hello folks welcome back to our IBM 1401exhibit our dinosaur computer from the 1960'sand today I want to take you through one of the challenging repairs we had todo on this tape unit.

That will give you a chance to see how it is built inside, it is quite amazing actually.

So this tape is called a vacuum column tape and the way it works is that instead ofgoing directly under the read heads, the tape is first is sucked in by vacuum inthis vacuum column right here, that Carl just opened for us, and then it allows the tape to coil into a reservoir here and the capstan can pull on it very fast.

It can start andstop so fast that it can just read one record at a time and of course that's done so this can beoperated like disk drives which didn't exist at the time In a second we are going to hitthe load button and you will see how the tape is sucked in the vacuum column.

You'll hear and vacuum coming on first So Carl if you can load it.

here we go and then you see the reels movinghere.

It's just the reel trying to keep the level of the tape at the right placeover here and behind this door is a little hole that senses vacuum and actuates a vacuum contact.

So all the reels do is react toreplenish the supply of tape in the column, and the tape itself is driven by another motor right here ata much faster rate So now let's look at it from the backto see how it's constructed First there is a little bit of electronicsin it In this cabinet are the same,rather simple SMS cards that we have in the main 1401 and lots of relay logic whichis almost impossible to follow on the diagrams.

Quite a few lights to tell youthe state of the tape Then that big thing actually swings out and reveals all the motorswe have here There is no less than eightmotors.

There are the two motors for the main hubs, two motors the capstans, the ones that actually move the tape, two motors for loading the tape one for the fast rewind, so that's seven.

And down there, I don't know if you can see it, a super big one for the vacuum.

So no less than eight motors to make thisthing work.

I want to draw your attention to thisassembly here.

This is a series of magnetic clutches and they are used to clutchin and out the different motors that we just talked about.

This one is used just for load, and these are the two main clutches for backward and forward The motors run continuously, the big ones there, and they are just clutched in and out whichallows the hubs to accelerate so fast and give it this jerky motion.

So Carl is going to load a tape and we're going to try to see it from the back That's the loading clutch and now you see the tape doing theirfunny danse here and its just this clutch coming in and out on a regular basis until the tape is in position.

Now we are reading and writing, you can hear the noise of the capstan starting and stopping You can see the hubs trying to catch up to replenish the vacuum column around this sensor point over here.

and the other lower sensor point over there That's what gives them this incredible random motion And our specific problem here has to do with the way the clutches are actuated Since they are rotating you need a slip ring to provide the current and those are these two copper rings and right on them you have a block of brushes that provides the current and of course with the time they have spent riding on traces, are failing and we need to replace them This is the original brush block with two brushes.

This one is still there the other one is totally gone So here is a bunch of old brushes, they are all worn out and IBM won't send us any new ones from 1959.

So the only solution is to make new ones So here we go, our little machined brush with a hole in it good enough to put the wire in it.

And voila, a carbon brush like in 1959.

And another thing we need to reproduce the brushes is the very flexible wire at the back.

We had a really hard time finding the right wire, but fortunately it turns out that thecompany I work for actually makes wire in Oregon.

We make it very high-end wirefor RF applications and one of the engineers over there Shashi heard my plight and I sent him a sample, and he made me the exact same wire, a whole spool of it, except he ugraded it tosilver-plated copper.

So we have a the wire also faithful to the IBM original Here are the brushes with the wire epoxied OK, that's the repaired brush block Two nice brushes.

Now It should work perfectly fine.

That's the clutch – whao super heavy – and that's the slip ring and that's where our brushes go, they slide on those two copper rings.

Putting the belts back together Here are the connectors Power is on.

That' clutch is working! That's good, it means the new brushes work.

There we go.

No sparking? No sparking.


I liked the sparking, that was pretty spectacular.

We don't need that! [laughs] But I liked it! Alright, our brushes are doing their job Reading cards.

OK, program is in.

Tape not happy Oh yes [adjusting knob] Better when it's in normal mode Iggy, I'm gonna use that drive! Just a second! Just do it on tape one that's the one we want to know if it reads without errors Alright got the printer on There it goes The tape is working we are writing and reading on the tapehopefully Is it supposed to print some errors in there? There we go.

What does it say? Tape demo program, zero error counts.

(Iggy) Wow.

Zero error counts? So that did it! That did it.

It was the sparking.

So what happened is that we just wrote a whole bunch of stuff on the tape and it tells us that we didn't get an error So the brush sparkng was causing the error and now we have good brushesand I we're good to go.

We are going to try a high speed rewind.

so it just unloads the tape to do the high-speed rewind and zip, off it goes And there is special motor to do that, this guy At one point it thinks it's full enough, it reloads the tape and continues the rewind in low speed using the crutches and the regular motor It's using that one and clutching in and out thanks to our brushes.

There we go it's unloading.

Here we go.

We've done it!.