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Macintosh 128k Mouse: X-Positioning Flaky; tends to only move to right

T-Squared

T-Squared

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Joined
May 29, 2011
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657
Location
San Antonio, TX
Today I was able to physically repair the mouse I got for my 128k (I had accidentally snapped off the supports, closest to the photodiodes, for the wheel axles.) I put some removable aluminum clips so if the axles ever become dirty again, I can take them out and clean them easily.

Now I have a new problem. The mouse works, but only partially. I have Y-Axis positioning, but the X-axis wants to stick to the right. (It tries to move left, but it only causes it to jitter.) I don't know what is wrong, or if I have caused a short circuit.

One cause I'm considering is that the Y photodiode is bad, or there is a dry solder joint I'm not seeing.
 
Ok, I've made some progress.

I found a few incomplete solder joints, but that didn't seem to help. So, I reflowed all the solder points on the board. Still nothing.

I did a continuity check of all of the connectors from the computer side to the PCB side. They seem OK.

My next guess is that despite the durability of 1980s electronics, the capacitors have dried up. I'm going to see if I can replace them right now.
 
I've made some more progress. The replacement capacitors have made improvements. The motion is smoother, but now the cursor does something very strange.

 
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I can assure you it was not a cap issue.
Likewise, if you were able to track left but not reliably track right, either something inhibits movement of the strobe wheel in that direction or alignment issues. The fact that it will work fine one direction is a clear enough indication that the light source and photodiode are good. If in doubt, swap the photodiodes on the X axis around.

Another thing is that the mouse will do weird things if it is not firmly plugged in on the macintosh side.
 
NeXT said:
I can assure you it was not a cap issue.
Click to expand...

Well, changing the capacitors did seem to make the motion better. It moves smoothly instead of snapping to the right.

NeXT said:
Likewise, if you were able to track left but not reliably track right, either something inhibits movement of the strobe wheel in that direction or alignment issues.
Click to expand...

The strobe wheels are moving correctly. I cleaned both of them, and even polished the grime off of the ball contact wheels. (Rubber grips polished surfaces well.)

NeXT said:
Another thing is that the mouse will do weird things if it is not firmly plugged in on the macintosh side.
Click to expand...

I plugged in the cable firmly. It still had problems.

As I've conjectured in my video, the controller chip inside the mouse may be faulty, but I don't know yet.
 
Unless I'm corrected, in the macintosh mouse the only IC deglitches the photodiodes. I've never seen one of those fail.
 
NeXT said:
Unless I'm corrected, in the macintosh mouse the only IC deglitches the photodiodes. I've never seen one of those fail.
Click to expand...

It may be possible, because as I described in the video, the cursor does some strange things:

  • The cursor goes to the right, even when I pull the mouse to the left.
  • When the mouse is pulled to the right afterwards, the cursor goes back to the left a little bit, then corrects itself and continues going right.

Because of what you have told me, I can only ascertain that the IC is faulty.


EDIT: I have tested the Macintosh with another mouse, a trackball, to make sure it was not the computer being faulty. It's not. The trackball worked perfectly.

I think the IC on the standard mouse has failed.
 
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I was going to suggest you could purchase the cheaper Platinum version of this mouse and just swap the innards, but I don't know if they're the same or not.
 
The problem is most likely a poor design choice. I've seen this before from several mouse manufactures. In a proper design, they'd have used a LM339 to detect changes in the photo interrupters. They'd have then designed in the proper positive feedback to keep it from being noisy right at the cross over point. In fact, this was the way they were first designed. Then someone said, "all we are doing is using the expensive LM339 to create hysteresis. Why not use a 7414/74LS14 that has built in hysteresis." I don't think anyone really thought about why this was a bad idea.
Make a plot of a sine wave and a cosine wave. Make marks at the 3/4 voltage points and then look at what the desired quadrature points look like versus the actual quadrature points are. Now take one of your sine or cosine waves and shift it just a little and see what happen to the quadrature signals.
Using the 74LS14 part reduces the margin to almost nothing to make the circuit work. Likely what you are seeing is aging of one or both of the photo interrupters. Look at the signals from these on an oscilloscope. Move the mouse slowly, but smoothly. Watch the points that the photo input device triggers up and triggers down. Try to adjust the vertical such that it triggers evenly amplitude wise up and down. Now you need to either increase the resistor in series with the output of the photo interrupter or decrease the resistance of the resistor for the LED. You need to do this such that as you move the mouse smoothly, the width of the high levels match those of the low levels.
Do be careful about decreasing the LED's resistor. You don't want to use such a high level that it blows the LED up. Calculate a safe maximum current and use an appropriate resistor to protect it.
If the LED has aged so much that you can not bring the signal to even levels, you'll most likely have to replace the photo interrupter.
Before giving up, do try cleaning the LED and Photo sensor lenses with alcohol, especially if the previous user was a smoker!!
Usually it is the LED that is failing. If you can't find a matching LED, I once cut the LED off and used a red side LED and a piece of tape to make the tiny slot.
Dwight
 
I had this exact same problem on a macintosh 512K model. The issues was bad resistors inside the mouse (model M0100) replaced the 4 resistors and the mouse behaved as normal. I suspected the mouse was exposed to liquid damage.
 
I know this is old, but before I reassemble my Macintosh (512k now, thanks to the memory upgrade I did), I still want to get some help with this, as it seems the replacement mouse I bought still wouldn't work properly when the computer was still previously assembled; same symptoms, same problem. I'm not sure if there's a component on the motherboard that is faulty.

I'm not pointing to the 6522 VIA chip yet. (which is what controls the mouse polling, and thankfully is still being made. Very expensive for an IC component at $10, though) I still seemed to get a working cursor from the trackball I used to use, but I want to see if attaching the M0001 mouse to the available open port on the trackball will produce the same symptoms.
 
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