Grey +2 RGB SCART help wanted

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
Post Reply
schneil
Drutt
Posts: 29
Joined: Mon Aug 17, 2020 9:07 pm

Grey +2 RGB SCART help wanted

Post by schneil »

Can anyone confirm which what needs to be set on a grey +2 so the RGB socket functions the same as a +2A/+3?

I would like to use the RGB output on a modern-ish TV. My ZX spectrum +3 works great with it's RGB lead from retro computer shack.

I would like to use this same lead with my +2. I have had the following mods performed:
- LK 2 fitted instead if LK 4 to provide 12V on pin 1 of RGB socket
-R9 changed to 470 ohm
-The +2 outputs sound on pin3 of it's RGB socket ?LK7 fitted instead of LK8?

However the picture on my portable TV is not steady.
1. Grey +2 on portable Tv. https://photos.app.goo.gl/MSTGRh5yefL44Ej19
2. Grey +2 playing Barbarian. ULA snow on the intro? https://photos.app.goo.gl/qfJZ7qFk6fwaAkAZA

Trying the +3 I get a nice steady picture
3. Plus 3 loading rainbow islands. https://photos.app.goo.gl/VDQeM4jDfwcWLrwr6
4. Plus 3 playing rainbow islands https://photos.app.goo.gl/Me1MP6Kta7h9syi19

On the big TV downstairs the +2 picture is nice and clear:
5. https://photos.app.goo.gl/EFuAcWihno5tQ1dQ6

The +3 displays a nice clear picture also
6. https://photos.app.goo.gl/XVstSFj5KnY2sQWq9

Can anyone help me on this? The two computers should now behave the same.
User avatar
Guesser
Manic Miner
Posts: 639
Joined: Wed Nov 15, 2017 2:35 pm
Contact:

Re: Grey +2 RGB SCART help wanted

Post by Guesser »

Yep, fit links 2, 3, and 8 for SCART output. (remove 1, 4, and 7)

That still won't be exactly the same because the +3 shorts the two 12V lines together meaning you can't use aspect/function switching (unless you cut the track on the motherboard and add a second resistor), but your cable won't be wired for that anyway.

There's also a difference in the composite sync output which means one needs a resistor in the cable and the other doesn't. Again you could mod one of them if you wanted to be able to use the same cable on both.
zx64
Manic Miner
Posts: 211
Joined: Sat Jul 11, 2020 3:25 am
Location: Australia

Re: Grey +2 RGB SCART help wanted

Post by zx64 »

schneil
Drutt
Posts: 29
Joined: Mon Aug 17, 2020 9:07 pm

Re: Grey +2 RGB SCART help wanted

Post by schneil »

Guesser wrote: Tue Sep 15, 2020 11:31 pm Yep, fit links 2, 3, and 8 for SCART output. (remove 1, 4, and 7)

That still won't be exactly the same because the +3 shorts the two 12V lines together meaning you can't use aspect/function switching (unless you cut the track on the motherboard and add a second resistor), but your cable won't be wired for that anyway.

There's also a difference in the composite sync output which means one needs a resistor in the cable and the other doesn't. Again you could mod one of them if you wanted to be able to use the same cable on both.
Ah I see so you're saying even though the RGB ports are now similar in the two machines. they are still slightly different?
Which machine would need the resistor mod?
schneil
Drutt
Posts: 29
Joined: Mon Aug 17, 2020 9:07 pm

Re: Grey +2 RGB SCART help wanted

Post by schneil »

zx64 wrote: Wed Sep 16, 2020 3:00 am Have you seen this link?
https://www.benophetinternet.nl/hobby/v ... cables.pdf
I've just been looking at that.
My +2 now has on it's RGB port:
1 +12V
2 GND
3 Audio
4 composite sync
5 +12V
6 Green
7 Red
8 Blue

Wheras the +3 has
1 +12V
2 GND
3 Audio
4 composite sync (different voltage?)
5 +12V
6 Green
7 Red
8 Blue

so that could be the reason for the instability in the picture?

also I heard some +2s have incorrectly placed transistors. Which transistors are affected? And what problems would it cause?
User avatar
Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Grey +2 RGB SCART help wanted

Post by Ast A. Moore »

schneil wrote: Thu Sep 17, 2020 9:15 pm I heard some +2s have incorrectly placed transistors. Which transistors are affected? And what problems would it cause?
It’s the three 2N3904s: TR4, TR5, and TR7. TR7 and TR5, even if mounted incorrectly, will generally cause no trouble with the video signal. TR4 is part of the circuitry that forms the composite video signal, and needs to be flipped for it to work properly. TR4 is the one sitting next to the RF modulator. The collector must be connected to the +12V rail (that’s the via closes to the modulator). The pins of the 2N3904 (looking at the flat side facing you are: emitter, base, collector. The silk screen shows the incorrect orientation.

That’s all moot, since you’ve decided to confiture your +2 two for the Peritel option, and it’s not using the composite video signal at all.

The composite sync levels are different on a +2 and +3. The former is a TTL-level signal, whereas the latter is a 0.48V signal. You might want to experiment with the value of the series resistor going to PIN 4 of the DIN socket to bring the TTL level down.
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
User avatar
Guesser
Manic Miner
Posts: 639
Joined: Wed Nov 15, 2017 2:35 pm
Contact:

Re: Grey +2 RGB SCART help wanted

Post by Guesser »

schneil wrote: Thu Sep 17, 2020 8:59 pm Ah I see so you're saying even though the RGB ports are now similar in the two machines. they are still slightly different?
Which machine would need the resistor mod?
It depends if you were removing it or adding it :)

The +3 (and black +2) divides the composite sync voltage internally.
schneil
Drutt
Posts: 29
Joined: Mon Aug 17, 2020 9:07 pm

Re: Grey +2 RGB SCART help wanted

Post by schneil »

Ast A. Moore wrote: Thu Sep 17, 2020 11:10 pm
schneil wrote: Thu Sep 17, 2020 9:15 pm I heard some +2s have incorrectly placed transistors. Which transistors are affected? And what problems would it cause?
It’s the three 2N3904s: TR4, TR5, and TR7. TR7 and TR5, even if mounted incorrectly, will generally cause no trouble with the video signal. TR4 is part of the circuitry that forms the composite video signal, and needs to be flipped for it to work properly. TR4 is the one sitting next to the RF modulator. The collector must be connected to the +12V rail (that’s the via closes to the modulator). The pins of the 2N3904 (looking at the flat side facing you are: emitter, base, collector. The silk screen shows the incorrect orientation.

That’s all moot, since you’ve decided to confiture your +2 two for the Peritel option, and it’s not using the composite video signal at all.

The composite sync levels are different on a +2 and +3. The former is a TTL-level signal, whereas the latter is a 0.48V signal. You might want to experiment with the value of the series resistor going to PIN 4 of the DIN socket to bring the TTL level down.
Yes that is one option - I change a resistor on the +2. Stuart (the chap who did the soldering) is a trained electronic engineer so he could figure it out. However it might take him a while to do, and he's doing the work for me as a favour. So I've got three options:

1. Further mod the +2 so it's output is the same as a +3
2. Buy a new cable or have one made that will work with the +2.
3. Buy another TV

I might go with 3, as there is a 20" Sony Bravia local to me on gumtree for £25. If the seller agrees I could bring the +2 round, hook it up and if it works all good.
User avatar
Ast A. Moore
Rick Dangerous
Posts: 2640
Joined: Mon Nov 13, 2017 3:16 pm

Re: Grey +2 RGB SCART help wanted

Post by Ast A. Moore »

schneil wrote: Fri Sep 18, 2020 12:42 pm there is a 20" Sony Bravia local to me on gumtree for £25. If the seller agrees I could bring the +2 round, hook it up and if it works all good.
That’s certainly an option. It’s ultimately your choice, but if you’re thinking of purchasing a dedicated TV set for you Speccys, may I suggest you get a good-old CRT one, rather than a (relatively) modern flat panel?
Every man should plant a tree, build a house, and write a ZX Spectrum game.

Author of A Yankee in Iraq, a 50 fps shoot-’em-up—the first game to utilize the floating bus on the +2A/+3,
and zasm Z80 Assembler syntax highlighter.
schneil
Drutt
Posts: 29
Joined: Mon Aug 17, 2020 9:07 pm

Re: Grey +2 RGB SCART help wanted

Post by schneil »

Ast A. Moore wrote: Fri Sep 18, 2020 1:17 pm
schneil wrote: Fri Sep 18, 2020 12:42 pm there is a 20" Sony Bravia local to me on gumtree for £25. If the seller agrees I could bring the +2 round, hook it up and if it works all good.
That’s certainly an option. It’s ultimately your choice, but if you’re thinking of purchasing a dedicated TV set for you Speccys, may I suggest you get a good-old CRT one, rather than a (relatively) modern flat panel?
I'd love a CRT, but I'm limited by space. Mrs Schneil will not be pleased with more clutter!
The other reason not to get one is the fact they will be pushing at least 20 years old. That's pretty impressive innings for a 14" portable! Especially since they were built on the cheap towards the end of their run.

The Sony near me is from 2007, and at that age it might truck on for years, or just break within months. Who knows with modern(ish) electronics?
Post Reply