Amiga 2500 Mouse PotY, PotX problem

[Prime]

I have an A2500, it seems to work fine except for the mouse movements. The mouse buttons work fine, but the X, Y axis don't work at all. I read on NewTek's site that it could be a fuse, but I don't see a fuse (Small green soldered one that looks like a resistor.) I checked with an ohm meter from pins 5 and 9 all the way to the paula, which seems like a straight shot with no resistance. The only thing inline that I see is a ferrite bead. I guess the next thing I can check to see is, if the mouse port is receiving 5+. I already tried to replace the Paula and the CIA's.
Any Ideas?

Thanks,
Joe

[save2600]

Look REAL close at your motherboard, just to the left of the mouse port IIRC. Might be smack dab in the middle of the two also. There is indeed a TINY ceramic cased .5 amp fuse. Typical for these to blow (never plug joys/mice into an Amiga when it's on!). I have a few if you discover you need one. Try "jumping" the fuse first maybe and see if the optic sensors spring to life on your mouse...



Since this is a commonly failing part, I'd solder wires and a regular fuse holder to her - which is precisely what I did on mine. Haven't had to replace the fuse since, but will make it that much easier if/when it blows again.

[Zac67]

Forget PotX & PotY, they're the analogue inputs used for paddles and such. You need to check pin 7 for +5V. Check FB215 and especially F1 (possibly green, resistor-like) between keyboard and mouse port.

[Prime]

I still don't see the fuse, so here is a picture.
I am going to test for 5v.
If there is no 5v on pin 7, what then?




Joe

[Chain]

If nothing of above helps, locate U74 and replace it (74HCT157), preferably wit a socket

[JimS]

The fuse was added at Rev 6 motherboards. That looks like a 4.5. So, it could be a blown trace, if there's no 5v at the joystick port.

[save2600]

Ahhh yes... the good 'ol mobo revisions. Shoulda thought of that, shoulda thought...  LOL!    Indeed his mobo does not look like my 6.2.

Before changing out that U74 chip though, I'd test all of those diodes and tantalum caps running about the ports. Tantalums fail QUITE often (especially after 15-20 years) and when they short, chances are they would take out something else in line - like U74 if it's on that particular mobo. FYI: you MAY replace tantalums with electrolytics to be on the right side of preventative maintenance.

And a blown trace could have happened because of battery acid leakage you're thinking JimS?  Or sloppy techs dropping their screwdrivers on the mobo too I guess. lol  

[JimS]

save2600 wrote:
And a blown trace could have happened because of battery acid leakage you're thinking JimS?  Or sloppy techs dropping their screwdrivers on the mobo too I guess. lol  

Possible... but I was thinking more like the trace acted as a fuse and vaporized. I've seen it happen on some machines that came in for repair. That's why they added the fuse, both to the joystick ports and the external video port as well.

[Prime]

Actually, there is 5v to pin 7.
I could change to electrolytic caps, what are the caps
rated at?
There is no U74 that I can find anywhere on this mobo.

Figures, it couldn't be something simple!

Joe

[save2600]

Sometimes it really *IS* the simple stuff... do you have another mouse to test?  There are components and an I.C. inside the mouse that can fail (unlikely, but I have run across "dead" mice). Are the female pins okay? Not completely recessed causing lack of good contact from some oaf clumsily inserting it in and out?  lol

Before mucking up your mobo, do you know how to check those caps? Use your DMM and make sure they are not the culprit. Look for a reading of (going by memory here) .1-.4 on the diode test (not continuity) function of your DMM. Basically, measure both sides of each capacitor and look for a shorted or way out of spec cap compared to the others. That'll be your culprit. Same with testing the diodes. Reverse the leads though for them. + to - will read .4-.5 (or whatever the consensus is), but when reversed, your DMM should test open. When reversing the leads and the DMM shows anything other than an opposite effect (ie: small current from the DMM itself OR no current the other way) - you've got a bad diode.

Not sure where else that 74HCT157 could be, should be close to the ports though I'd imagine. Still, check the caps and diodes first before attempting to replace an IC - for which you really should be using a logic probe for anyway. You know... hi-lo, pulse - that sort of thing.

[Prime]

How can you test caps and diodes when they are mounted on a mobo? Doesn't this give bogus results?

Joe

[save2600]

Measuring resistance  - yes, of course. But you're not going through loops, coils and resistors here (or entire circuits). We're talking about different components: Capacitors and Diodes, which test differently. I never said to put your DMM in ohms (resistance) mode. Rather... you'll want to test in 'diode' mode. If components always needed to be removed from their respective circuits, the electronics service man would have completely and utterly died a LOOOOOONG time ago ;-)  

Now... what are you waiting for? Break out that DMM and get to work or send your board to me!!  lol

BTW: do you not have another mouse?!?

[Prime]

Yes- I have tried other mice...

I love this stuff, so any little gems you can throw my way, be my guest!

I will check it tonight when I get home from work.
I am excited, hopefully we can get this bad boy working.

Joe

[JimS]

In the little gems department, I once saw a 2000 where the mouse failed every time I tightened down the screw next to mouseport. Never did figure that one out... finally just put a plastic washer under the screwhead.

[save2600]

@ JimS:

ROTFLMAO! Maybe you had an Amiga 2000 'especial' where someone from South America douched down the mobo with WD-40   ;-)    LOL!