+2B RGB output

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
User avatar
Morpheus
Dizzy
Posts: 62
Joined: Thu Nov 16, 2017 4:18 pm
Location: Hurworth-On-Tees, UK
Contact:

+2B RGB output

Post by Morpheus » Fri Jan 11, 2019 12:02 pm

Hi
Just bought a +2B and it was a bit problematic at first but seems okay now and the self test ROM passes the main tests. Problem I am having is with the RGB output it is very dark almost black/negative with no sound, the RF is fine. I brought a new lead and have no reason to suspect it is faulty, I have ordered another one from Ian Priddey to check but I noticed there is a resistor mod for the RGB (680 Ohm or 1k piggyback) that can be done whereas this has not.
I don’t know if this has been modded before as there is some heath robinson fixes on the PCB going to a logic chip below the AY and some underneath following the same path as above.
There is a capacitor on two pins of the Z80 as well, very odd. My old +2B had none of this.
0 x
R Tape loading error, 0:1

User avatar
Ast A. Moore
Dynamite Dan
Posts: 1075
Joined: Mon Nov 13, 2017 3:16 pm

Re: +2B RGB output

Post by Ast A. Moore » Fri Jan 11, 2019 1:21 pm

Morpheus wrote:
Fri Jan 11, 2019 12:02 pm
There is a capacitor on two pins of the Z80 as well, very odd.
That’s a factory mod. Mine has it, too. Your old one probably had it on the bottom side of the logic board.

If you have a good signal via RF, then the problem is likely in the cable. The TEA2000 encoder forms a composite signal from the RGB and brightness signals coming from the gate array. I suspect your TV expects a composite signal, while the +2A can’t generate it (well, it doesn’t have it present on the DIN connector). Instead, it generates a composite sync signal, which can sometimes be interpreted by the TV as a very dark, “almost negative,” as you put it, image.

Unless you can manually force your TV to accept an RGB signal, you’ll have to generate a signal for it to switch to RGB video automatically. For that, you need to feed 1–3V to pin 16 of the SCRAT connector. You have 12V available on pin 2 and 5 of the DIN connector on the Spectrum, but normally, it is fed via a 1K resistor, and combined with the TV’s internal 75Ω resistor they form a voltage divider with an output of just below that which is required by the TV.

The solution is to reduce the resistance on the logic board of your Spectrum (R44) to something like 330–470Ω either by replacing it, or by soldering another resistor in parallel with it. (Yes, 680Ω is a good value. It’ll give you an equivalent resistance of about 400Ω.)

Not sure why you’re not getting any sound, although this might have something to do with your TV being too “smart,” and muting all audio input if it doesn’t receive a proper video signal. However, no factory mods (that I know of) exist for the AY chip, so that might be the culprit.
Last edited by Ast A. Moore on Fri Jan 11, 2019 2:17 pm, edited 1 time in total.
1 x
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
Morpheus
Dizzy
Posts: 62
Joined: Thu Nov 16, 2017 4:18 pm
Location: Hurworth-On-Tees, UK
Contact:

Re: +2B RGB output

Post by Morpheus » Fri Jan 11, 2019 1:33 pm

Thanks, as always you are the goto man for help. Will change the value of R44 and see what happens.
0 x
R Tape loading error, 0:1

User avatar
1024MAK
Manic Miner
Posts: 404
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: +2B RGB output

Post by 1024MAK » Fri Jan 11, 2019 2:31 pm

Lots of information on the Amstrad versions of the ZX Spectrum here. What particular factory modifications were done depends on which board issue you have, and when it was made.

Mark
1 x

User avatar
Morpheus
Dizzy
Posts: 62
Joined: Thu Nov 16, 2017 4:18 pm
Location: Hurworth-On-Tees, UK
Contact:

Re: +2B RGB output

Post by Morpheus » Fri Jan 11, 2019 4:14 pm

Changed to value of R44 to 470 Ohms and nothing has changed so maybe the cable is the problem as the TV normally switches to RGB itself. Tried it on two Televisions with the same result. So waiting on the new cable arriving to see.
0 x
R Tape loading error, 0:1

User avatar
Ast A. Moore
Dynamite Dan
Posts: 1075
Joined: Mon Nov 13, 2017 3:16 pm

Re: +2B RGB output

Post by Ast A. Moore » Fri Jan 11, 2019 4:59 pm

Out of sheer semi-scientific curiosity (although I’m sure you’ve already tried that), can you manually change the TVs inputs? Sometimes pressing the AV button (or similar) on the remote or the set itself will go through several available options.

Another possibility (albeit extremely rare) is the TV itself might not have an RGB input on one of its SCART connectors (if it has more than one). I even used to have a TV that had a single SCART socket which also didn’t have any RGB signals wired to it. I was very surprised, to say the least.

I personally haven’t dealt with off-the-shelf AV cables for the Spectrum. I made two for myself and a couple more for friends. They’re not difficult to put together, except that fitting the diode/resistor network inside of the male SCART enclosure for the original 128K machine can be a little fiddly.
0 x
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
Morpheus
Dizzy
Posts: 62
Joined: Thu Nov 16, 2017 4:18 pm
Location: Hurworth-On-Tees, UK
Contact:

Re: +2B RGB output

Post by Morpheus » Fri Jan 11, 2019 5:58 pm

My Panasonic TV has two Scart inputs and one is RGB the other switchable composite/svhs I am using the right one and it works fine with my Grey+2, CPC464,CPC6128. When you power them up the channel flicks from AV1 to AV1-RGB and away we go, with this nothing...
0 x
R Tape loading error, 0:1

User avatar
Ast A. Moore
Dynamite Dan
Posts: 1075
Joined: Mon Nov 13, 2017 3:16 pm

Re: +2B RGB output

Post by Ast A. Moore » Fri Jan 11, 2019 6:20 pm

Some gray +2s were configured (via an onboard jumper) at the factory to output video the same way as the original 128K machines. If that’s the case, and you’re using the same cable, then yes, you won’t get any image. The +2A cable is simpler but quite different.
0 x
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
1024MAK
Manic Miner
Posts: 404
Joined: Wed Nov 15, 2017 2:52 pm
Location: Sunny Somerset in the U.K. in Europe

Re: +2B RGB output

Post by 1024MAK » Fri Jan 11, 2019 6:29 pm

Morpheus wrote:
Fri Jan 11, 2019 5:58 pm
My Panasonic TV has two Scart inputs and one is RGB the other switchable composite/svhs I am using the right one and it works fine with my Grey+2, CPC464,CPC6128. When you power them up the channel flicks from AV1 to AV1-RGB and away we go, with this nothing...
Which specific cable do you have?

Mark
0 x

User avatar
Morpheus
Dizzy
Posts: 62
Joined: Thu Nov 16, 2017 4:18 pm
Location: Hurworth-On-Tees, UK
Contact:

Re: +2B RGB output

Post by Morpheus » Sat Jan 12, 2019 3:34 am

I bought the +2/+3 cable for this machine. Managed to get the cable working last night by moving the 12v wire to pin 1 ( it was on pin 3 ) there is no sound yet but the audio wire is connected to pin 5 and not 3. I bought this cable from Cool Novelties and it's the first time I have used them. :evil:
0 x
R Tape loading error, 0:1

Post Reply