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Old 15 December 2018, 20:22   #1
SpocksBeer
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A1200 mouse movement issues - check my working out

Hello all,

While I'm waiting for what I hope are the correct parts to arrive, I figured I'd start this thread to catalogue a problem I'm having with an A1200 mouse port. Perhaps it will be helpful to someone else. I'm also hoping more knowledgeable people might chime in to let me know if I'm on track or not.

I received a new (to me) A1200 motherboard recently, a rev 1b. It boots to Workbench fine, but the mouse only moves up and down. Any horizontal movement results in a tiny (1 pixel) movement, but it immediately returns to the hard left of the screen. Both buttons work fine, fortunately.

Based on this, I deduced what was not my problem: the 5v line was ok (confirmed on pins 7 and 8 of the mouse port) and Lisa was probably ok (some data was getting to it for vertical motion at least). This left, I think, two possibilities: the IC at U34, shift register 74LS166, or the R370 pull up resistors.

Checking each resistor, they all measured the correct 4.7kohm value, except one at R370F. This one measures only 540ohms. I think this adequately explains the problem I'm seeing, as it is connected to one of the parallel inputs on 74LS166 and is thus associated with mouse movement. On another working A1200 motherboard I had available to test, the same resistor returns the correct 4.7kohm value.

So now I'm waiting for some new surface mounted parts to arrive. My only concern about my "fix" is that loss of resistance seems to be a strange failure mode for a SMD resistor. My Google sleuthing tells me open circuit or short circuit is more likely, and this looks more like another resisotr in parallel. I can't see how that's possible though.

Anyway, thought I'd do a brain dump while it's relatively fresh in my mind. Be cool to hear if anyone thinks I'm right, or completely wrong. Guess we'll know for sure soon enough.
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Old 24 December 2018, 02:09   #2
SpocksBeer
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Whelp...not the resistor. Removing it and measuring it showed its nominal 4.7kohm value, and putting a new one in resulted in more or less the same ~600ish ohms value.

So...shift register? Measuring the open pads with R370F removed showed I think about 1.3Kohm, which I thought was a little odd. Also just noticed that E355C is missing from the board but this should, if anything, affect vertical movement of the mouse. I'm confused.
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Old 24 December 2018, 02:13   #3
SpocksBeer
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Added image of the area i'm working in...

I will reinstall a cap at E355C also, and see what that does.
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Old 05 January 2019, 00:58   #4
SpocksBeer
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Sigh...the saga continues. Re-added the missing cap at E355, no change. Replaced the shift register at U34 and...success! For a little while, at least. Proper horizontal and vertical movement - yay!

Then it died again. Exact same thing. One moment the mouse is fine, the next I can only move up and down. Something killed the shift register again, somehow.

Does anyone have any ideas, because I'm reaching the limits of my meager skills.
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Old 05 January 2019, 04:28   #5
SpocksBeer
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Nevermind. I spent a little more time on it, and I think I've fixed it. It seems that capacitor E356 was also bad. It showed some odd value when measured in circuit, about 220kohms. Replaced it, and everything seems to work again.

Between good quality schematics, the Amiga PCB explorer (http://amigapcb.org), and the ReAmiga 1200 BoM (for correct Mouser part numbers) I finally managed to nut this one out. Hope this helps someone else one day.
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