If you are the proud owner of a Tektronix 2445 / 2465 oscilloscope, you are probably aware of the infamous U800 horizontal output IC that so often fails in these fine instruments. The cause of this failure, among other things, is insufficient cooling usually caused by a bad fan, so if you have a 2445/2465 with fan trouble you MUST get the fan fixed before continuing to use the oscilloscope. However, depending on exactly what model you have that may be easier said then done. If you are lucky enough to own a 'letter model' oscilloscope, that is a 2445A, 2445B, 2465A, or 2465B, then you are in luck - your oscilloscope uses a standard square computer fan and so it is trivial to get a replacement. However, if you have one of the original models (2445 or 2465, no suffix) then you are in for a project. You see, these models used a sophisticated hall-effect sensor controlled motor designed by Siemens (P/N 1ad3001-0a) which is now totally unavailable as is the custom Tektronix-designed fan assembly it is used in (P/N 670-7390-00 or 670-7390-01). Worse, due to the fact that this fan assembly uses a squirrel-cage type impeller, you can't just substitute a standard computer fan without somehow getting a hold of a rear plastic cover from a letter model oscilloscope.
That leads us to the project of rebuilding the existing motor. You would think this would be easy - just take it apart, replace the worn bearings, reassemble, done - and you would be right if the motor was any normal motor design. But it is not. You see, the only way to get at the bearings is to remove the windings! Yes. That is correct. You have to rewind the motor to replace the bearings. I have succeeded in rebuilding one of these and plan to offer a rebuilding service in the near future. I will describe the process in detail below, and you are welcome to attempt it, but be warned. This is not for the faint of heart!
---Here we go---
The first step is to remove the power supply from the oscilloscope and then remove the fan motor assembly from the power supply. This is detailed in the manual and so I won't go into it here. Just be very careful when removing the fan impeller. The collet used to hold it to the shaft is VERY easily broken and the part is not available (although I am working on a replacement - stay tuned.) Once you have the fan motor assembly removed from the oscilloscope, unsolder the fan motor from the board. The motor has thirteen pins (yes, that is 13!) soldered directly into the board, so the only way I know of to get it out is with a vacuum de-soldering station. YMMV.
Once you have the motor out it will look the picture below. To start disassembly, use a small screwdriver to un-crimp the aluminum from the top and bottom end caps. Then, using a pick, remove all the gray adhesive from the hole in the bottom of the motor housing. (Note that I will refer to the end of the motor where the shaft comes out as the top and the end with the pins as the bottom thought this discussion.)
With this done, pry off the top end cap and place it aside. Take a flat piece of material, thicker then the length of the motor shaft, and drill a hole in it just big enough for the shaft of the motor. Place this piece on a firm surface and put the motor shaft into the hole. Then, grip the body of the motor and press it down against the material. This should force the armature and bottom end cap part way out of motor casing. Once the process is started as described, it should be easy to complete by hand. However, DO NOT PULL ON THE BOTTOM END CAP. Rather push the armature out from the top end. With the outer casing thus removed, the motor will look this this:
Take this opportunity to label the winding supports as shown. This is essential for rewinding the motor later. Use permanent marker and touch up the labels if they start to wear off later in the process. After labeling the winding supports, remove the thin strip of tape holding the hall-effect sensors / bottom end cap to the armature. Do this slowly and carefully to avoid damaging the very delicate hall effect sensors. Save the tape for reuse later by sticking it to the inside of a Zip-Lok bag. With these steps done, you can carefully pull the bottom end cap off of the armature. If you removed all the gray adhesive, it should come off without breaking. The armature should now look like this:
Take a moment to compare your motor to the wiring diagram at the bottom of this document to verify that you understand how to rewire the motor, then unsolder or cut all the wires off of the terminals. Once you have done this, carefully remove the tape covering the armature, again saving it for re-use. The armature will now look this this:
Decide at this point if you want to save the winding wire for re-use. Doing so will take more care in unwinding the motor, but will save tracking down the correct replacement wire. This is what I did. If you do choose to replace the wire, you will need to source two rolls of #34 magnet wire of different colors. Originally they used red and clear. Once you have made your decision, remove and save the strip of tape holding the loose end of the winding in place and unwind it. You shouldn't need to count turns as I have already done that for you. There are 135 +/- 1 turns per winding. After removing the first two windings (they are wound together with two parallel continuous strands of wire) your armature should look like this:
Now remove and save the last piece of tape and remove the last two windings. Once you have done so, the two halves of the plastic housing around the permanent magnet rotor will separate and the rotor can be pulled out. The bearings are held in with a metal clip with four tabs. Each of these tabs must be depressed at once while rotating the clip counter-clockwise to remove it. I found that a nut-driver does a nice job of compressing all the tabs at once and that there is enough friction between it and the clip to rotate the clip for removal. See the picture below.
With the clip removed, the bearings can be removed for replacement. The bearings themselves are oil-impregnated spherical bronze bushings. I have not found anyone who stocks these parts, but I was able to get a small run of replacements made by LM-Tarbell (lm-tarbell.com). They are a really helpful and friendly company which I highly recommend. PM me if you want to buy a pair. If you want to have some made for yourself, the dimensions are 4.45mm diameter, 3mm length, 2mm ID. Below is a picture of the new bearings next to the old:
Before installing the new bearings, be sure to clean all the parts carefully, including the felt oil washers. Here I am soaking them in isopropanol to remove residual dried oil.
Install the new bearings the same way as the old ones were removed. For reference, here is a photo of the bearing, washer, and clip together ready for the clip to be installed with the nutdriver.
That completes the replacement of the main bearings, but the thrust bearing probably also needs attention. The thrust bearing is a modified set-screw installed under all the gray adhesive in the hole in the bottom end-cap. With the adhesive gone, it can simply be unscrewed with a small flat-blade screwdriver.
Once it is removed, check it for wear which will show up as a worn dimple in the middle. Mine was pretty bad so I decided to lap it flat again. Here is the bearing before lapping:
To lap the bearing, I made a small jig out of Delrin plastic. The jig just consists of a 'puck' of the plastic with a hole drilled in it which is just barely smaller then the OD of the setscrew. This way the setscrew can be threaded into the hole and will bind enough to stay put. To lap the part, the setscrew is advanced such that its end just barely protrudes through the bottom of the puck. The bottom of the puck is then polished against a piece of sandpaper placed on glass (to assure a flat surface) until the setscrew is lapped down to the level of the bottom of the puck at which point the setscrew is advanced a little bit and the process is repeated. See the picture below.
Once there is no more evidence of the dimple, the sandpaper is replaced with a fine polishing stone and the end of the setscrew is polished to an almost mirror finish:
In a perfect world, the end of the shaft which meets the thrust bearing would also be resurfaced, but I don't know of an easy way to do this which preserves the correct hemispherical shape. Thus, this is as far as we can go. Reinstall the thrust bearing. Before reinstalling the rotor, check for and remove any rust or dings on the shaft which might damage the new bearings. Also, be sure the bearing surfaces on the shaft are clean and have an almost mirror-finish to them. Then wet the oil washers, bearings, and shaft with just a drop of motor oil. I think 50W is about right for this application. See the photo below. Reinstall the rotor (make sure no filings are stuck to it - it is a magnet) and reassemble the two halves of the plastic rotor housing. You are now ready to rewind the motor!
Rewind the motor according to the diagram below. Reinstall the tape in the same places you took it off of - one piece to hold the loose ends of the first two windings and another to hold the loose ends of the second two windings.
Due to stretching, if you reuse the old wire you will have a bit of extra. Put on a couple extra turns or cut it off - it doesn't really matter. When finishing a winding, make sure the loose ends end up toward the outside of the end of the armature as shown in the picture below. They should not end up coming out near the shaft end in the center.
Before reinstalling the large piece of tape that covers the entire armature, use your fingers to work each winding such that none of the wires stick out past the winding supports. If you don't do this, the armature won't fit back into the casing! You can carefully test-fit the armature once you have the tape on, but don't do it before then as you will damage the windings. Getting it to fit will take some iteration. With the winding done and the armature taped, carefully reconnect all the wires to their corresponding terminals per the diagram below. Note that when I refer to the inner winding I mean the two windings you made first, while the outer winding refers to the two windings you made second.
With the connections made, carefully snap the armature back into the bottom end cap, paying attention to the fact that the hole it fits into is 'D' shaped and thus it can only be (correctly) assembled one way. Then press the housing back over the armature, snap on the top end cap, and re-do all the crimps with a small screwdriver. Before soldering the motor into the board, adjust the thrust bearing (accessible though the hole in the bottom end cap) such that the shaft can move in and out of the motor by about 0.3mm. A dial caliper with a depth gauge works well for measuring the play. Then secure the thrust bearing with a bit of lacquer or similar removable adhesive. Solder the motor back into the board, reinstall the assembly into the oscilloscope and watch it work! If done correctly, the motor should be completely silent with only the very slightest air noise when the fan is attached. You can try heating up the thermistor on the board with a soldering iron to see the motor accelerate. In case you run into trouble, you can use an ohm-meter to verify that each winding is about 25 ohms and that they are connected correctly per the diagram above.
Good luck! (and I hope this guide actually helps someone)
(In the spirit of full disclosure, I must admit that the photos in this post were taken during reassembly of the motor, not disassembly as they are presented, but for all intents and purposes they are the same.)
-Matthew D'Asaro
.
If that cools it down too much, I'll install a thermostatically controlled damper door. Sheesh!! 
