiot.... I had a composite source still physically connected (but not turned on) at the front panel which shares input 5 on the back.... Now it's fine (although still not on the CRT which is weird but not a problem)

[Ast A. Moore]

Doesn't work at all on an old Panasonic CRT still.

Are you absolutely positive the Panasonic treats it as an RGB signal, rather than a composite signal? Without a simple modification, the blanking signal coming out of the Spectrum usually doesn’t have quite enough voltage. If you can manually switch the TV to RGB, it should be fine. If not, replace R9 with a 490 Ohm resistor on the +2 PCB.

Alternatively, desolder the wire from Pin 4 inside the male DIN 8 connector of the cable and solder it to Pin 1 instead (composite video). You may want to add a series resistor to it, too, something in the range of 15–39 Ohms. You can leave this mod there permanently, since the TV will pick up the sync from the composite video anyway.