Return to 729 tapes

729 Helpful Maintenance Notes

from Eric Schlaepfer (September 2026)

The steps to remove the 729 clutch and reel shaft assembly:
1. Loosen the 4 bolts on each reel motor so you can slide them up and remove tension on the belts. I like to finger-tighten two bolts so the motor stays in place. Then remove the belts and let them lay slack.

2. Remove the quick-change lever and hub assembly by unscrewing them, ensuring not to drop any of the washers. They have to go in a specific order: Thick washer against the lever, several (4) thin shims in the middle of the thick washer, a spring washer facing up (inner rim against the thick washer), a spring washer facing down, a spring washer facing up, and the metal ring washer last.

3. There is a goof plug on the right side wall of the chassis. Pop it out, and look through it, and you should be able to see the taper pin that holds the remaining part of the hub assembly to the reel shaft.

4. Feed a long flat-ended punch through the hole left by the goof plug and put it up against the *narrow* end of the taper pin. Then give it a single confident whack with a hammer to knock the pin out. A weak tap could mushroom the end of the pin, making it harder to remove, so try to do it with one whack.

5. Inside the unit, loosen the two set screws on the magnet assembly on the back of the shaft. These are splined, not allen wrenches, so be sure to use the right tool.

6. Remove the three screws holding the mercury switch assembly to the rear bracket. Remove the whole mercury switch assembly (stationary and rotating part) and gently (!) set it aside. You can leave the wires attached, but do not break the mercury capsule!

7. Go to the workshop and locate the plastic storage bin holding the 729 clutch parts, and retrieve the plastic bag with the four long bolts.

8. Going one bolt at a time, use an allen wrench to remove the bolts that hold the rear bracket assembly. Replace each bolt with the long bolt as you remove it.

9. Loosen the allen (or spline) set screw on the rubber coupling that fastens the high speed rewind motor to the left reel shaft.

10. Yank hard on the rear bracket assembly to pull it free of the two reel shaft bearings. The bearings stay with the shafts. The bracket should slide out on the four long bolts you installed earlier. (Marc is going to yell at me here for forgetting to install the support zip ties that hold up the ends of the reel shafts.)

11. Remove the two screws holding the bracket for two of the brush assemblies, and remove the two brush assemblies and tuck them out of the way.

12. You may want someone to hold the belts and wires and stuff out of the way, but gently slide the entire shaft and clutch assembly to the rear of the machine and lift it out of the machine. Support it with both hands since it is heavy.
I like to put fasteners back in their holes to keep them in the correct spots and prevent them from getting lost, wherever possible.
To reassemble, basically you do these steps in reverse order. It's also a good idea to add more IBM grease to the brake clutch's gear teeth.
Use Frank's special glass container of IBM grease that can be found in the Leibert room next to the toolkit.

Notes about 729 clutch felt washers/seals:
There's a giant bag of mixed felt washers in the 729 bin in the workshop.
I took a quick inventory and catalogued the following styles, placing them into separate marked bags to make things easier for folks.
There are two main categories of them: felt washers go between the back plate and the rotor, and felt seals go between the bearing and the rotor.
Don't mix them up!

Felt washers for installation on the back cover:
Thin washer: 1 1/4" OD, 13/16" ID, 1/8" thick:
This is to be used only on the back cover of certain clutch assemblies.
This type of back cover plate has a metal washer on top of the felt washer and a C-clip on the outside to keep it all together.

Medium washer: 1 3/16" OD, 13/16" ID, 1/4" thick:
This is a very tight fit in the inside recess of the back cover of certain clutch assemblies.
This back cover plate does not have the metal washer/C-clip combo. I suppose over time, friction from the rotor will wear down the surface and make it fit better, so it's probably OK to use.

Medium washer: 1 1/8" OD, 13/16" ID, 3/16" thick:
This fits well in the inside recess of the back cover of certain clutch assemblies (the ones that do not have the metal washer/C-clip combo).

Felt seals for installation between the rotor and the bearing on the inside of the housing:
Thin seal: 1 1/8" OD, 11/16" ID, 3/16" thick:
This seal was found in some of the existing clutch assemblies and is generally too thin to effectively seal and prevent the magnetic powder from getting into the bearing. Avoid it.

Medium seal: 1 1/8" OD, 11/16" ID, 1/4" thick:
This seal works pretty well, doing a good job keeping the magnetic powder out of the bearing and not imposing too much friction between the rotor and the housing.

Thick seal: 1 1/8" OD, 11/16" ID, 3/8" thick:
This seal is just a bit too thick and creates quite a bit of friction between the rotor and the housing. I'd avoid using it.

Keep in mind that different bearings will have different widths and may require thicker or thinner felt seals.

Notes on reinstalling a 729 clutch:
Fun details discovered while reinstalling the clutch on CT 729 #3: the little magnetic speed sensor with the mercury switch on it uses splined set screws. Actually a couple of shaft fittings do, like the motor that lowers the head. The splining has 4 lobes on it, not the 6 I was expecting. I found a single wrench in our little bag of wrenches that would fit, so I put a little tape marker on it so we can find it easily.

After powering up the machine, there were some tense moments when the coast brake seemed to be engaged on both reels even though the machine was unloaded, and the motors were running, which was puzzling. The reel release button, which usually causes a relay to click, wasn't doing anything. We poked around at the NFP and NFP-1 relays, and discovered that they are supposed to work in a very clever way: one relay is mechanically coupled to the little pin that senses the file protect ring on the supply reel. If you hit the reel release button or load the machine with the pin pushed it, it *latches* the reel and holds the pin in place so it doesn't rub against the protect ring!

It turns out all of that stuff was fine. I noticed during our troubleshooting that the left capstan wasn't retracted all the way. Pushing it back in got the machine back into business. I added some oil to the two flip-cap oilers on the capstan motor and the shaft, and that unstuck it. For future reference, it's much better to oil the capstan motor flip caps in addition to the motor shafts (both front and rear). Unfortunately it does mean taking off the side panels.