Other Apple II "Disk II"-type Drive Repairs
Having repaired two Apple disk drives (which I'd originally zapped by accident) I turned my attention to the two remaining drives from the Apple Haul.
Photo 1. Naked "Newtronic Co. Ltd." Drives with blown capacitor removed (as explained below)
These drives had never worked since they were dropped off. They have crosses on the case in black felt pen which suggest they may have been broken even when put away by the original owner? When I fired them up initially one blew a capacitor (in a most spectacular fashion) while the other one didn't show any signs of life at all. I did some initial dabbling, replacing the blown cap in the former drive, with one from the dead-as-doornails latter one. On application of power that capacitor blew up too!.
Deciding these two were in the "too hard" basket, I then put them to one side while I pondered what to do with them. They were Disk II clone drives, not genuine Apple ones. The circuitry differed from the other drives I had and I couldn't find any information about them on the Net. In particular, I couldn't find a circuit diagram. I had felt I'd probably just have to toss them.
Photo 2. Blown capacitor
Drive 1. Diagnosis and fix
I started with the drive that blew its capacitor. At least that one had shown some sign of life!
As both the original and replacement caps had blown I suspected a cross-circuit was the problem. However, some of the guys on the Vintage Computer Forums suggested the cap may have blown due to a long period of non-use and similar behaviour from the replacement cap may not be atypical. After all, it was from the second drive and so was the same vintage as the first cap. It could just have given up the ghost due to old age.
I figured there would be nothing to lost by getting a new cap and trying it, so that's what I did. I soldered it in, plugged in the drive and booted!
No bang, and no smoke!? What's more the drive actually started spinning! I put a disk in and the computer loaded ProDos without a hitch!! Problem fixed. Just like that!
Drive 2. Diagnosis and fix
Ok on to drive number two. I soldered a new cap into the holes left by the one I'd borrowed and booted.
Nothing. Zilch. No sign of life at all. Dead-as. This one wasn't going to be as easy.
The first thing I did was to swap the main PCB board over with my (now working-) drive one, in an attempt to isolate the problem. Doing this revealed the problem moved with the main PCB board. OK, at least that's a start. I now knew where the problem lay.
How about the ICs? There was only four of these so started to swap them over with those on the good board, to check if they were ok. Ah ha! with the first one I noticed an issue (see photo 3). A bent pin! This was probably the cause of the problem. I straightened it, re-inserted and re-booted.
Photo 3. Bent pin noticed when swapping out ICs
Still nothing. Damn. I wasn't out of the woods yet. Swapping out the rest of the ICs didn't cure the problem either. Time to break out the test equipment.
One thing that concerned me was running my Apple IIe computer in a boot mode while messing around trying to find this fault. The fault seemed serious in that not only did the drive show no signs of animation, but the computer didn't even show the "Apple IIe" logo at the top of the screen. The drive was completely hanging the machine. My intuition was whispering to me of a serious short-circuit somewhere. I felt it would be safer to remove the board, and probe around it with a multimeter. That may bring something to light without risking any working computers.
And in fact it did reveal something! I found a + test point on the card near the edge connector. On testing this against earth, the meter revealed a closed circuit! This didn't seem right at all. There was complete connectivity. I checked the same on the working board and found an open circuit, as you would expect between +ve and earth, when there is a lot of circuitry the current has to go through in between.
But where was the short circuit? I examined the track running from the +ve test point. It didn't run very far, and terminated at a transistor (Photo 4). Measuring across the transistor revealed lots of resistance, so the short must be occurring between that component and the test point. There were a couple of capacitors and a coil off this track. I removed their legs from the track, measuring each time to see if each removal made a difference.
Photo 4. Path of Positive (+) rail (in red) terminating in a transistor
Hmmm...now I was stumped. I'd removed every component from that positive track, yet I was still getting a closed circuit between that track and the earth rail? How could that be??
I spend about 30 minutes looking for a reason. There HAD to BE a reason. It just didn't make sense??
Finally I found one. I had failed to notice a track, UNDERNEATH THE BOARD, going from a connecting point on the positive track in question to another part of the board (Photo 5).
Photo 5. Connection from +ve track underneath the board. Passage of track on top of board marked with dots.
This popped up again on the top, and continued towards the front of the board, finally meeting the leg of a large tantalum capacitor.
Photo 6. Full pathway of +ve track to large 22u capacitor
I measured across that cap and found no resistance whatsoever. I measured cross the same cap on the working board. Complete resistance! The first cap was faulty! Fault found!
The next day saw a new cap inserted, and the drive working as well as the first one!
Photo 7. Two now-working disk drives
The two drives have undergone a head clean and speed alignment and now join the other 4 drives from the Apple haul. That means ALL six of the 5.25 inch drives donated in that lot are now working! That makes me feel very satisfied and gives each of my four "haybarn"-sourced Apples at least a drive each, sometimes two!
This exercise taught me to always consider BOTH sides of a circuit board. It's also taught me that you can do a lot armed with a simple multimeter and patience, even without the aid of a circuit diagram.
28th March, 2009