Amiga 1200 RGB gremlin

[arttu80]

Hi all!

I have this spare A1200 machine with Rev 1D1 motherboard in it. The only real problem I'm having with this machine is related to RGB output.

It behaves strangely, because when I power this on, it doesn't provide any picture output, but when WB or similar disk is inserted it gives me crystal clear image with no errors watsoever. It had flickering issue, screen would go on and off, but after caps replacement ( I did all video related C408, C407, C409, C235, C236, C237 and couple audio too C324, C334) it stopped flickering, but problem remains.

Also is very strange that when some demo disk is inserted (NDOS disks for that matter), it can't show proper image, but tries desparately as I can see some erratic graphics and sometimes none.  Also this baby has ancient scandoubler Scandy by Micronik, which is showing graphics right away when connected to VGA monitor. The quality of this VGA picture is not so good (colors are not vivid and it has some vertical ghost lines), so I suspect it's related to my RGB output circuit.

So please someone please give me troubleshooting tips for this baby! Is there some typical smd caps that I should replace close to RGB port?

BTW, motherboard had fried pin 22 trace (+12V) which I jumpwired from E264R, so now it measures +12V ok.

[arttu80]

It's in default PAL resolution, because as I stated it gives me correct picture everytime I boot from any DOS disk. I mean if I connect CF IDE card, it runs perfect, if I boot from any utility disk, it runs perfect. So I'm little confused, what can be cause for this problem. I tried my other A1200 with same output cables and screens, it's okay. I even can get to Early Startup Menu, no problem.

[arttu80]

Hi!

Today I checked some voltages again at RGB port and discovered PIN 22 has actually 12,7 volts. I compared this reading to fully working A1200, which had 12,1volts. I read at http://www.ianstedman.co.uk/Amiga/amiga_hacks/Amiga_SCART/amiga_scart.html ,  that CRT and LCD TVs need 11-12volts at SCART pin 8 (PIN 22 at Amiga side) to autoselect AV input.

Should I try and lower this voltage closer to 11-12 volts range? What do you think?

Anyway, tomorrow I dig up my 1084 monitor and try it on this machine. I think RGB monitor isn't that touchy on small voltage deviations.

[arttu80]

Tried my C= 1084S monitor, the startup screen is visible, but heavily distorted and blinking. So can this be caused by defective CSync signal? If so, how can it be determined? Already tried another capacitor (from spare mobo) on CSync and +5V lines on RGB -port. Anyone please?

BTW I think this Scandy scandoubler is pretty crap compared to Indivision AGA in function and it look like some crazy hack with all these little CIA boards and cables cobbled around mobo. But then again it isn't flickerfixer, so no real comparison here.

[arttu80]

Oh, well I post my success here just for the record, so maybe someone could use this info.

 Finally managed to correct this video problem, it required spare U32 (74HCT244) chip which I removed from nonworking mobo and swapped it on, it solved the problem. So now I have fully working machine AT LAST!

Old one had constant 4,5V on pins 3, 5, 7, 9 (digital RGB signals), compared to fully working chip, which had below 1V values.

[arttu80]

You may be correct, but when I compared values between working machine and nonworking, they both stayed in startup screen state (where is animated insert disk), with no cables connected, so I figured this voltage difference. I too suspected CSync signal, but couldn't came up with any test I could perform on it. Only these RGBI values gave me hint of a problem. I consider myself "common sense" type of guy to correct these problems (and many times that's really what it only takes), because I know very little about electronics in depth.