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Vintage Computers Any vintage computer systems, calculators, video games etc., but with an emphasis on 1980s and earlier equipment.

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Old 18th Jan 2019, 8:13 pm   #1
Ryan_1993
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Default BBC Micro RGB - SCART INPUT VIA VCR

Hi all,

I've got a really old Portable Sony Trintrion from 1971. It is a colour TV, but unfortunately I haven't been able to get colour output over RF into it it just stays as black and white.

I know nothing is wrong with the TV as my VCR displays in full colour when hooked in over RF. However, I was going to use one of those RGB-SCART adapters for my Beeb hook it into the back of my VCR and the RGB port on the back and hopefully get colour image into my TV that way with the VCR hooked into the TV via RF.


Would there be any issue in doing this? would it even work into the back of my VCR? Its a 2004 Panasonic Model

If not I'm thinking about buying one of the MicroVitec CUBs for it instead.
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Old 18th Jan 2019, 9:30 pm   #2
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

It would work with a SCART socket on a TV (at least, the first one; usually AV1 was wired for RGB or composite, and AV2 was composite only or composite and S-VHS); but VCRs don't usually have RGB inputs, only composite.

But there is a simple modification (detailed in the Advanced User Guide, if my memory serves me well; on early issue motherboards it requires adding a capacitor but later machines have the capacitor already fitted, and a jumper to connect or disconnect it) that will give colour on the BNC output. (It was normally mono-only to prevent colour dot effects from showing up on high-resolution mono monitors.)

If you can get one of the MicroVitec CUB monitors in good condition, that would be the perfect thing to use with it. But they are elderly and probably in need of some TLC.

THOUGHT: Does your Beeb actually give a colour picture on another TV set, when connected via RF? It may be faulty .....
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Old 18th Jan 2019, 10:16 pm   #3
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Your VCR won't encode the RGB signal from the Beeb into a PAL composite signal for the TV, I'm afraid. You'll get no picture that way, just a blank screen. The Beeb's own RF output should be in colour unless the machine is faulty. However, you'll get vastly better picture quality from a proper Cub monitor, or even the RGB SCART cable in to a modern TV.

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Old 18th Jan 2019, 10:49 pm   #4
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Thanks all

I’m going for a CUB monitor i suppose its all matching then as well. I think the issue definitely lies at my TV Somehow. My 4K TV displays the beep in Colour over BNC, although its very fuzzy and saturated it was in colour. I’ll keep my wood Trinitron for VHS and BetaMax stuff

There is a guy round corner from my work who refurbs BBC Micro, Masters and its associated components so I’m getting a refurbished CUB display from him.
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Old 19th Jan 2019, 10:00 am   #5
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

The non-standard PAL decoder in early Sony TV sets does not always work well with the rather crude approximation of a PAL signal that a home computer gives. I'd try it with a proper PAL-D set (e.g. almost every other one other than an old Sony) before suspecting anything wrong with the computer. The BBC Computer doesn't work with every version of the Sony KV-1612UB either, some of the early ones just give a locked blank screen.
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Old 19th Jan 2019, 10:25 am   #6
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Yeah it was Colour on my 4K Samsung when I put in the jumper cable. I had heard about the implementation of PAL on early Sonys before something to do with not wanting to pay the PAL Licencing to Telefunken or something.

Having seen a cub monitor on period photos i always thought they looked like some kind of massive Broadcast style monitor, but saw one in person yesterday and they are quite a lot smaller than I imagined. Ill keep my Sony for Beta and VHS work

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Old 19th Jan 2019, 12:05 pm   #7
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

There were many versions of the Cub monitor, with several CRTs types fitted. Broadly, you can tell them apart from looking at how dark the screen is when not powered.

The dark ones have a very fine shadow mask, giving excellent results when using MODE0 and MODE3, whereas the lighter ones were essentially the same sort of pitch as a domestic CRT, meaning that the high resolution screen modes are rather fuzzy.

I'm generalising slightly for brevity. When you look at the manual, you'll see the range that was fitted - of the 14" models, there's 0.64mm, 0.43mm and 0.31mm, with the latter being the one to go for if you have the choice.

These monitors have a TTL RGB input, so are directly compatible with the BBC. If you're going into a Scart, don't forget that you need to resistors in series with each signal, as the Scart input will be expecting a smaller voltage.
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Old 19th Jan 2019, 12:26 pm   #8
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Quote:
Originally Posted by Ryan_1993 View Post
I had heard about the implementation of PAL on early Sonys before something to do with not wanting to pay the PAL Licencing to Telefunken or something.
That is correct and Sony overcame the problem by using a decoder that worked on the 'Simple PAL' principal. It was basically a modified NTSC (American) decoder that didn't use normal PAL switching. It worked OK for normal TV and probably for VCR's but was a bit temperamental with anything else.
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Old 19th Jan 2019, 1:08 pm   #9
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Good that the OP has decided to go for a historically correct monitor, but for the record, many of the later generation HDD/DVD recorders did have RGB -input- via SCART. I think this was intended to be used for connection of RGB-out from a VCR so you could transfer VHS original recordings to DVD or HDD, bypassing the Macrovision 'protection' which prevented similar activity via a composite connection.

I've used my Pioneer 545 HDD/DVD recorder (which has RGB input via SCART) as a way of converting RGB SCART in to HDMI out, to view the output from an Atari ST on an HDMI monitor.

I can't remember if that Pioneer machine also outputs a modulated RF out signal or whether the RF-out is just an aerial passthrough signal.
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Old 19th Jan 2019, 1:36 pm   #10
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Quote:
Originally Posted by Sideband View Post
It was basically a modified NTSC (American) decoder that didn't use normal PAL switching.
That's not quite correct; full story here:

https://www.vintage-radio.net/forum/...d.php?t=136086
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Old 19th Jan 2019, 2:11 pm   #11
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Quote:
Originally Posted by mhennessy View Post
There were many versions of the Cub monitor, with several CRTs types fitted. Broadly, you can tell them apart from looking at how dark the screen is when not powered.


These monitors have a TTL RGB input, so are directly compatible with the BBC. If you're going into a Scart, don't forget that you need to resistors in series with each signal, as the Scart input will be expecting a smaller voltage.
(I have removed part of mhennessy's post, hopefully without changing the sense).

Not all Microvitec Cubs have TTL RGB inputs. There was a composite PAL version (I think the input on that was a BNC socket). And you could set internal links on the TTL RGB model for analogue RGB inputs. There may have been other versions too.

If the input connector is a 6 pin DIN socket (like the RGB socket on the BBC Micro) then most likely the Cub can be set for TTL RGB and you need a straight-wired 6 pin DIN to 6 pin DIN cable to connect it.
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Old 19th Jan 2019, 2:39 pm   #12
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

I remember one of the schools I went to had Philips RGB monitors for their BBC Micros.

These had plastic cases with a flap on the front for the screen controls.
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Old 19th Jan 2019, 2:48 pm   #13
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Quote:
Originally Posted by Richard_FM View Post
I remember one of the schools I went to had Philips RGB monitors for their BBC Micros.

These had plastic cases with a flap on the front for the screen controls.
Yes, that Philips monitor was also supplied by Acorn later in the 1980s, in the 'Master' era. They're also excellent and can give results even better than the Cub (the frame linearity always seemed better to me) but they don't have quite the same vintage Beeb feel. I think the Philips is a derivative of the ubiquitous CM8833 which dominated the Amiga and Atari ST world. I've got a slightly later one on a tilt/swivel stand which I use with an Archimedes, though with the right cable it would also work well with a Beeb.

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Old 19th Jan 2019, 4:03 pm   #14
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Yes I remember the same monitors being used with the Archimedes, which replaced the BBC Micros during my time at secondary school.
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Old 19th Jan 2019, 4:24 pm   #15
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

AKF50 multisync hi-res and AKF60 standard res. No CVBS on them unlike the CM8833.
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Old 19th Jan 2019, 5:31 pm   #16
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Quote:
Originally Posted by TonyDuell View Post
Not all Microvitec Cubs have TTL RGB inputs. There was a composite PAL version (I think the input on that was a BNC socket). And you could set internal links on the TTL RGB model for analogue RGB inputs. There may have been other versions too.
Yes, I did say there were many variants, and provided a link to the manual so people can see some of them for themselves (not all variants are covered by that version of the manual). Upon re-reading my post, I guess that I could have added a weasel-word or two to the "These monitors" part of my last sentence to cover myself against pedantry

I will say that I have seen many of these with composite inputs (yes, on a BNC), but all of those also had RGB inputs. I have one in front of me now - a 1431APMS4. I believe that the "A" means audio, and "P" means PAL. I wondered if "S4" implies Series 4, but the internals are definitely Series 3, complete with separate tripler.

So did composite-only models exist? I've seen dozens (perhaps over 100) of Cubs over the years and have yet to come across one, but I know that there must be many variants that I haven't met, so wouldn't pretend to be authoritative about this. Personally, I don't really see the point of omitting the RGB input, given that it only saves an MC14551 analogue mux and the socket and switch, but perhaps they made some for marketing reasons. Who knows? They made a ZX Spectrum version, but it's not mentioned in any of the manuals or brochures that I've seen: https://www.flashbackgames.co.uk/Sho...s/IMG_2820.jpg

Still, should you find one, it ought to be easy enough to convert a non-RGB example to RGB if you have the appropriate manuals or a suitable example to copy.

Back to my one, the RGB input is very definitely TTL only. That's because where the jumpers would be (TL103), there is a Molex socket that connects to the PAL decoder PCB - this is how the TTL RGB signals travel to the 14551 mux on the PAL board. It might be possible to re-wire that connector, perhaps, but that's not detailed in the manuals I have here - I'll have a look when I'm back at work next week.

It ought to be safe to say that all RGB-only models that use the Series 3 chassis can be switched between TTL and analogue with TL103, but as it's possible that some might be made with soldered links rather than jumpers, perhaps even that might be a risky statement to make on a forum.

It's worth saying that the analogue RGB option is 0-4V rather than the more usual 0-700mV - that's true of the series 3, 4 and 5 versions, according to the data I have in front of me - no-doubt there will be an exception just waiting to prove me wrong, but for anyone hoping to plug their DVD players into these, they will probably be disappointed with the results.

Oh - it's also worth saying that many of these monitors have a fuse in the Neutral as well as Live. The Neutral fuse should be replaced with a wire link. Somewhere, I have a copy of the letter from Microvitec telling us about that.
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Old 19th Jan 2019, 6:09 pm   #17
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

Three posts split to a new thread: https://www.vintage-radio.net/forum/...d.php?t=153272
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Old 21st Jan 2019, 6:40 pm   #18
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

I've been reading a bit more of the service manual.

There's (yet) another variant that has RGB inputs on separate BNCs. These go through the PAL decoder IC, which amplifies them, meaning that these are "normal" 700mV signals.

Retrofitting these looks to be quite easy, as it's just wiring and sockets to add (the rear panel is pre-punched under the label). Tempting to consider a Scart socket

Sync on green is supported by moving a jumper on the decoder board (TL1).

Studying the diagrams, for those sets with PAL decoders, it looks like it might be possible to re-wire the connector that plugs into TL103 to get analogue rather than TTL in via the DIN socket, but without testing it I wouldn't guarantee it. I also don't see the point as it's 0-4V. It's not mentioned as a possibility for those sets that have the PAL decoder - only those without. So instead, simply go in to the PAL decoder IC as described above. Sets that have all 3 options use a 3-position centre-off toggle switch to choose between TTL RGB, linear RGB and PAL - all the logic for this - such that it is - is already present.

Either way, should you come across one of these that is composite only, adding TTL or linear RGB is trivial.

Happily, I can add that the manual I'm looking at does cover the Spectrum interface, amongst others. That's the first time I've seen those detailed, and is very lucky, given that this particular manual was about to be recycled (we haven't used them for quite a while). As and when I eventually get some quality time with the scanner, I'll make them available for all to see. Just don't hold your breath

Cheers,

Mark
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Old 21st Jan 2019, 6:44 pm   #19
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

For nostalgic reasons I'd be interested in seeing that manual when you get round to scanning it.

But just for nostalgia. The only Cub-like thing I own is the one built into the Acorn Cambridge Workstation.
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Old 22nd Jan 2019, 12:29 pm   #20
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Default Re: BBC Micro RGB - SCART INPUT VIA VCR

If you're having trouble getting colour out of the beeb it might be that the colour subcarrier frequency is out. This could be either a dead/dirty crystal, or more likely it just needs a twiddle - there's a small variable capacitor trimmer next to the crystal, about 1" soutwest of the modulator try giving that a twiddle. My recently exhumed Electron needed that doing to get my Sony Pro monitors to believe it was colour!
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