Spectrum +2 Dark RGB

For experts to discuss very technical stuff and newbies to ask why the Spectrum they bought off ebay doesn't work.
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Morpheus
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Re: Spectrum +2 Dark RGB

Post by Morpheus »

Ast A. Moore wrote: Sun Aug 04, 2019 2:17 pm If you add the extra circuitry, you could replace the 3.5 mm jack with a single TRS one and wire it the same way it’s wired on the +3. If you don’t want to bother with all that, you can just buy a generic cassette adaptor. They’re cheap and work well.
If I remember you made a video of this using something called a Schmitt trigger?
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Ast A. Moore
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Re: Spectrum +2 Dark RGB

Post by Ast A. Moore »

Morpheus wrote: Mon Aug 05, 2019 4:52 am If I remember you made a video of this using something called a Schmitt trigger?
Nope. Definitely wasn’t me. Perhaps you’re referring to this video from Joules per Coulomb:

https://youtu.be/qUIv-A_DOc0?t=1014
Every man should plant a tree, build a house, and write a ZX Spectrum game.

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Re: Spectrum +2 Dark RGB

Post by Morpheus »

Yes, sorry I think you posted it a while back and I thought you had made it. My bad :oops:
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Ast A. Moore
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Re: Spectrum +2 Dark RGB

Post by Ast A. Moore »

Morpheus wrote: Tue Aug 06, 2019 9:33 am Yes, sorry I think you posted it a while back and I thought you had made it. My bad :oops:
No worries. I’m actually flattered. :D
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.
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Re: Spectrum +2 Dark RGB

Post by Treguard »

Sorry for reviving an old thread, but I have this exact same problem with a +3. Very dark, grey output from the RGB port.

Looking at the motherboard I can’t find TR8 or TR9 at all, so I’m assuming the +3 is different or I’m being stupid in some way!

Anyone got any suggestions for a +3?

Not sure if it related but the output from the RF unit is completely black. The RGB scart compatibility modification has been done by the previous owner.

Thank you.

Image
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Ast A. Moore
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Re: Spectrum +2 Dark RGB

Post by Ast A. Moore »

Treguard wrote: Fri Jan 01, 2021 8:35 pm Sorry for reviving an old thread, but I have this exact same problem with a +3. Very dark, grey output from the RGB port.
Your TV expects composite video, instead of RGB. The +3 doesn’t have a composite video signal, and what you’re seeing is the composite sync signal fed into the composite video input. If you can manually force your TV to accept RGB video via SCART, do so. If not, you’ll need to make a small modification to your +3 (replace R44 with a smaller-value resistor, such as 330–470Ω) for the TV to accept RGB video automatically.

Here are a few places to peruse: one, two, three.
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.
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Re: Spectrum +2 Dark RGB

Post by 1024MAK »

A +2 grey is uses a completely different PCB with very different circuitry. So the details about TR8 and TR9 are not relevant to your board.

Are you are trying to connect via a SCART lead? If yes, do you understand that a SCART connector can carry a choice of various different video signals? TVs normally default to composite video, but the +3 does not (as it left the factory) output a composite video signal on it’s monitor/RGB port. It only produces RGB video signals.

Hence I tend to agree with Ast A. Moore.

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Re: Spectrum +2 Dark RGB

Post by Treguard »

I bought it on ebay, and it came with a RGB - Scart cable. I also bought a Scart to HDMI convertor that was advertised as RGB and +3 compatible. That also doesn't seem to be able to lock onto a signal.

Image

Looking at the circuit, board and based on the listing, I think the modification you suggest has already been done (photo attached) but I'm colourblind and can't read the resistor coding so I don't know if it's correct.

Does this circuit board look right to you?

If RGB isn't going to work, has anyone got any idea what might cause the RF to produce a completely black screen?

I might send it back, but the rest of the machine is immaculate, so I'd like to fix it if I can.

Perhaps I just need an older telly. This is the only one I have with SCART at all. All the rest are HDMI only.

Thanks.
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Ast A. Moore
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Re: Spectrum +2 Dark RGB

Post by Ast A. Moore »

Treguard wrote: Sat Jan 02, 2021 1:06 pm Looking at the circuit, board and based on the listing, I think the modification you suggest has already been done (photo attached) but I'm colourblind and can't read the resistor coding so I don't know if it's correct.
Hmm. It’s an odd choice to use a high-precision resistor for R44, but it looks like it’s a 610 Ohm one with a ±1% tolerance. It’s still too high, though. I’d replace it with a 330 Ohm one.
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.
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Re: Spectrum +2 Dark RGB

Post by Guesser »

It's 680 shirley.
That is theoretically enough, though on the high side so less margin if the 12 volt rail sags a bit. I used 560 in my +3 for that reason.
330 is lower than I've seen anyone recommend before. It seems like increasing current for no benefit to me.
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Re: Spectrum +2 Dark RGB

Post by Ast A. Moore »

Guesser wrote: Sat Jan 02, 2021 4:19 pm It's 680 shirley.
D’oh, you’re right. It’s a gray band.
Guesser wrote: Sat Jan 02, 2021 4:19 pmThat is theoretically enough, though on the high side so less margin if the 12 volt rail sags a bit. I used 560 in my +3 for that reason.
330 is lower than I've seen anyone recommend before. It seems like increasing current for no benefit to me.
Well, the spec calls for 1–3V. As it is, we have 12/(1,000/75), which is 0.9V. 330 Ohm will give us 2.7V, which is, I agree, on the higher end, but still within spec. 560 gives you 1.6V. I think I stuck 470 in my +2A originally. Can’t remember off the top of my head, because in my case it’s moot; I select the RGB input on my TV manually. (Haven’t tested if the TV can switch inputs automatically, actually.)
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.
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Re: Spectrum +2 Dark RGB

Post by Guesser »

That's not how you calculate a potential divider :?
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Ast A. Moore
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Re: Spectrum +2 Dark RGB

Post by Ast A. Moore »

Heh, no it’s not, but the ballpark values are close enough. :D

Okay,

12×75/(1000+75)=0.83V
12×75/(560+75)=1.4V
12×75/(330+75)=2.22V
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.
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Re: Spectrum +2 Dark RGB

Post by 1024MAK »

Okay, time to ask the slightly obvious questions...

Is this TV known to work with other devices that produce a SCART RGB signal? E.g. DVD players?

If the computer has been modified, it’s likely that that cable is correct, but it is still worthwhile checking. Either visually by checking the wiring after removing the connector covers. Or electrically by using a multimeter.

Does the disk drive spin a disk?

If no, it’s possible that the PSU is not producing a +12V output. This would affect the +12V supply to the monitor/RGB port. It would also affect the RF/UHF TV output.

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Re: Spectrum +2 Dark RGB

Post by TMD2003 »

All this talk of resistors.

Have I got a surprise for you lot... hopefully soon...
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Re: Spectrum +2 Dark RGB

Post by 1024MAK »

TMD2003 wrote: Sat Jan 02, 2021 8:47 pm All this talk of resistors.

Have I got a surprise for you lot... hopefully soon...
But can we resist? Or is resistance futile...

Mark
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Re: Spectrum +2 Dark RGB

Post by TMD2003 »

1024MAK wrote: Sat Jan 02, 2021 11:52 pm But can we resist? Or is resistance futile...
It may very well be...
Spectribution: Dr. Jim's Sinclair computing pages.
Features my own programs, modified type-ins, RZXs, character sets & UDGs, and QL type-ins... so far!
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Re: Spectrum +2 Dark RGB

Post by JollyRoger »

I've also run into this issue with my +2B. Here's the SCART cable that I'm using. As you can see it's certainly the correct type:

Image

When I connected the computer to my Panasonic TX-P50ST50B via SCART and switched to AV1 which provides RGB, I was met with a darkened image. The TV's SCART terminal definitely has RGB enabled and it always accepted an RGB signal from my Wii. These are the SCART pinouts and AV options from the manual:

Image

Image

There doesn't appear to be a way to "force" RGB.

My Polaroid LED TV fared even worse and displayed a "no signal" error. :(

Out of the three flat panel TVs at my disposal, it was the oldest of the bunch - a (2001?) 4:3 Cello that played ball and my goodness, compare the difference between in quality between RF and RGB on the same TV! :D

RF

Image

RGB

Image

Image

Image

Of course I'm delighted with the picture quality but I'm also miffed that the cable only worked with 1 out of my 3 flat panel TVs. Has the cable not been configured correctly or are my TVs at fault?
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Re: Spectrum +2 Dark RGB

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JollyRoger wrote: Sat Jan 16, 2021 12:06 pm Of course I'm delighted with the picture quality but I'm also miffed that the cable only worked with 1 out of my 3 flat panel TVs. Has the cable not been configured correctly or are my TVs at fault?
I'm sure it will be a combination of how the cable has been wired, and the signals generated by the spectrum.
SCART is an old and well defined standard, which no model of Spectrum conforms to properly without modification.
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Re: Spectrum +2 Dark RGB

Post by JollyRoger »

Guesser wrote: Sat Jan 16, 2021 4:10 pm I'm sure it will be a combination of how the cable has been wired, and the signals generated by the spectrum.
SCART is an old and well defined standard, which no model of Spectrum conforms to properly without modification.
What exact modification would the +2B require in order for SCART to work correctly?

[media]https://youtu.be/98_BN5zZZqc[/media]

Something along what's carried out here?
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Re: Spectrum +2 Dark RGB

Post by Guesser »

At a minimum, lowering the resistance of R44.
If you want automatic aspect ratio and input switching you have to cut a trace on the bottom of the PCB and add a second resistor so that the second "12V" pin of the RGB socket is usable.

I should take a photo of how I did that on my +3 some time.
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Re: Spectrum +2 Dark RGB

Post by 1024MAK »

The basic problem is that two pins on the SCART socket of the TV tell the TV what type of signal is to be expected (well, actually to select between the ‘default’ of composite video via SCART and RGB via SCART, it’s actually far more complex as some devices also allow for S-Video and/or component video but this is not completely standard, and between different aspect ratios).

Most old TVs (the less fancy CRT type) and some modern-ish allow the user to select RGB overriding any automatic switching.

But most modern TVs don’t provide such user controls. Hence because NONE if the ZX Spectrum models can provide the required voltages on their monitor/RGB ports, it does not matter what cable you use, it won’t work.

There are then a couple of choices. Modify the ZX Spectrum or modify the cable. The cable modification involves wiring in a 12V feed from a suitable PSU (or from a suitable battery for testing purposes).

Oh, and if there are two SCART sockets, in most cases only ONE will accept RGB signals.

Mark
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Re: Spectrum +2 Dark RGB

Post by JollyRoger »

1024MAK wrote: Sun Jan 17, 2021 12:51 am The basic problem is that two pins on the SCART socket of the TV tell the TV what type of signal is to be expected (well, actually to select between the ‘default’ of composite video via SCART and RGB via SCART, it’s actually far more complex as some devices also allow for S-Video and/or component video but this is not completely standard, and between different aspect ratios).

Most old TVs (the less fancy CRT type) and some modern-ish allow the user to select RGB overriding any automatic switching.
Understood. On the Cello TV, I just selected SCART from the menu and it automatically detected the RGB signal, which was fortunate.
1024MAK wrote: Sun Jan 17, 2021 12:51 am But most modern TVs don’t provide such user controls. Hence because NONE if the ZX Spectrum models can provide the required voltages on their monitor/RGB ports, it does not matter what cable you use, it won’t work.
Now I understand why it's recommended that if you need a SCART cable for an Amstrad CPC that you buy a powered one.
1024MAK wrote: Sun Jan 17, 2021 12:51 am There are then a couple of choices. Modify the ZX Spectrum or modify the cable. The cable modification involves wiring in a 12V feed from a suitable PSU (or from a suitable battery for testing purposes).
Modifying the cable sounds far less daunting than the prospect of modifying the computer. :D I'm going to contact the seller, explain the issue and ask if they'll alter the cable for me to provide the feed.
1024MAK wrote: Sun Jan 17, 2021 12:51 am Oh, and if there are two SCART sockets, in most cases only ONE will accept RGB signals.
I remember that from the 90s when I was using a TV that had two SCART sockets and one accepted RGB whilst the other was composite & S-Video IIRC. My previous TV was a flat screen CRT Sony 100hz set with three SCART sockets and only one of them had the RGB pins. Thanks for the heads up with this. Let's see if the seller will help me out...
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Re: Spectrum +2 Dark RGB

Post by Guesser »

JollyRoger wrote: Sun Jan 17, 2021 9:52 am Modifying the cable sounds far less daunting than the prospect of modifying the computer. :D I'm going to contact the seller, explain the issue and ask if they'll alter the cable for me to provide the feed.
It's a trivial mod if you don't care about about automatic input/aspect ratio switching (which would also need a different cable anyway), it's just a single resistor change.
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Re: Spectrum +2 Dark RGB

Post by JollyRoger »

Guesser wrote: Sun Jan 17, 2021 1:01 pm it's just a single resistor change.
As shown here? :)

https://spectrumforeveryone.com/technic ... compliant/

Image

I think that with a bit of practice beforehand, I could pull this off. :D
Guesser wrote: Sun Jan 17, 2021 1:01 pm It's a trivial mod if you don't care about about automatic input/aspect ratio switching (which would also need a different cable anyway)
Meaning that I'd have to switch to AR to 4:3 manually on the remote if the TV doesn't detect it correctly? I think that I can live with that. :)

Thanks.
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