Annoying A1200 screen flicker / wavy on disk access

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Sorry, I should have watched your video and read your last post properly! Glad you got the colour fixed on the monitor - it's so often something simple like a connector or cabling.

Has your A1200 been recapped? Leaked electrolyte can weave all kinds of evil magic on the motherboard, including making large areas become conductive when they shouldn't be. If it has been recapped (or had other work done), check carefully for solder bridges; I had an A600 with ticking noises in one audio channel after recapping, which was caused by a tiny solder bridge where the solder mask was deteriorating, which was shorting one of the interrupt lines into the audio signal.

[mykrowyre]

Yeah... I recapped it long ago... It only happens on disk access so I'm certain its not the monitor.  I noticed its much worse with DF1 disk access than DF0.  It's clearly something to do with noise on the power rail but I still haven't found the source.  It's possible both drives need to be recapped. I may try recapping DF1 to see if it has any effect.

[F0LLETT]

Yeah... I recapped it long ago... It only happens on disk access so I'm certain its not the monitor.  I noticed its much worse with DF1 disk access than DF0.  It's clearly something to do with noise on the power rail but I still haven't found the source.  It's possible both drives need to be recapped. I may try recapping DF1 to see if it has any effect.

Is D215 populated?
If not, add a ceramic cap or ferrite across the bottom pads.

[mykrowyre]

Is D215 populated?
If not, add a ceramic cap or ferrite across the bottom pads.

I previously installed a new 1uF cap across 215A which is unpopulated.  No improvement.

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Now that I think about it, the later Escom floppy drives might use only the +5 V line, which would be consistent with the voltage dropping on the +5 V line. I was thinking that +12 V runs very close to the video signals, both at the output connector and on the motherboard. Still, I wonder if it's worth scoping the video output signals while the floppy drive is active but without a monitor cable connected. And of course lubing the drive. Any chance of testing the floppy drive from a bench power supply that would show current draw?

Just to confirm, since you mentioned checking resistors and diodes at the video output: R250-R253 read the expected 4.7k?

[F0LLETT]

I previously installed a new 1uF cap across 215A which is unpopulated.  No improvement.

What about C215 and R215. If you have them both populated, try removing R215. If its still same, try putting it back and removing C215. If not, then put it back on.
Just when had this issue in past, either D215A, R215 or 215 solved it.

[mykrowyre]

I wanted to follow up here, it's been a very long troubleshooting session and I think I finally found the answer.

Programs which stop the drive from ticking didn't work and besides if you plan to never use a floppy drive it's pointless.

I finally came to the conclusion that this may not be the computer itself.

I purchased a VGA LCD monitor which supports 15khz modes, and surprise! No screen jumping.  I also tried a VGA CRT which did not support 15khz, and it also had no screen jumping.

Lastly, I tried a scan doubler on the VGA CRT and no loss of sync.

I started going thru the main board with help from grok, because the schematics which are available on zimmers.net do not actually match my 1084S-P monitor.

In fact the schematic for the PCB is a different CM8833 based monitor which looks similar to the Daewoo monitors.  It took many many rounds of correcting AI to finally get the correct schematic and despite the frustration I confess I am grateful for AI's help.

I finally managed to determine which capacitors may be failing and found one which had no signs of leaking but was completely dried up and tested as open, then I found another 250V polyester film capacitor which tests good but has a burn mark on the side of it. 

I replaced the first capacitor and this greatly reduced the flicker and jumping of the screen, but caused the screen to shrink vertically beyond the vertical size control is able to correct. Apparently this is a side effect of the now working capacitor, but also a remaining bad capacitor further along which just so happens to be the one with the burn mark on it.

I'm waiting for that cap in the mail but I can clearly see I am on the right track.  The most interesting thing about this issue is that it is the worst when the amiga is playing music.  Apparently it's just noise in the sync signal which the 1084S is no longer able to filter because of the bad capacitors.

I will provide more details once I confirm the repair.  Even though Grok was wrong about 80% of the time, it did finally help me find the correct schematic and explain the role of the capacitors I discovered which were bad, confirming they did have an effect on vertical picture size.   I saved the entire grok conversation which is sort of funny if anyone is interested in just how many times AI got it wrong.

[F0LLETT]

Thanks for update.
That is very interesting, never ever had caps go bad causing that issue (and believe I have seen hundreds of motherboards).

[mykrowyre]

Just wanted to clarify the bad caps are in the 1084S not the A1200.  I spent a year doing everything from replacing caps to replacing the DAC, adding caps to the supply rail, etc with no luck.

Strangely the video is only effected when the Amiga is connected via RGB.  I'm not sure why composite doesn't have the same issue, maybe because the composite sync doesn't have as much noise or is better filtered, not sure.

So anyway, I discovered that the short vertical height was actually caused by the vertical adjust pot which was broken. It looks like the section of the carbon track broke off, and it could not longer reach full range needed to expand the size to fill the screen..  grok and schematics insisted this was a 220ohm pot, but that is impossible because the remaining section (about 3/4 of the track) measures 275 ohms. After several rounds of arguing with AI and describing the broken pot it finally gave in and agreed that it's possible a 500 ohm pot was used in some boards.  Pretty funny that AI was wrong 90% of the time during this repair.  I'm tempted to upload a youtube video of the dialog which got a little heated at times as grok sent me in circles.

Anyway.... these pots are somewhat difficult to find, the original impossible, but I believe I found one at digikey which might work.

What I found after replacing the other caps in the monitor was that the loss of vertical sync (screen rolls once) on disk access went away and I just got the slight screen standard warble at the top of the screen ... something every A1200 I've seen has suffered from. I remember complaining about it in the 90s lol.

Will update in a few days when the new vertical size adjustment pot arrives.

I'm trying to save someone else from the hassle I've gone thru to resolve this issue. Really hoping this fixes the issue because I am really tried of using an LCD with horrible blocky distorted pixels. Also external PC speakers do not sound as good as the 1084s.  The 1084s just sounds amazing.