80-Bus News

and swell. The mechanical temperature compensation of the drives was coping quite well with the high temperatures as proved by the fact that disks could be formatted and used normally; what it couldn’t cope with was disks, which having drunk most of the atmospheric moisture around them and swollen up, became a few thousandsth of an inch larger in diameter than usual. Of course the greatest affect of this swelling would be at the outer edges of the disk, and guess where the boot and system tracks are.

So what was the cure. Well we did nothing, we managed to copy the old disks to new disks prepared under the elevated temperature and humidity and used those for the duration. We didn’t chuck the old disks, because as soon as the weather returned to normal (sub-zero and rain), we updated the old disks from the new, and went back to using the old disks with no further trouble. However, one fact did emerge from that episode, and that was that Dysan disks were not affected anything like as badly as the others. I have since learned that Dysan lacquer their disks to stop them absorbing moisture and to improve their wearing capabilities, the only trouble with Dysan is that they are twice the price of the competition.

Different disk prices

Now on to the differences in price of double and single sided disks, and quad, double and single density disks. It is my opinion that all disks (except those coated on one side only for single sided use) are created equal, that is double sided, double density, 80 track (96 t.p.i.) disks. In the process of manufacture, they are tested and those which pass the 80 track test become quad disks, those which fail the 80 track test are rechecked at 40 track (48 t.p.i), those which pass becoming double density disks, and then the failures are checked as single density disks. Those that fail then end up in the bin. By this time most, if not all, of the disks have been accepted. Now the vagaries of disk production are not likely to bow to the dictates of the marketing boys whose market profile shows that the majority of disks required are the 40 track double sided breed. In fact, if the production process is going well and not too many lumps are in the pudding before coating, then the majority of disks that fall off the end of the production line are going to be of the double sided 80 track breed and not what is required for sale. So I guess a fair proportion of 80 track disks are diverted and labelled as 40 track disks, simply because the disks being made are better than specification.

So what is this all leading up to? Well we’ve stopped selling 80 track verified disks for a start. We sell (and use ourselves) only 40 track disks for the 80 track machines. This saves about 70p per disk with no apparent difference of performance whatever. We still get about 1% of duff disks, be they 80 track or 40 track, so as duff disks are sent back for exchange I know we would rather have the 70p’s and so would you. All this is borne out by the fact that the original box of ‘single sided single density’ disks I bought for testing the original Henelec disk controller at the time when it was fitted with single sided Shugart SA400’s are still seeing useful daily life on my present machine as ‘quad density double sided’; and not one of them shows any signs of either wearing out or giving verify errors.

My Advice

So my advice? Find out what brands of disk suit your machine, and when you’ve found that out, find out how cheap you can get them. Never buy 80 track verified disks just because you’ve got an 80 track machine, it seems a waste of money to me. The only exception I make is Dysan, they certainly behave better in high humidity environments and because of the lacquer, should wear