Amiga 3000 Video Corruption when Cold

[dkedrowitsch]

Greetings!

I've got an Amiga 3000 here I recently purchased on eBay that had suffered some motherboard damage from a leaking battery. Unfortunate because the system was in flawless condition otherwise, not even dusty inside. I replaced the motherboard with an undamaged one from another 3000 with a beat-up chassis, so now I'm in the process of getting the other system back in presentable condition.

I repaired the battery damage and everything is working perfectly except for one issue. When the system is first powered on after sitting for a while, the video output is "glitchy" with what appears to be some kind of horizontal noise. After the system is on for only a minute or two the issue completely clears up. In fact, once it clears up the video quality is even cleaner than my undamaged 3000 motherboard.

Video of the problem here, and you can see by the end of the video the noise completely disappears: [ Show youtube player ]

One oddity I noticed is the issue only happens when Agnus is jumpered for NTSC mode. If I move the jumper to PAL, the display is rock solid even when cold.

I do not think it's related to the battery damage, I was very through in my testing and repair. The corrosion was confined to only a small space, no effected traces under the Denise socket or anywhere near Agnus. Although Denise got a new socket since I had to remove it to properly treat the corrosion.

The issue happens both with the Amber circuit enabled or disabled, but I have not tried the DB23 port yet.

The only troubleshooting I've done so far is to swap Denise with a spare but that has no effect.

I'm considering the use of freeze spray to try and narrow it down, but to be honest I'm not too keen on shocking circa 1992 components with blasts of cold--especially the irreplaceable Agnus. Although I may swap Agnus from my other A3000 however, to rule it out. I actually suspect Agnus to be the issue, or perhaps the chip ram PALs but I have a spare set of those I can try until I get around to programming a set of GALs to replace them with.

I should note I had an A3000 in the 1990s that exhibited the _exact_ same symptom, so this must be a somewhat common issue. It never got worse and never caused any problems, but I'd like to try and solve it if possible.

Does anyone have any experience with this issue?

[r.cade]

Have you adjusted the little pot on the back near the VGA port? The symptom you are showing is usually able to be adjusted out with that control...

It can float due to temperature, but you should be able to find a sweet spot.

[dkedrowitsch]

I have not, but I was under the impression this pot is only for cleaning up the Amber output when the scan-doubler function is enabled? If that is the case I would expect the video output to look OK when Amber is disabled. In this case the problem is present both with and without Amber enabled...so I assumed that pot adjustment to be irrelevant.



But if it can affect the non-scandoubled output too, I will certainly try adjusting it.

Cheers!

Originally Posted by r.cade

Have you adjusted the little pot on the back near the VGA port? The symptom you are showing is usually able to be adjusted out with that control...

It can float due to temperature, but you should be able to find a sweet spot.

[r.cade]

Yes, it's only a deinterlacer adjustment. Won't hurt to try, though.

[dkedrowitsch]

I just RTFM and you're right, the documentation says you need to adjust the "display enhancer" both with it enabled and in bypass. So they affect the VGA output whether or not Amber is enabled.

Part of my repair was to replace U477 which is a CMOS hex inverter that's part of the clock regen circuit supporting Amber. I'm sure any differences in the propagation delay or other electrical properties of the new component will have some affect on those adjustments.

I will follow the adjustment procedure in the manual and report back with the result!

[hth313]

If that does not help, you could try some cold spray, it usually pinpoints temperature related issues quickly.

[dkedrowitsch]

I am pleased to report the adjustment pot needed about half a turn to completely clear up the issue.

So this Amiga 3000 is now restored to perfect working condition.

Thanks!