I recently purchased a couple of A1200 motherboards. They had been recapped, and confirmed working, and indeed they were, except one had an odd issue - when using a USB to PS2 Mouse adapter the right mouse button would not function. Left was fine, X and Y movement were fine.... but no right mouse button. Plugging an old style 9 pin Amiga ball mouse in and suddenly the right mouse button was working again.
Weird.
Upon some investigation I found this is a common problem that effects certain Amiga 1200's. The right mouse button is read by the Paula chip so many assume it is a fault of the Paula. People have tried many fixes including replacing the Paula (not so easy on an A1200), with mixed results.
I went through all my recently refurbed A1200's and found another unit (out of 8) that had the same problem, so it is not a rare problem by any stretch of the imagination.
So, I did some more searching and found a website that seemed to have a different theory as to what causes this issue - https://shred.zone/cilla/page/482/amiga-1200-mouse-button-fix.html
Basically, Commodore started replacing several ferrite based resistors on A1200 motherboards with the slightly cheaper standard resistors and whilst they essentially do the same job, they effect how low the line going to Paula from the right mouse button pin on the DSub can be pulled. Because of this, the line when pulled low for a right mouse click is right on the verge of being low enough for Paula to register the action. However it is still pulled low enough to work with the original mechanical switch mice that we used back in the day.
The problem arises when you use a modern adapter that uses a logic switch instead of a mechanical one and thus fails to pull the line low enough for Paula to register.
So, to fix the problem you simply have to replace four resistors on the A1200 motherboard with four ferrite resistors..... and blow me down, it works! I have done this simple fix to two of my A1200 motherboards and both now work flawlessly with a USB adapter and optical mouse.
To save anyone in the same boat some time, you need [4] x 68 ohm SMD resistors, though 70ohm is fine to use and the only ones I could find. Both 0603/1608 or 0805/2012 package sizes will fit and the four resistors that need to be replaced are E353R, E354R, E363R, and E364R which are clustered together in the same area on the underside of the A1200 motherboard. The same four resistors seem to be present and carry the same lines in all A1200 motherboard revisions (that I have seen), so the same resistors should fix this issue on any A1200, though it seems to be the later versions that have the problem.
I hope this quick write up will help others with this annoying issue.
Weird.
Upon some investigation I found this is a common problem that effects certain Amiga 1200's. The right mouse button is read by the Paula chip so many assume it is a fault of the Paula. People have tried many fixes including replacing the Paula (not so easy on an A1200), with mixed results.
I went through all my recently refurbed A1200's and found another unit (out of 8) that had the same problem, so it is not a rare problem by any stretch of the imagination.
So, I did some more searching and found a website that seemed to have a different theory as to what causes this issue - https://shred.zone/cilla/page/482/amiga-1200-mouse-button-fix.html
Basically, Commodore started replacing several ferrite based resistors on A1200 motherboards with the slightly cheaper standard resistors and whilst they essentially do the same job, they effect how low the line going to Paula from the right mouse button pin on the DSub can be pulled. Because of this, the line when pulled low for a right mouse click is right on the verge of being low enough for Paula to register the action. However it is still pulled low enough to work with the original mechanical switch mice that we used back in the day.
The problem arises when you use a modern adapter that uses a logic switch instead of a mechanical one and thus fails to pull the line low enough for Paula to register.
So, to fix the problem you simply have to replace four resistors on the A1200 motherboard with four ferrite resistors..... and blow me down, it works! I have done this simple fix to two of my A1200 motherboards and both now work flawlessly with a USB adapter and optical mouse.
To save anyone in the same boat some time, you need [4] x 68 ohm SMD resistors, though 70ohm is fine to use and the only ones I could find. Both 0603/1608 or 0805/2012 package sizes will fit and the four resistors that need to be replaced are E353R, E354R, E363R, and E364R which are clustered together in the same area on the underside of the A1200 motherboard. The same four resistors seem to be present and carry the same lines in all A1200 motherboard revisions (that I have seen), so the same resistors should fix this issue on any A1200, though it seems to be the later versions that have the problem.
I hope this quick write up will help others with this annoying issue.
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