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ZX Spectrum 128K +2 grey RGB cable problem

Posted: Thu Jul 20, 2017 7:41 am
by psanchez
Hello, I have a ZX Spectrum +2 128K grey model, when I connect it to the tv using RGB out and RCA input on the TV, the image is with some kind of noise (waves), no color (sometimes appear flashly), when loading tapes the image is shaking, an after loading a tape, the image is shaking again but worst.

I've attached some photos. I you need more information or photos, please let me know.

Any help is welcome, thank you in advance!

Re: ZX Spectrum 128K +2 grey RGB cable problem

Posted: Thu Jul 20, 2017 11:26 am
by 1024MAK
Hello and welcome to our forum :D
psanchez wrote:
Thu Jul 20, 2017 7:41 am
Hello, I have a ZX Spectrum +2 128K grey model, when I connect it to the tv using RGB out and RCA input on the TV, the image is with some kind of noise (waves), no color (sometimes appear flashly)
The Monitor/RGB socket has two different video output signals: "RGB video" and "composite video". As you say you are using a "RCA" input on your TV, I presume you mean you are using the composite video input connector. Normally this is a RCA/Phono/Cinch socket. These days, often using yellow plastic. The composite video system uses a single cable.

RGB video uses a cable with a number of wires, as three colour signals are carried by separate wires (R=red colour channel, G=green, B=blue). On most TVs, if the TV supports RGB inputs, this is via a SCART socket.

Some +2 (grey) machines use a PCB where due to a manufacturing fault, three transistors are fitted the wrong way round on the board :(
From your description and pictures, it is very possible that your machine has this problem. Unless your TV has a RGB capable SCART socket, the only solution is for the manufacturing fault to be corrected. This involves removing the three transistors and soldering replacement transistors to the board.

Details of this problem (plus some other problems) are in this thread on World of Spectrum forums .

Mark

Re: ZX Spectrum 128K +2 grey RGB cable problem

Posted: Thu Jul 20, 2017 1:10 pm
by psanchez
Thank you very much, 1024MAK, actually I tried already the "Composite Mod" procedure, with no luck (I think the transistors hardware problem is there). Then I bought a RGB to RCA (3 => video, audio L, audio R), but no SCART used, so I connected directly to RCA inputs in the back of the TV (4K samsung tv, recent model).

The results are in the photos I've posted already.

So, despite of using the RGB procedure, the image is crap, and I have to proceed to replace the transistors (using the procedure you mentioned), correct?

Thanks!

Re: ZX Spectrum 128K +2 grey RGB cable problem

Posted: Thu Jul 20, 2017 2:15 pm
by 1024MAK
psanchez wrote:
Thu Jul 20, 2017 1:10 pm
Thank you very much, 1024MAK, actually I tried already the "Composite Mod" procedure, with no luck (I think the transistors hardware problem is there). Then I bought a RGB to RCA (3 => video, audio L, audio R), but no SCART used, so I connected directly to RCA inputs in the back of the TV (4K samsung tv, recent model).
The socket is labelled "RGB" but the lead you are using is not using the RGB video signals. For some reason Amstrad (Sinclair) labelled that socket "RGB" even though it provides a BOTH a composite video signal AND RGB signals. On later machines they labelled it differently (see photo below). Your lead has a composite video signal plus left and right audio (keeping in mind that the computer only outputs mono sound, so left and right audio will be exactly the same signal).

A true RGB signal produces a much better quality picture than a composite video signal.

By the way, where in the world are you?

I can't say for certain, as there could be another fault. But if you do have a +2 computer where the three transistors are incorrectly fitted, then, yes, they need to be replaced.

Image
Image

Details of the pin-out of the socket are here

Mark

Re: ZX Spectrum 128K +2 grey RGB cable problem

Posted: Thu Jul 20, 2017 2:32 pm
by psanchez
Thanks again, Mark, I'm in Greece but I'm from Spain... a long story :)

Ok, I understand, I'd read about the different fixes for zx spectrum models, so probably I'll proceed with the fix in order to get video working. I'll let you know after the fix, I hope not destroy the machine!

I come from Amstrad CPC but I've always wanted to have a spectrum :D

Thanks, we'll keep in contact!

Re: ZX Spectrum 128K +2 grey RGB cable problem

Posted: Fri Jul 21, 2017 8:45 am
by psanchez
Hello Mark, finally I made the fix related to the TR4-TR5-TR7 and the video signal is far better than before, I've attached some pictures in order to show it, please let me know if this quality is normal for composite signal, or maybe I should fix some other issue in the board.

I changed position the transistors and performed some meterings, I don't know if they have normal values, more or less .940 (ohms scale 2K), one of them was .850 (see picture).

Thanks again for your valuable help!

Re: ZX Spectrum 128K +2 grey RGB cable problem

Posted: Fri Jul 21, 2017 1:25 pm
by 1024MAK
Yeah, that's looking more normal.

If you remove ceramic disc capacitor C31 and rewire/reconnect or wire a new capacitor in as described on WoS here, that will give a further increase in quality.
Beyond that, the limitations of the composite video signal format and the actual implementation in both the computer and the TV limit the quality.

If your TV has a RGB capable SCART input, the best quality is via a RGB SCART lead. I use RGB SCART leads for all the retro computers that I have that have suitable outputs and only use composite video for computers that don't have suitable outputs (48k Spectrum, ZX81 etc).
For the +2 grey, a ready made lead is on eBay (UK site) or eBay.com, link to PDF Sales Brochure.

If you prefer to build your own, details are on Paul's Fruitcake web site here ;)

Mark