Amiga 1200 - Weak RGB

[Steve T]

noticed since changing to a different display, my A1200 has weaker video than other machines, all using RGB/SCART, the TV itself is a bit weak too, not sure if its just a worn tube or the electronics, the colour (saturation) adjustment on that doesn't work at all for example & the contrast is not great.

anyway, i'll most likely replace that TV and i'm not sure why that might only be affecting the Amiga, wondering if its just the SCART cable? I don't want to open the cable unnecessarily though, does anyone know the typical resistance on a colour line of an Amiga RGB-Scart? the cable is maybe just over 1m long.

if that is OK, would it be a good idea to measure the voltages between each colour pin and ground & is it better to do this on a connected socket which is displaying video at the same time or disconnected at the TV end?

Or is this just normal and I should just turn the brightness up a bit on the TV?

Btw This machine is still on its original caps and all components, original PSU, expanded with ACA1231 and CF to IDE storage.

Tl;Dr, does anyone know the typical resistance of a colour line of an Amiga RGB-Scart? Thanks!

[turrican9]

Originally Posted by Steve T

noticed since changing to a different display, my A1200 has weaker video than other machines, all using RGB/SCART, the TV itself is a bit weak too, not sure if its just a worn tube or the electronics, the colour (saturation) adjustment on that doesn't work at all for example & the contrast is not great.

anyway, i'll most likely replace that TV and i'm not sure why that might only be affecting the Amiga, wondering if its just the SCART cable? I don't want to open the cable unnecessarily though, does anyone know the typical resistance on a colour line of an Amiga RGB-Scart? the cable is maybe just over 1m long.

if that is OK, would it be a good idea to measure the voltages between each colour pin and ground & is it better to do this on a connected socket which is displaying video at the same time or disconnected at the TV end?

Or is this just normal and I should just turn the brightness up a bit on the TV?

Btw This machine is still on its original caps and all components, original PSU, expanded with ACA1231 and CF to IDE storage.

Tl;Dr, does anyone know the typical resistance of a colour line of an Amiga RGB-Scart? Thanks!

I noticed that my Amiga 500s and 600 has a better quality RGB picture VS my 1200. Using my BenQ BL 702A's with RGB to VGA cable. I confronted the forum with this and sure enough the A1200 is using some kind of videodac chip producing a different quality RGB picture VS the A500 and 600s. I had to increase gamma alot on my 1200 VS my 500/600 and even then the 500/600 had superior picture quality. In my opinion the 1200 is overrated and overpriced. Those few AGA games are not good enough to justify the 1200. 1200s and 600s have poor/cheap build quality VS Amiga 500. Amiga 500 will always be my favourite out of these machines. And 99% of all the classic Amiga games was made for the Amiga 500.

[Steve T]

so, you're saying you don't like the 1200? A topic for another thread perhaps, honestly one I'd rather not get involved in but I hope you feel better for sharing anyway.

I'd rather talk about the DAC itself if that's the cause, thanks for that info! I guess changing the passives in the cable or something simple like that is not going to work to create some kind of gamma adjustment. I probably wouldn't bother going that far but if it really bothered someone enough to make a modification I guess you could replace it/upgrade it.

But if nothing is really wrong other than a slightly lame converter then maybe i'll just hunt down a better CRT, while it sounds like it's always going to be there, how noticeable it is may have a lot to do with contrast ratio.

[Daedalus]

The A500 and A600 both have different video output circuits, with the A600's one similar to the 1200 but with a different encoder. Obviously the A500 and A600 video encoders aren't capable of the subtleties of the full 24-bit palette with only four bits per gun, so a totally different encoder was used for the A1200. I wouldn't equate output levels with quality - I haven't noticed any difference in brightness or contrast range between my Amigas and other systems for example, so it could be that the A1200 (and possibly the A4000) are designed with a different RGB voltage range in mind. Not all RGB is the same, and while it's mostly the same there are subtle differences, the main one being VGA requiring voltages from 0-0.7V, whereas SCART uses 0.3-1.0V. The range is the same, but the offset is different, meaning that display devices that don't correctly compensate for this difference are likely to give different brightness and possibly grey offsets, giving the impression of poorer "quality".

Please note that this is speculation on my part going on the relevant schematics as I haven't actually measured the signals with a scope, and remember that the Amiga RGB output was never really designed for SCART usage so the DAC selected might not have been chosen with that in mind.

To answer the OP's question about resistance, there should be 200 ohms between the DAC and the RGB port on each of the RGB lines on the A600 and A1200, with the A600 also having an analogue transistor stage in between. The cable itself should have no resistance on the RGB lines, though other lines should have resistors in order to match signal levels to the SCART specification.

[john1979]

Originally Posted by turrican9

I noticed that my Amiga 500s and 600 has a better quality RGB picture VS my 1200. Using my BenQ BL 702A's with RGB to VGA cable. I confronted the forum with this and sure enough the A1200 is using some kind of videodac chip producing a different quality RGB picture VS the A500 and 600s. I had to increase gamma alot on my 1200 VS my 500/600 and even then the 500/600 had superior picture quality. In my opinion the 1200 is overrated and overpriced. Those few AGA games are not good enough to justify the 1200. 1200s and 600s have poor/cheap build quality VS Amiga 500. Amiga 500 will always be my favourite out of these machines. And 99% of all the classic Amiga games was made for the Amiga 500.

Seems a bit of a weird anti 1200 rant there. Just thought I chime in and say the RGB picture quality from my 2 1200s are perfect, on par with my SNES and PS1. I have an A600 too and the picture is identical to the 1200s.

So perhaps you just have an unlucky 1200.

[turrican9]

Originally Posted by john1979

Seems a bit of a weird anti 1200 rant there. Just thought I chime in and say the RGB picture quality from my 2 1200s are perfect, on par with my SNES and PS1. I have an A600 too and the picture is identical to the 1200s.

So perhaps you just have an unlucky 1200.

Many people have confirmed that the Amiga 1200 and CD32 have a much lighter black output VS Amiga 500/600. So I don't think I have a unlucky Amiga 1200.

I don't rant over the Amiga 1200 but the RGB output from Amiga 500/600 simply looks better. Amiga 1200s output is not horrible but I notice the Amiga 500/600 RGB picture is better. But it's a fact that most of the classic games was made for the Amiga 500. It's also a fact that Amiga 600/1200 have cheaper build quality VS Amiga 500. 500s rarely/never needs recapping as an example. While the A600/1200 is known for it's caps going bad. In general you can see and feel that the build quality of the 500 is much more solid. For me personally I don't think Amiga 1200 is worth the price just for some extra colours and AGA. This is my personal opinion in hindsight of me getting the Amiga 1200 one year ago. Using it with AGA MK2, ACA 1233 and compact flash. This is the dream machine for many people. I actually prefer to load floppies on my Amiga 500. It has ACA 500 + ACA 1221. But lately I prefer to load floppies on it. I have several 500s btw.

Appart from the facts I wrote above part of my reason is probably nostalgia. Amiga 500 is the real Amiga for me.

[Steve T]

Thanks all, I appreciate you all taking the time to reply, even if you have strong feelings I don't necessarily agree with.

John, possibly but i'm not the only one to notice this, though I have noticed that the only people complaining are perhaps using devices like LCD monitors which might have a low contrast range, or perhaps aren't capable of compensating for a weaker signal, if that's the issue.

Again i didn't notice any difference with any machines on my previous TV.

Originally Posted by Daedalus

I wouldn't equate output levels with quality - I haven't noticed any difference in brightness or contrast range between my Amigas and other systems for example, so it could be that the A1200 (and possibly the A4000) are designed with a different RGB voltage range in mind. Not all RGB is the same, and while it's mostly the same there are subtle differences, the main one being VGA requiring voltages from 0-0.7V, whereas SCART uses 0.3-1.0V. The range is the same, but the offset is different, meaning that display devices that don't correctly compensate for this difference are likely to give different brightness and possibly grey offsets, giving the impression of poorer "quality".

This is pretty much what i suspected, that the actual signal power is just reduced compared to some other machines... and that a fix (if even required) might be easy in that case or simply a matter of pairing it with the right display... Its also hard to say if it's a poor quality DAC, its a Sony chip so OK i haven't really looked at the data, but I wouldn't expect it to be shit, all the best stuff is made in Japan, right Doc?

I found the thread that T9 was probably referring to, the DAC gets a lot of flak, Commodore accused of cheaping out and not rolling their own chip, but it makes sense to me to use that Sony part if it meets the spec, probably saved them a lot of money... I'm not sure if thats fair or means the chip is bad quality either.

Originally Posted by Daedalus

To answer the OP's question about resistance, there should be 200 ohms between the DAC and the RGB port on each of the RGB lines on the A600 and A1200, with the A600 also having an analogue transistor stage in between. The cable itself should have no resistance on the RGB lines, though other lines should have resistors in order to match signal levels to the SCART specification.

Interesting, so i should essentially measure no resistance in the cable on the colour lines. I was expecting some as I had to put 75ohm resistors in the cable i built for my megadrive - one on each colour channel, which produces a great picture - comparable to my PS2 with its sony/stock RGB cable... I guess thats MD specific thing as it has little resistance on the board - no doubt to save money, megadrives having a proprietary cable for SCART.

looking at stedy's plans I see just a capacitor on each colour.

http://www.ianstedman.co.uk/Amiga/am...cable_V1.1.png

The cable i'm using for the Amiga is one bought off Ebay, intended to work on any Amiga... So i've been hoping that its simply a matter of the resistance being too high in it, due to some added passives that shouldn't be there for the 1200... if not perhaps changing the resistance on the board itself might be a good idea. I'm not going to do anything like that until i get a better display though, to see if its really worth doing.

Another thing i considered is that the DAC just isn't getting as much power as it would like for whatever reason, psu maybe.

[Daedalus]

Originally Posted by turrican9

Many people have confirmed that the Amiga 1200 and CD32 have a much lighter black output VS Amiga 500/600.

To me this sounds like the output is offset differently to what the display can handle, which corresponds with what I was saying earlier about SCART versus monitor RGB use. In that case, a capacitor in series with each of the RGB lines could help remove the DC offset and bring the signal into specification with what the display expects.

Edit: Just saw your reply there Steve. Yes, the capacitors in that SCART cable would indeed remove any DC offset that was present in the signals. This could be the case if the image you're getting is grey when it should be black. It's a simple mod to try anyway. 75 ohm resistors in the cable are common for consoles - they seem to require them to correctly balance the signals, presumably in some sort of effort to ensure you only buy cables from them. The N64 has a similar requirement which means that cables from the SNES sort of work, but the video is washed out, meaning that you really need two different cables.

Reference voltages for the DAC can easily be checked with a multimeter. The chip is powered from the video voltage rather than directly to +12V, and the reference voltage is derived from there so it's worth checking those and the +12V to ensure all is in order. I recently repaired an A600 that had no audio, simply because the resistor providing power to the op-amps had died. But given that your output is working and just lacking in contrast, I'd say try the capacitors in Stedy's diagram and see if it helps.

[ajk]

@turrican9

The capacitor issue has nothing to do with build quality; they are soldered in place perfectly fine. It is a question of the early SMD components not being as good long-term. But Commodore could not have known that at the time. They didn't design the A500 to last longer without maintenance, it just happened to turn out that way.

Luckily capacitors are nowhere near the disaster the old RTC backup batteries are...

As far as the RGB output goes I can't say I'd have noticed a significant difference between any of my systems from A1000 to A4000, but it's always possible that some combination of monitor and Amiga don't work so well together.

[turrican9]

Originally Posted by ajk

@turrican9

The capacitor issue has nothing to do with build quality; they are soldered in place perfectly fine. It is a question of the early SMD components not being as good long-term. But Commodore could not have known that at the time. They didn't design the A500 to last longer without maintenance, it just happened to turn out that way.

Luckily capacitors are nowhere near the disaster the old RTC backup batteries are...

As far as the RGB output goes I can't say I'd have noticed a significant difference between any of my systems from A1000 to A4000, but it's always possible that some combination of monitor and Amiga don't work so well together.

You have a point there. Still most chips is socketed in the 500 and soldered to the board on 600/1200. Also, the whole casing on A600/1200 feels more fragile and much cheaper built VS a 500.

[Steve T]

Originally Posted by Daedalus

To me this sounds like the output is offset differently to what the display can handle, which corresponds with what I was saying earlier about SCART versus monitor RGB use. In that case, a capacitor in series with each of the RGB lines could help remove the DC offset and bring the signal into specification with what the display expects.

Edit: Just saw your reply there Steve. Yes, the capacitors in that SCART cable would indeed remove any DC offset that was present in the signals. This could be the case if the image you're getting is grey when it should be black. It's a simple mod to try anyway. 75 ohm resistors in the cable are common for consoles - they seem to require them to correctly balance the signals, presumably in some sort of effort to ensure you only buy cables from them. The N64 has a similar requirement which means that cables from the SNES sort of work, but the video is washed out, meaning that you really need two different cables.

Reference voltages for the DAC can easily be checked with a multimeter. The chip is powered from the video voltage rather than directly to +12V, and the reference voltage is derived from there so it's worth checking those and the +12V to ensure all is in order. I recently repaired an A600 that had no audio, simply because the resistor providing power to the op-amps had died. But given that your output is working and just lacking in contrast, I'd say try the capacitors in Stedy's diagram and see if it helps.

Hey, thanks again, I have a crapola MM, so no way of measuring capacitance on the cable, i'll have to open that up to see if any caps were put in there, possibly not, just measured about half an ohm on the red line in the cable so that seems fine.

I was wondering what effect capacitance has on the signal, so perhaps even the electrolytics in the amiga itself (as i said, i'm on original caps) could be causing the issue.

Would i be correct in saying though, if nothing is out of spec, if thats just how the machines are, and you're getting this washed out weaker look to the picture, *and* if it really bothers you, it would be worth reducing the resistance on the A1200 motherboard itself? by replacing or bypassing components - In order to bring a weaker signal up into the SCART range that was described earlier. Or would that be a terrible idea/wouldn't work because i've wondered if it might be signal power (in which case just turn it up) or perhaps dynamic range, in which case...?

I wonder, are people with re-capped 1200's seeing this issue?

Ajk, yes perhaps its simply a matter of getting the right display for the machine, like i said on a better CRT TV, this wasn't even apparent, i'm chasing one down now on facebook, looks pretty good and just £10, free VCR thrown in too

[turrican9]

Originally Posted by Steve T

Hey, thanks again, I have a crapola MM, so no way of measuring capacitance on the cable, i'll have to open that up to see if any caps were put in there, possibly not, just measured about half an ohm on the red line in the cable so that seems fine.

I was wondering what effect capacitance has on the signal, so perhaps even the electrolytics in the amiga itself (as i said, i'm on original caps) could be causing the issue.

Would i be correct in saying though, if nothing is out of spec, if thats just how the machines are, and you're getting this washed out weaker look to the picture, *and* if it really bothers you, it would be worth reducing the resistance on the A1200 motherboard itself? by replacing or bypassing components - In order to bring a weaker signal up into the SCART range that was described earlier. Or would that be a terrible idea/wouldn't work because i've wondered if it might be signal power (in which case just turn it up) or perhaps dynamic range, in which case...?

I wonder, are people with re-capped 1200's seeing this issue?

Ajk, yes perhaps its simply a matter of getting the right display for the machine, like i said on a better CRT TV, this wasn't even apparent, i'm chasing one down now on facebook, looks pretty good and just £10, free VCR thrown in too

The Amiga 1200 I bought last year was re-capped prior to shipping. It looked professional. Very nice solder job.

[john1979]

Originally Posted by Steve T

John, possibly but i'm not the only one to notice this, though I have noticed that the only people complaining are perhaps using devices like LCD monitors which might have a low contrast range, or perhaps aren't capable of compensating for a weaker signal, if that's the issue.

Again i didn't notice any difference with any machines on my previous TV.

I think you have a point here. I'm using a decent 90s Trinitron for my Amiga. The pictures of the 1200 and 600 are identical, there's no problem with lighter blacks on either machine.

I think the issue is flat panels, they are terrible for retro computers.

[Steve T]

well, perhaps check the cable, the plot thickens.... So I've dismantled both ends and while its pretty well constructed there are no capacitors at all in this ebay cable, just one resistor.

It also has really shitty RCA connectors and is un-shielded - the picture suffers ghosting which I always thought was the cable, i sense a rebuild is in its future.

[turrican9]

Originally Posted by john1979

I think you have a point here. I'm using a decent 90s Trinitron for my Amiga. The pictures of the 1200 and 600 are identical, there's no problem with lighter blacks on either machine.

I think the issue is flat panels, they are terrible for retro computers.

People with CRT screens have reported the same difference between A1200 and A500/600. I could always try it on one of my CRTs one day.

Flat panels are not horrible for retro computers if you have the right types of flat panels. The BenQ BL 702A using RGB to VGA is excellent for Amiga use and I actually prefer it over any of my CRTs.

[Steve T]

Originally Posted by john1979

I think you have a point here. I'm using a decent 90s Trinitron for my Amiga. The pictures of the 1200 and 600 are identical, there's no problem with lighter blacks on either machine.

I think the issue is flat panels, they are terrible for retro computers.

I realise some just can't justify the space for a CRT, or may like the picture for whatever reason, but imho, if you're using LCD for retro stuff you're doing it wrong

Yeah a Trinitron is going to have a much better range, darker glass and more power/efficiency to get the brightness through it, I've gone from a similar type to a much older spherical tube with much greyer glass, while that would be fine, the contrast is kinda disappointing all round, its not really nice even for the other machines... I tried tweaking it, improved it a bit, the convergence is really nice and the geometry is quite good, maybe the circuitry is a bit tired and the faults could be fixed but people are basically throwing CRT's out all the time, i'll just pick up a better one, i think it will be off to the recyclers soon.

[Daedalus]

The electrolytics failing in the Amiga could produce all sorts of strange symptoms, but unless it's affecting the power supply (and therefore the reference voltage and other 12V parts), it's probably not the capacitors. At least, not the suspect ones anyway... Still, it might be an idea to have them changed anyway.

Personally I really miss my Trinitron monitor. It was 21", comfortably did 1600x1200 at 75Hz, and weighted about as much as me. But it died not long before I moved house, and just couldn't justify keeping it. Now I have a nice plasma which is a reasonable substitute for a CRT, and an IPS Dell monitor that gives excellent calibrated colour (for an LCD) and supports the Amiga native screenmodes.

P.S. Even a rubbish multimeter will tell you there's no capacitor on a wire since they use DC to measure the resistance. If the resistance measured stays constant, there are no in-line capacitors. Just FYI

[Steve T]

Originally Posted by Daedalus

The electrolytics failing in the Amiga could produce all sorts of strange symptoms, but unless it's affecting the power supply (and therefore the reference voltage and other 12V parts), it's probably not the capacitors. At least, not the suspect ones anyway... Still, it might be an idea to have them changed anyway.

Personally I really miss my Trinitron monitor. It was 21", comfortably did 1600x1200 at 75Hz, and weighted about as much as me. But it died not long before I moved house, and just couldn't justify keeping it. Now I have a nice plasma which is a reasonable substitute for a CRT, and an IPS Dell monitor that gives excellent calibrated colour (for an LCD) and supports the Amiga native screenmodes.

P.S. Even a rubbish multimeter will tell you there's no capacitor on a wire since they use DC to measure the resistance. If the resistance measured stays constant, there are no in-line capacitors. Just FYI

thats a good point, made far too late visual inspection wasn't as painful as i expected though, i.e. i didn't destroy the connector casing getting into it.

A proper Trinitron monitor would be awesome, but as i only really want low res screens, games or making low res graphics in Dpaint, a TV is I think best for that, and its nice for the eyes to soak up some more err, radiation, i mean it looks different, which is nice compared to the LCD's i stare at all day.

In some ways i prefer an old fashioned spherical screen to a cylindrical, or a flat, I think the domed effect adds an interesting depth/perspective to games!

I bet if i add these caps to the cable, it will still have the same issue - perhaps it will clear up the ghosting a bit?

[Daedalus]

I doubt it will do anything for the ghosting, that's probably purely down to the cable construction. All the capacitors will do is block the DC component of the signal, eliminating any offset that might be present.

[Steve T]

Originally Posted by Daedalus

I doubt it will do anything for the ghosting, that's probably purely down to the cable construction. All the capacitors will do is block the DC component of the signal, eliminating any offset that might be present.

hmm, yeah i know it wouldn't fix it but I thought caps just sorta smoothed things out & that the analogue signals here were all DC too?