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Amiga computer related discussion => Amiga Hardware Issues and discussion => Topic started by: mykrowyre on April 22, 2024, 09:22:22 PM
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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
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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
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?
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@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?
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From memory D215A. Is there a component there?
Fix is normally, ceramic cap across collector and emitter pads.
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@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.
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oops replied to myself
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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.
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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.
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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.
Panasonic Drives are known to require a re-cap. Could be a failing cap putting out too much noise on the motor. Worth a try.
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Thanks for confirming and the updates on your progress. From your other replies you've clearly got much more electronics knowledge than me, so just a few more questions for clarification, to try and help diagnosis.
At the end of your original post you wrote this happens even with the floppy unplugged, so is the Panasonic drive your floppy drive or your hard drive?
You wrote that you tried without accelerator, have you also tested with just floppy drive and no hard drive, no accelerator?
What are the ratings of the power supplies you've used? Are they all official Amiga PSUs or do you have something more powerful? (I was recently given 5 PSUs and the specified outputs vary, even for units with the same part number.)
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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.
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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.
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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.
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In my experience it's quite common to get one or more colour channels going intermittent on the 1084 monitors. The input connector at the back gets stressed (especially when cables are squished against a wall or left dangling) and the solder joints get cracked. Touching up the solder joints with a soldering iron to reflow them is often enough.
In older Panasonic floppy drives the leadscrew grease can harden and cause a lot of mechanical resistance, which could cause excessive current draw, though I think the leadscrew stepper is normally driven by +12 V rather than +5 V (though I've seen some drives that are the reverse, and of course external drives only use +5V). In addition to recapping the drive, you could try degreasing and regreasing the leadscrew, paying particular attention to the tricky ball bearing setup at the tip of the leadscrew (watch out for balls falling out!). I'd suggest testing without the Panasonic drive connected at all.
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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.
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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.
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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.
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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?
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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.
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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.
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Thanks for update.
That is very interesting, never ever had caps go bad causing that issue (and believe I have seen hundreds of motherboards).
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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.