Annoying A1200 screen flicker / wavy on disk access

[mykrowyre]

I have a 1D4 mainboard, and I have this annoying problem that the screen flickers and become a little wavy with any disk access. The floppy drive tick will cause the screen to jump, especially when inserting or removing disks.

I'm aware of the A1200 "flicker fix" on the Video DAC reference line where a cap is installed to make sure the 1.2V is clean.  That cap always exists on my board, and the 1.2V is clean and does not have any trace of noise.

The 5V rail however which runs the floppy drive is very noisy.  Each time the drive ticks the voltage dips.  When the floppy tick is disabled (sticking a disk in), the screen stops jumping, but is still wavy any time I'm accessing the HD so the noise on 5V is still there just not as bad.

Since the 5V rail is used to pull up the Horizontal and Vertical sync pins, I think this may be where the video is being effected as 5V rail sags.

I've tried placing various capacitors across that 5V line to ground but it seems to have no effect at all.

I thought it might be the power supply, so I removed my accelerator and it didn't seem to have any effect on voltage. 

Obviously it still could be a the power supply.

I considered that I could have a bad floppy drive which is pulling excess current, but even when unplugged the screen is wavy on HD disk access.

Has anyone run into this before?

Thanks

[mykrowyre]

@mykrowyre, I've seen this to varying degrees and it has varied for me by screen and connection. Sorry if I've missed it on previous posts but how do you connect e.g. RF modulator, composite, RGB, and to what type of screen?

RGB.  I see the disk drive pulse on composite signal also but it doesnt effect the composite display only RGB because I think the issue is the sync signals.  On RGB it causes the sync signals to dip and the screen jumps.

[mykrowyre]

oops replied to myself

[mykrowyre]

From memory D215A. Is there a component there?
Fix is normally, ceramic cap across collector and emitter pads.

Yes a cap is there. The 1.2V reference is perfectly stable.  Anything pulled up to 5V pulses with the floppy drive access.

I measured about a 150mv dip each time the floppy ticks.

[mykrowyre]

Hi,

Simple answer is “Yes”, have had this even on a Rev 1D1 mainboard. The answer maybe that the top RF shielding is needed. Or the drive motor has become too noisy on the drive.
Is the drive by chance a Panasonic one?

Actually yes it is a newer panasonic drive.  This is an Escom Amiga made in France.  I've suspected it could actually be the floppy drive drawing too much current on motor start.  Its not a shielding problem, its the 5V rail dipping and the monitor loses sync, I can see it on my scope.  It happens even with all shielding installed.  I've tried different power supplies so its not the supply.  I've tried a different video cable.  This is a bare bones mainboard no accelerator.

[mykrowyre]

I appreciate you taking the time to try to help.  Right I was mistaken there. The screen jump only occurs when the floppy drive is accessed.  If I insert a disk to stop the drive ticking, the display is stable.  I read somewhere that the 1D4 boards have the main ground and video grounds tied together, where previously they were isolated.  Not sure of the details or where I discovered that.

It happens with or without accelerator, and I tried a modern power supply plus an ATX power supply.

Is there a schematic or document showing the 1D4 circuit changes?

I have an external floppy drive arriving Monday. My plan is to disconnect df0 and see if the jumping happens when only df1 is connected. The could possibly tell me if its the panasonic drive which is the issue.

[mykrowyre]

Ok, I've tested with an external drive and the problem persists. Each time drive is accessed my screen jumps, so its not my floppy drive or my power supply.  Must be a bad cap somewhere.

[mykrowyre]

I wanted to give an update on this. I had another problem pop up... my 1084s died!  I repaired it, but afterwards I found my screen was pink!

https://www.youtube.com/watch?v=EHSURD7paxU

I did a bunch of troubleshooting and nothing made sense. I saw output on the green line at the dac, but I wasnt sure what I was seeing was correct. I checked all resistors and diodes on the RGB output and decided to just take a chance and replace the DAC, VP101-3BA.  This was a little scary, I've done many repairs and some surface mount but not a large chip like this on a multilayer board.  I failed the first time and had lots of shorts between pins.  I removed the dac and tried again. I checked the pcb for pin bridges before installing the new chip so I could be sure I had no new bridges between pins.  I soldered in the new DAC, succesfully this time, checked for bridges and found nothing new. Result: screen still pink!  !?  I  checked the video cable and found rgb green wire disconnected, so I re-soldered it and was rewarded with perfect color in both rgb and composite. Well that sucks lol. All that chip soldering for nothing.

The screen jump on disk access appears to still be there so the DAC is not the problem, at least I can be certain of that.

I installed a new 1uF can at 215A.  No improvement of screen flicker, so frustrating! But, at least I can use my Amiga again without a pink screen.

[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.

[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.

[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.