DEC PDT-11/150 – Part 7. The disk drive belts.

I ordered a pair of 590mm belts from and  they arrived very quickly. The original belts were 588mm so 590mm should be fine. However, they are too small and don’t fit.

How can a belt that is 2mm bigger than the original be too small? The answer is in the detail or more specifically the tolerances. It says on the website the belts between 500mm and 980mm can be out by +-1.5%. So there’s the problem. The bigger belts may actually be smaller and these ones are.

The motors in the drives are mounted on slides and so can be adjusted to compensate for belt variation but even with the motor adjusted as far as it will go, the new belts still wouldn’t fit.

Back to the supplier. Next I ordered two 600mm belts and these too arrived very quickly.

These fit just fine.

Try again.

So how are we doing now? Mixed results I’m afraid.

The in depth software test is still telling me that there is a problem with drive 1. However, things are a little different with drive 0.

With no disk in drive 0 I see the image on the left, “Read error”. I also see it with an unformatted disk in drive 0 and even with the only PDT disk I have. There are differences however.

With either no disk or an unformatted disk, the message appears very quickly. With the system disk, it retries seven or eight times before showing the message.

I am reasoning then that the drive is able to read something but doesn’t like what it’s reading. I am taking this as progress.


Leave a Reply

Your email address will not be published. Required fields are marked *