Falcon crashing when moved

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derkom
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Falcon crashing when moved

Postby derkom » Sat Sep 22, 2018 9:12 am

I am working on a Falcon, and discovered yesterday that it was crashing, complete with screen flicker, whenever I inserted a floppy. It quickly became clear to me that it had nothing to do with the floppy drive, but with the physical shock to the board from inserting the floppy. I was hoping that there was a simple short to the shielding happening somewhere, but unfortunately this seems not to be the case.

When it crashes, I get one frame of bad video, then the desktop returns to normal, but with the system frozen...

vlcsnap-2018-09-22-11h47m12s478.png

One such frame. (The whole display is changing to the garbage pattern. The desktop is still visible because my camera isn't in sync with the display.)

I have stripped the system down to the bare board, and it will fairly consistently crash if I let it warm up (it does not crash cold) and then tap lightly with my finger on various ICs, to send little shockwaves through the board. (Don't worry, I'm talking about quite light tapping here, nothing that should cause any problems on its own.)

There is no HDD present, no floppy present, and I've changed the RAM and PSU both to eliminate these factors. No change.

Sadly, I think this may point to microfractures in the board. I have visually inspected with a magnifier and don't see anything, but of course it doesn't take a visible crack on a multilayer PCB to cause problems.

One other thing about this board... When looking at the bottom, I saw this unusual capacitor arrangement...

IMG_20180921_140950.jpg


I don't think I've ever looked at the solder side of a Falcon board before, and I don't find any pictures of it online either, so I have no idea if this is typical or unusual.

Does anyone have any thoughts on the crashing problem, or remarks on the capacitors? This board is starting to make me sad. :?
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Re: Falcon crashing when moved

Postby dhedberg » Sat Sep 22, 2018 10:58 am

I would be surprised if the work on the back side is from the factory. Seems like somebody tried to replace components with the wrong (too large) package size, like 1206 instead of 0805.
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Re: Falcon crashing when moved

Postby derkom » Sat Sep 22, 2018 11:03 am

dhedberg wrote:I would be surprised if the work on the back side is from the factory. Seems like somebody tried to replace components with the wrong (too large) package size, like 1206 instead of 0805.


Agreed on the size change. It all tests good with a quick multimeter check though, so I don't think it is related to the crashing, just figured I would throw it out there. :)

Edit: Does make one wonder why this was done, of course.

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Re: Falcon crashing when moved

Postby dhedberg » Sat Sep 22, 2018 11:09 am

Yeah, well it would be interesting to know why they were replaced in the first place. Do you know the history of this machine?
You could try and press gently on the motherboard at various spots in an attempt to identify where on the board the problem is located. Has any chips been replaced? Check the pins carefully and reflow the pins. Also check for anything stuck in between pins.
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Re: Falcon crashing when moved

Postby derkom » Sat Sep 22, 2018 11:28 am

I know very little about it, unfortunately. The person who gave it to me was not the user, and didn't really know much about it. I know that the former user was a musician (it came with Cubase Audio and a Cubase Audio Clock), and there are a few stickers from a long gone Atari shop in London. I know virtually nothing else.

The board has this clock patch...

clockpatch.jpg


And I see a tiny it bit of corrosion here, just to the right of the RF modulator, which I have not yet cleaned up...

trace.jpg


I have tried isolating the problem by pressing at various locations, and haven't gained much knowledge there. Seems it will crash if I press around the CPU area and also if I press below where the floppy should be. Frustrating.

Nothing component side appears to have been replaced, nothing obviously shorted. I haven't gone as far as reflowing yet.
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Re: Falcon crashing when moved

Postby mpattonm » Sat Sep 22, 2018 11:55 am

Try heating up running board part by part with reasonably set heat gun (150stC) to see if you can isolate the problematic part.

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Re: Falcon crashing when moved

Postby derkom » Sat Sep 22, 2018 11:57 am

mpattonm wrote:Try heating up running board part by part with reasonably set heat gun (150stC) to see if you can isolate the problematic part.


Good thought. I'll try that.

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Re: Falcon crashing when moved

Postby mpattonm » Sat Sep 22, 2018 12:40 pm

Oh and you might wanna try wiggle all ferrite beads, they were hand soldered, mostly sloppy job. There could be one with cold solder joint.

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Re: Falcon crashing when moved

Postby derkom » Sat Sep 22, 2018 5:44 pm

mpattonm wrote:Oh and you might wanna try wiggle all ferrite beads, they were hand soldered, mostly sloppy job. There could be one with cold solder joint.


Noted. I've managed to run out of time today, but I'll try these things out tomorrow. :)

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Re: Falcon crashing when moved

Postby Rustynutt » Sun Sep 23, 2018 12:10 am

Same area on this board. S/N Y43702008816, looks to be in the "meat" of the production run. REV sticker is long gone.

Remember (again some time back) the area around the monitor connector mount on the PCB was suspect. The machine seemed touchy like you're describing. Reworked the solder job, didn't have much luck resolving the "touch" issue. Imagine going years with a dongle for a monitor adapter took it's toll, somewhere layers in-between must be real iffy.
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Re: Falcon crashing when moved

Postby mzry » Sun Sep 23, 2018 3:44 am

The corrosion would suggest a previously humid environment. I would get a can of Pcb cleaner and an antistatic brush, give it a thorough wash down and let it dry. Just to make sure that any environmental factors are eliminated.
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Re: Falcon crashing when moved

Postby mikro » Sun Sep 23, 2018 9:16 am

derkom wrote:will fairly consistently crash if I let it warm up (it does not crash cold) and then tap lightly with my finger on various ICs

Sadly, I think this may point to microfractures in the board.

Don't be too pessimistic here. From your description it doesn't look like a PCB damage to me (how else would it work in the cold state, right?)

I'd start with replacing the strange capacitors and before that, narrowing down the area where taps makes the difference (obviously, around the floppy drive). The usual suspect would be the SDMA, for instance.

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Re: Falcon crashing when moved

Postby mpattonm » Sun Sep 23, 2018 11:57 am

Yes, large ICs could be the cause, as mikro says.

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Re: Falcon crashing when moved

Postby derkom » Mon Sep 24, 2018 7:37 am

A small update... I have isolated the problem to the area around/below the SDMA chip, which of course means it is also the area with the mysterious caps on the flipside. Since I don't happen to have a supply of replacement caps at home, I think I will first try reflowing the existing caps. The solder connections look pretty well done, but since they are maybe not factory, it's worth a shot.

Edit: Just to add a little intrigue, check out this photo from the wiki: https://temlib.org/AtariForumWiki/image ... k_Base.jpg
Pretty similar wacky caps arrangement there, and that is not my board.

Edit2: Nope. No change there.

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Re: Falcon crashing when moved

Postby tzok » Fri Sep 28, 2018 8:25 pm

The trace on right to C87 is not "slightly corroded" it is completely dissolved... same with 3 vias below it - they're probably gone as well.


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