gene@uicsl.UUCP (07/17/85)
After 18 mos it happened; my observations & fix follow: head misalignment only occurs due to "clockwise" rotation of the stepping-motor cam-wheel. It gets hammered against a hard stop (i.e. "brick wall") which is the little metal tab that is mounted firmly to the main casting with a single screw and adhesive. Either , or both of two things can happen....the metal stop-tab is hammered out of its original fixed position, or the 1/4" stepping-motor shaft can slip in thecam-wheel (press fit). If your're fortunate enough to have only the metal tab move, it's a fairly easy task to return it to its original position simply by running any known good disk, or disk-check program (I used the "Safe & Fast Disk Checker" program in June '85 "RUN" magazine, which reads all sectors and tracks). I carefully re-connected the electronics board by temporarily re-rout- ing one of the cable connectors. Be sure to note the orientation of all con- nectors before removing, and make sure to keep everything suspended safely with card-board insulation. This way you can run disks while everything is exposed for easy trial-and-error adjustment of the stop-tab. (Since part of the heat- sinking function is lost when you suspend the electronics board from the case, you might prefer to set a room-fan nearby although I doubt if it is necessary). Due to the fixed relationship between the "cam-wheel"(irrespective of whether it has slipped on its shaft) and the read/record head (via the steel drive- band), there will be "only one" correct position for the metal tab (i.e. this cannot be compensated by rotating the slotted stator-mounts of the stepping- motor). The latter should be used to compensate only in the event the cam- wheel has slipped on its 1/4" shaft. Since in my case, only the metal tab had moved, I will restrict my remarks to that case and, I believe, my solution to prevent it from recurring.... perhaps some users might like to consider this approach as a preventive meas- ure: First of all, since my shaft had not slipped, I scribed a sharp line across the 1/4" shaft-end and the cam-wheel for future reference in case it should. However, I believe my "spring-tensioned" metal tab-stop will preclude this possibility in the future. As most people know, there is a tremendous differ- ence in trying to break loose a press-fit when it is a "solid impact" as com- pared to even the slightest compromise away from that condition. Thus, instead of restoring the metal stop-tab to its original state, I in- stead improvised a little brass-bushing to ensure that it swung freely. Then I fabricated a sheet-metal hood-like structure to pass over the top of the swinging-tab and provide a bent-down tab as a firm stop for the swinging-tab to rest against in its "calibrated position". I also added some solder to stiffen the metal and strengthen the bent-down tab (although the head-banging does not hammer against this stop). There are two unused, tapped holes in the casting which I used...a 4-40 thread can be persuaded to thread into them quite easily. They are a little awkward to utilize but you can drill your own if you prefer. Then for the spring-tension, I used a strip of .015" flat-metal spring- stock...a strip about 1/8" to 3/16" wide, and long enough to reach between the swinging-tab and the side of the case (perhaps 2" or 2 1/2"). It should have some "bow" in it. Then file small notches in the swing-tab (opposite from where the cam-wheel strikes it) and also in the end of the flat spring. These mesh together to hold the spring in place. On the case-side of the spring, put a bed of solder, or epoxy to trap the spring in place. The spring, of course, holds the swing-tab against the new tab in the proper calibrated position, but provides impact-absorption when the cam-wheel is "banging" against it in the clockwise direction. With this spring in place, my 1541 passed perfectly all the known-good disks and disk check programs that I have. Some further notes: 1. When I re-installed the electronics board, I added some heat-sink com- pound where the heat-sink flange attaches to the side of the case. 2. I have no interference problems (video does not go thru TV-antenna ter- minals) so I left off the large 1541 internal metal shield. 3. I drilled 16 5/16" holes in the rear area (upper sides & rear), and some in the bottom of the plastic case....routing air flow up and to the rear to minimize possibilities of dust reaching the disk-head area. 4. I pressed dowels into the recessed bolt-cavities in the bottom to ele- vate it further from the table.