The memory chip replacement I mentioned in the previous post was routine and got the motherboard up and smiling.
I figured that since I had the machine apart, I should clean and grease the floppy drive. This turned out to be an adventure that took a good part of three days.
Since I had never successfully restored a floppy drive to working condition in a couple of previous attempts on more modern macs, I was a bit concerned. First I found a couple of web sites that described what to do, or so I thought. I followed the directions cleaning some of the gunk off of the worm gear and the mechanism and then lightly greased it. However when I tried to boot the machine, it barely worked. I was able to boot the machine only two times between countless failures. Once booted, it seemed stable, so I was fairly certain that the problem was in the drive, not the motherboard. At this point I disassembled and cleaned the drive, some more. This repeat effort also resulted in a drive that barely worked.
At this point, I really studied the mechanism and determined that a clean worm gear is critical to proper head positioning. A dirty worm gear or worm gear follower could really mess positioning up. As it rotated, any grime or dried grease could shift the follower forward or backward a bit, ruining head positioning. I went back and cleaned more gunk out of the gear and removed piece of grime that had attached itself to the follower.
I also examined the circuit board to see what kind of debugging or calibration was possible. The parts all seem to be off the shelf parts, that should still be attainable, and their data sheets are on line, so I felt comforted by that. There are only two two calibration points. One is the read amplifier gain, and the other is the differential input phase adjust. There is no adjustment for the motor speed control, which is somehow controlled by software, anyway. For now, I left both adjustments alone. I did measure resistance of the adjust pots, so I could put it back to factory settings, in case I decided to change either setting later on.
I also partially disassembled the sliding mechanism and really cleaned it out, which you can’t do without disassembly. The red arrows in the image denote slip rings and a spring that need to be removed in order to do this. Don’t do this in a carpeted area, since you may never find a dropped slip ring in a carpet. At this point the mechanism worked as smooth as new and I thought I had a chance for success.
floppy drive disassembly points
This time, rather than reassembling into the carrier housing, I just attached via the ribbon cable and used some paper to insulate the drive from the chassis. Once again, the drive didn’t work. Somewhere around this time, I placed the drive upside down outside of the chassis in order to get a better view of things. Lo and behold, it booted several times in a row. I played with the machine for a while before going to bed.
The next day, I moved the drive back into the chassis, just prior to reassembling and it started behaiving badly again. Moving it back out of the chassis, it started working again. Maybe it was sensitive to position, since it was on it’s back when outside the chassis and in it’s normal position when inside the chassis. Moving the drive right side up, and setting it carefully into the chassis, I immediately encountered more errors. Trying different positions only revealed that it didn’t work when in it’s normal position inside the chassis.
I was stumped. This drive is very well built, I didn’t see anyway in which gravity could affect operation. In fact, it is so well built that I thought that it would be very unlikely that it could ever get out of mechanical adjustment. Do to this, I resisted the great temptation to fool with mechanical adjustments. I know there is a website that suggests raising the head could help things. Based on what I saw, I figured that that was unlikely to significantly help matters, especially when the problem for that person really could have been a dirty worm gear or follower throwing the head positioning out of kilter.
I even got out my scope and checked the phase adjust test points. The phase relationship between the two signals was dead on. I thought it was unlikely that there was a problem with the circuit, though gain could still be off. I had no real way to know what gain should look like, so I left well enough alone.
I put things back together and tried some more experiments with positioning the unit, only to find that putting it in the chassis brought bad performance.
Finally, it struck me, the CRT was right next to the floppy when I had the floppy in the enclosure. In addition, I had removed the shielding/mounting bracket for my testing. EMI from the high voltage CRT was screwing up the drive read and write capability! Loosely putting the mounting bracket over the drive to shield it from the CRT, quickly changed performance from awful to normal! I had solved the issue.
A couple of lessons learned about Mac 400K drives should be shared here.
Never muck with mechanical adjustments – these devices are very well built and never should need adjustment
You should not need to muck with gain or phase, if you do, make sure you record factory settings before you change anything
Never operate it loosely in the chassis without proper EMI shielding from the CRT
If you are mechanically skilled you should take the mechanism apart to really get it cleaned and lubed properly
The worm gear should be as clean as you can get it
Clean the follower arm where it follows the worm
The good news about this, is that I believe that these mechanisms are extremely well built and should last a long time with a bit of careful maintenance.
