mouse ports not working

[smariotti]

I have this same problem on my A500.  It started happening shortly after I moved my computer from one desk to another and seemed to just spontaneously stop working.  

Like the OP the mouse buttons work fine, but I can't move the mouse at all.  I tried my mouse on another Amiga and it works there, and I tried another Amiga mouse on my Amiga and got the same result (buttons work, no motion.)

It sounds like it's a fuse that's blown on the mouse port. I haven't yet taken out my multimeter to continuity test the fuses at the port, but I disassembled the Amiga and see where there's a bank of resistors and fuses near port 0.  I suspect that one of the fuses is blown and I will have to replace it with another fuse.  It seems like the Littelfuse Pico II fuses are the same type.  I need a 100mA replacement from what I read... these seem to run about $5-6 USD which seems a little expensive for what it is.

Has anyone had this problem on an A500 or has anyone else replaced these fuses with something else?

[smariotti]

If you have a voltmeter, measure carefully between the center two pins of the bottom row of the joystick port. There should be 5v there. If there's really no fuse, a trace might have opened up.

I tested both joystick ports and measured +5v between the two pins mentioned, so it would appear my fuses are ok.

Yet I still have no mouse, just buttons on port 1, don't know how to switch ports to try the mouse in port 2 if that's possible.

It looks like the resistor at EMI401 is burnt.  I can't make out the color bands, but the first one is green.  Available documentation suggests that this is a 1 Ohm resistor, though clearly mine is not.  The burned/scorched parts hide the colors of the other bands and I can't find anywhere that lists the resistor value for EMI 401 on my A500 rev 5 motherboard.  I suspect the value of the resistor is different on different revisions and I don't want to put the wrong value in there!

Anyone else have a rev 5 that could give me the color bands of EMI 401 ?  (It's near the JOYSTICK 2 port)

I'm guessing based on a few pictures available (all hard to read, sadly) that it's GREEN-BROWN-SILVER-GOLD or something close to that, which suggests 0.51 Ohm.

http://www.bboah.com/download_photos/a500mb_rev5_2.jpg
http://www.bboah.com/download_photos/a500mb_rev5_3.jpg

[smariotti]

According to the service manual, EMI401 is a noise filter listed as 100pf. On the actual schematic, its drawn as a resistor with an extra lead going to ground... I'm assuming that's a 100pf capacitor to ground. It says 4.7 next to it, but no multiplier... In any case, the 5v to both joystick ports goes through it, so if you've got the 5v on the ports, I'd say EMI401 is OK.

That's interesting, as on the motherboard it's clearly a 1/4 watt resistor.  There are three holes at the site of EMI401 on the silkscreen, though the resistor just connects between two of them.

[smariotti]

Well, that fixed it!

I replaced the charred resistor with a closely matching (1.1 Ohm) resistor from my parts box and voila!  Mouse works again like a charm.

What was misleading is that the burned resistor still read approx 1 Ohm impedance across it, even though it was "bad."

Here's the newly soldered resistor that got my mouse back in action.