Amiga 1200 Rev 1D.1 RGB problem

Hello all!

This is my very first post here on EAB, I noticed here are very savvy Amiga people hanging around (with no B.S.), so may I ask my troublesome qustion;

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 on TV (C= 1084 monitor seems to wake up but show distorted startup image), BUT when WB or similar DOS disk is inserted or if I load WB from CF HDD it gives me crystal clear image of Workbench with no errors watsoever. It had permanent 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 this one problem remains. I also measured almost all critical resistors around RGB port with no luck.

BTW, motherboard had fried E264R to pin 22 trace (+12V) which I jumpwired straight from E264R, so now it measures +12V ok. This gives me hint that this Amiga maybe had wrong cable inserted at some point in time, which burned something along.

So what can it be that it can't wake up RGB output correctly? Please help me troubleshoot this fine little fella!

[johnim]

hi what screenmode you using

Well, what other than standard? I mean any DOS disk will "wake up" RGB signal and AOS screen will appear correctly. Even if I hold mouse buttons pressed I could see Early Startup Menu all right (with slight delay). But if NDOS disk such as demo is inserted, it doesn't show picture. Little strange. I tried KS3.1 and 3.0 ROMs with no success.

[johnim]

have you tried re-flowing the video CXA1145M RGB encoder encoder and surrounding components

others here

http://amiga.serveftp.net/datasheets.html

Sorry my simpliness, by re-flowing do you mean re-soldering?

Ok, now re-flowed this U12 Sony encoder chip, U23, U32, even U30, no results. Now, should I replace remaining original caps for the heck of it? These are C239, C214, C303, C304, C306, C307, C822.

[roy bates]

try to have a check after the video dac signals and before the the encoder,if you have the schematics you will probably find parts on the board thats not in the schematics.just check those as well.
have a look and see if you get the same distortion on compisite.

Thank you for reply. I connected composite output to 1084 monitor and I have very good color picture output, startup screen with no imperfections or flickering. So what do you think? Can this be Sony chip (U12) at partial fault?

And as I am no electronic wiz, I don't know how to test videodac and related components properly. I do not have oscilloscope, just basic multimeter.

edit:

I just found that U32 (74HCT244) chip has constant 4,5V on pins 3, 5, 7, 9 (digital RGB signals?), but for comparison, my fully working A1200 gives me way below 1V on these pins (actually pin 3 voltage fluctuates a little). I tested both without any cables connected and both in basic startup stage. Now, would this be some indication of fault?

[roy bates]

i dont think its the video dac simply because you have a picture.

you could try replacing u32 but check before it in the circuit as well you may have to replace the encoder.

Oh yeah now it's WORKING!

It really WAS U32 at fault, but I removed one spare from nonworking mobo and swapped it on, it solved the problem. So now I have fully working machine AT LAST!!!