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Old 16 May 2017, 03:49   #1
MartinW
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A1200 Video Issues

Hi,

I've been looking into a problem with one of my A1200's and I'm a bit stumped at the moment. It's a 1D4 board. I got it from a friend who has had it from new in the 90's and I had it recapped. I've done this before in the past but wasn't really in the right frame of mind so just paid a reputable chap to do it for me

SO, the problem is that in PAL screen modes, and only PAL screen modes the video display as way too much red. If I switch to an NTSC mode is it perfectly OK. I have a youtube video that really shows the problem, I'll try and dig out the link in the morning.

Another oddity is that if I go into a DBL mode (PAL or NTSC) the colours are fine but there is a green bar down the left side of the screen.

This is all on an LG TV that otherwise works fine with my other A1200 and other Amiga's, on my Microvitec CRT the red is there all the time.

Does any of this ring any bells with anyone?
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Old 17 May 2017, 02:04   #2
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I forgot to add the video which shows the issue:

[ Show youtube player ]

It may not be clear what you're seeing so I'll explain...

The video starts with me in an NTSC screen mode. I then switch to use PAL and then back to NTSC. The rest shouldn't need explaining, the difference is like night and day with the amount of red only visible in PAL mode. I don't have this on my other A1200 using the same cable.

I'm completely stumped at the moment which is annoying as I wanted to put this board in my new case!
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Old 17 May 2017, 06:30   #3
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How is it connected, RGB? Looks very blurry so not sure.

Also you don't have a lot of red, what I think you have (or rather, don't) is absence of green. Your display is purple.
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Old 17 May 2017, 12:41   #4
MartinW
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Sorry, I probably should have said

It's an Amigakit RGB scart lead. I've opened it up and it conforms exactly to the diagram on the net by Ian Steadman so the lead appears to be good. I think the blurriness is simply down to my rubbish video skills on my phone

Here's another video. This time it's my Microvitec 1438 which obviously has a dedicated cable. The 'off' colours are not as obvious here but because it can display the DBL modes it shows up another quirk of this board. On this monitor, NTSC and PAL are both as bad as each other, but when I go to DBLPAL (or DBLNTSC) the colours are great (nice grey background) but this time there is a pronounced 1/2" or so of green lines down the left hand side of the screen. There is another thread on this board from some years back where someone reports that problem but it didn't get any replies.

Quite bizarre!

[ Show youtube player ]

So far I've looked at the outputs on a scope and there doesn't seem to be a difference on any colour, but TBH I'm not sure if you should really be able to "see" the difference in colour signals anyway? A very obvious thing that I've completely forgotten to do is measure the SYNC frequencies in each mode to make sure they are OK and maybe I should crack open my really nice A1200 and compare some signals?

I found one oddity in that according to the schematics, Alice should have a 14Mhz input on pin 36. This looks like it is an output from Lisa on pin 23. On my board this is 14Mhz when it comes out of Lisa, but when it gets to Alice it is 7Mhz. If I bridge E377R then I get 14Mhz at Alice but appeared to get noticeably hotter and it made no difference to the issue. The additional heat may have been my imagination, Alice is tucked right under the keyboard after all but I have removed the bridge for now so the 14Mhz input on Alice is back to 7Mhz.
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Old 17 May 2017, 18:07   #5
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I would be scoping/tracing the green channel's travel. See what comes up with when you switch modes.
Oh, I see now the green bar too. When I had a similar issue, my Lisa's soldering joints had gone wrong and needed to be redone. Sometimes the screen looked likewhat you show, but not when changing modes.

Try pressing down on Lisa and see what happens.
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Old 18 May 2017, 01:23   #6
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Thanks for the tip on it being green not red, that's definitely the case. I can certainly see a voltage shift (lower) in the green inputs to the video encoder compared to the red and blue, but strangely it doesn't change when I switch modes. I don't see any difference when pressing on any of the ICs.

I forgot to check what the DR/G/B/I signals are doing but my scart cable isn't connected to those anyway. (What are they actually for?)

So not really any further forward at this point unfortunately.

[EDIT] Never mind about the DR/G/B/I - I can see from the schematics that they are for 16 colour. It's a bit of a shame that some of the other used signals are going through the lower part of the 244 buffer. If they weren't then I could remove the buffer to make sure it's not causing any problems. If it were DIP I could have swapped it out by now but I don't have any SMT IC spares.

Last edited by MartinW; 18 May 2017 at 01:37.
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Old 18 May 2017, 01:55   #7
Sgt Esterhaus
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The effect looks similar to that when switching colour space between RGB and component video (YPrPb). Whether that's what is happening here I'm not sure...


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Old 18 May 2017, 02:05   #8
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Would I be right in thinking that the only place something like that could remotely be happening is at the composite video encoder? If so, I could remove the capacitors from the RGB inputs of the encoder in order to isolate it from the equation. I can't see me ever using Composite out anyway really.
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Old 18 May 2017, 02:14   #9
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I'm not sure if the encoder can influence the RGB output, but that one usually goes bad because of nearby cap leakage. There's a shit ton of caps around it that can cause damage. So maybe give it a look!
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Old 18 May 2017, 02:41   #10
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In theory I wouldn't think the encoder should be able to influence the RGB out because of the capacitors but I guess it won't hurt to remove them and the encoder to check for damage in the area. Will give it a go and report back (will be at least Friday now).
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Old 21 May 2017, 00:01   #11
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OK, just to update this thread with the solution as far as I know it so far, it turns out I was correct to suspect the link between the RGB out from Lisa and the encoder. I measured the capacitors on the inputs of the encoder (C211, C212, C213 for r1 board) and sure enough, C211 and C213 (R & B) were around 0.22uF as they should be but C212 (G) was reading lower at 0.18uF. I pulled C212 and I now have a perfect picture in all modes, including the green bar down the left of the screen having cleared up in DBL modes.

Now the only small spanner in the works is that the only SMT spares I have kicking about are what was left over from my GBA build and so I don't have any 0.22uF caps. Until I can put a new cap in C212 I won't know if the encoder is feeding bad signals back up through to the analog RGB out or if it is simply the bad cap.

Given I can't see me ever using composite output, I'm tempted to just leave it as-is, but I suppose in case I ever sell it, when I order the parts for my Picasso build I'll stick on some of those caps and see what happens. Bit of a pain though as they could be a while and I want this back together now!
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Old 21 May 2017, 01:45   #12
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PM me your address and I'll stick one in the post for you.

I'm a bit surprised that the cap value just drifting by that amount caused the gross fault you had. Did you measure it's DC resistance too, I'm guessing it has properly failed and is a lowish DCR.
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Old 21 May 2017, 01:57   #13
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Umm, thank you very much - I'll drop you a PM in the morning

Basically, no, I didn't really approach this in a very technical manner and I agree, 0.6uF really didn't ought to make the difference I am seeing. I looked at the schematics, decided that either the output from Lisa and / or the DAC must be bad or the only other place that could possibly impact anything was the inputs to the encoder. Since it was easy to remove them, I removed all 3 of the caps, thus isolating the encoder from the RGB out. I powered it back up and bingo, all was good. Tried it on my Microvitec in DBLPAL and it was still all good.

While the caps were just sitting on the bench I noticed my meter has a capacitance range and got the readings. I put the two that were reading 0.22 back onto the red and blue and the picture is still good. I haven't connected the green back up yet.

So I didn't do anything more scientific than that I'm afraid.

[EDIT] I'm just happy that something went right for once! I've not had a lot of luck lately in terms of the old computers and tonight it looks like I've fixed that and I also swapped the CIA outputs for DF0 and DF1 on my GBA, so a couple of positive things for once. Tomorrow I might tackle the PSU in my BBC Master since I'm not a bit of a roll

Last edited by MartinW; 21 May 2017 at 02:04. Reason: Added a bit extra
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