DIY Reel Motor Rebuild for Pioneer CT-F800/900/950/1250 Cassette Decks

[pustelniakr]

Folks, every item of any design has something that will break first. For these decks listed in the title of this thread, at this age, it is most common for the reel motor to go intermittent. This shows as the take-up reel stopping while playing/recording a tape, or failing to start up after a stop or pause. This shows up as tape spilling out at the exit of the capstan/pinch roller contact point. Usually, the affected deck has spent quite a bit of time without being used. The motor can develop dead spots, due to oxidation of the armature contact surfaces.

Before you judge too harshly, these decks are in the neighborhood of 40 years old, and, in some cases have been playing faithfully all that time. The design is not bad. But something has to fail first, making the weak link a common one. Restore the motor correctly, and that is the key, you can have quite a faithful deck for a good long time more.

Note: This is not an invitation to go diving into a capstan motor. Those are completely different animals.

Again, here is the list of Pioneer Cassette decks affected:

• CT-F800
• CT-F900
• CT-F950
• CT-F1250

Again, this motor works great for decades, but, nowadays, after ~40 good years of operation, they all develop torque problems, and dead spots, where, if stopped manually, they will not start back up. The symptom for the overall deck is the play mode mysteriously stopping in the middle of a tape, for no apparent reason. What actually happens is the motor hits a dead spot while rotating slowly, and stops. This is because of the reduced torque dictated from the motor during play. The tape motion sensor detects the take-up reel stoppage, and stops the tape. These motors will tend to work fine during fast modes, where higher torques are dictated.

I've done many of these, and I still say, this is not a project for the faint of heart. The most difficult part is getting the little suckers open. Also, do not get dismayed if you have to go back in and repeat the procedure until it takes. More on that later.

The attached pic is of the motor we will rebuild...



Enjoy,
Rich P

 

[pustelniakr]

The first step is actually the absolute hardest in the process: Getting the outer case open. It is apparent that Pioneer did not intend for these to be opened or rebuilt. The outer case is swaged on, meaning the rear of the case has several swages, or retaining divets (distortions in the metal) driven into the edge of the main can, which keeps the end cap from coming off (it works too ).



You need 3 hands to do the job. That means you need someone who loves you to hold it for you (you don't want to clamp the motor in a vice lest you distort the case which would be disastrous). Then you need to use a collection of small punches and modified jeweler's screwdrivers, which are then used only for this purpose (because you pretty much ruin the tips for screwing), to drive the divets out.

The motor is rotated, on its side, till the divet you want to work on is on the bottom. Then you begin to try to get the divet to bend back out of the way, so the end cap can clear it. There isn't much meat to work with, so you must decide in advance to be patient.

Once the divets are cleared, then you take another small screwdriver, and pry the end cap off, starting with the exit hole for the wires. Be careful not to distort the end cap (it is very soft), or damage the wires or their insulation.

The pics show the end cap and divets of the outer case. Then the contents of the outer case are shown. The outer case houses a complete motor, a pair of silicone shock mounts (anti-vibration, so the deck does not hum, physically), and a mu-metal shield (to limit the magnetic field that gets out of the motor, important for tape decks with sensitive heads).



Enjoy,
Rich P

 

[pustelniakr]

The next step is to get the inner motor open (it actually has its own part number on a label). It is held closed in a similar manner to the outer case, except that the divets are much easier to access for driving out of the way. You will need to remove the motor pulley before you can open the inner case, but note its exact orientation and position, relative to the end of the shaft.

Be careful, as you open up the inner case, to ensure that you do not damage the brush assembly or lose any parts (especially the tiny polyethylene washers on each end of the motor). Note the exact arrangement of the little plastic washers on each end of the shaft. They have to go back on in just the same way.

The next pics show the inner motor parts: the end cap serves as the base for the brush assembly, the armature (with its contacts, and speed regulator assembly), and the permanent field magnet (attached to the inside of the inner motor housing.

(Note: the brush dust that has accumulated on the parts. Also, there was an excessive amount of oil all over the inside of this motor (not good). One drop of oil in each bushing is all you need, and there is one you can't get to untill you disassemble the motor as shown.)







Enjoy,
Rich P

 

[pustelniakr]

Please, before we begin the correction of issues, remove the polyethylene washers from each end of the armature, carefully noting how many there are and the order of installation. If you lose one of these, or put them back on in the wrong order or location, you will have a paperweight.

Now for the first step of actual rebuild: burnishing the speed regulator contacts. The motor speed is regulated by running in three torque levels, governed by 2 centrifugal switches (switches with weights attached to their cantilevers).

• 1st state: high torque, with both centrifugal switches closed.
  
• 2nd state: Normal run torque, with the startup centrifugal switch open, but the over-speed centrifugal switch closed.
  
• 3rd state: reduced torque, where the motor speed is too high, causing the over-speed centrifugal switch to open. The torque will be reduced till the over-speed switch closes again. The motor normally operates right at this threshold.

One problem, here, is bad centrifugal switch contact resistance, caused by oil on the contacts, or contact corrosion and/or pitting. This will result in bad startup and run torques (bad enough to be the cause of the problems, all by itself).

WE are cleaning the contacts here only. It takes a special set-up to adjust the switches in any way, so don't bend the cantilevers or change the position of the contact screws.

Both sets of contacts on this unit were dirty.

I take a GC Electronics contact burnishing tool (PN #GC 93337, the small thin one), and add a bit of extra contact pressure while burnishing the contacts till clean and polished. Burnish only until the desired contact finish has been obtained, and use a 10x jeweler's loupe to look. There are 2 sets of contacts to burnish. Actually, the plating on the contacts is too thin for most folks to be trusted with a burnishing tool. Better is a bit of blotter paper or such with some DeOxit D5 on it. lightly press the contacts together on the paper and pull the paper through several times. Look at the contacts. If they are clean, you are done.

The attached pics show the speed regulator assembly. Study it until you can see how it works.





Enjoy,
Rich P

 

[pustelniakr]

The next step is to clean up the cause for dead spots in the motor rotation: corroded or dirty armature contacts, with worn high places at each armature contact cut point. I cut a thin strip of 1500 grit silicon carbide paper, gently clamp the motor shaft in a Dremel tool, and spin the armature at the lowest possible speed. Then, I gently use the strip of silicone carbide paper and remove just enough of the armature contact surfaces so that they are level again. Be sure to take off enough, but not too much. Stop and inspect with a jeweler's loupe (10x) at regular intervals, and stop when you are done. Take off no more that is absolutely necessary.

Note: Be very careful with this part! If you so much as touch an armature wire, while the armature is spinning, it will snap and you will have a pretty paperweight till you get another motor (don't ask me how I know )

Wear eye protection while turning the armature contacts (not optional!). Also, be sure to rotate the armature in the Dremel as slowly as you can get the Dremel to spin. If you don't, the centrifugal weights will break off and fly to Neverland, maybe even by way of your eye, or the weight cantilevers will be distorted and not function properly. If you can run your Dremel out of a variac, you can get slower speeds than the normal minimum. Also, I carefully and gently wire the centrifugal switches closed, using 30 ga wire-wrap wire, to prevent a launch, in case of an inadvertent over-speed during this operation.

This motor had hosed up contacts, so I could see why the dead spots were there.

The attached pics show the before and after conditions or the armature contacts, as well as the contact polishing process.

Armature Contacts - Before


Armature Contacts - After


Armature Contacts -Polishing


Enjoy,
Rich P

 

[pustelniakr]

The next cause for poor motor torque and dead spots is gummed up or dirty brushes. This unit had both conditions.

I don't have a source for new brushes yet (I'm actively looking though), but this unit still has plenty of brush life left. Believe me, even after 30 years, there is very little brush wear. I can usually still see the curvature originally put on the brushes at the factory. Simply use a tiny pencil eraser, like for a mechanical pencil, and clean the brushes (rotating motion, back and forth). Use no fluids. you can get them clean enough dry, and you do not want to damage the brush composite.

Note: Under no circumstances try to reshape the brushes, with files or any other tools. If you do, you will not know if your rebuild is going to take until the brushes reform to the armature contact curvature, which could take weeks). If you square up the brushes, you will get burning of the armature contacts and brushes at the armature contact cuts (due to tangential contact points), and the motor will die and you will need to go back in

Also, do not modify the angle of the brush cantilevers, unless you must to increase the brush contact pressure (on the armature contacts, normally not necessary). You do not want to modify how the brushes lay on the armature contacts. If you do, you will have to run them in, and that may require you to go back in and re-perform the contact turning several times, to clean up the burn marks at the slots, due to tangential contact points.

The attached pic shows the brush assembly.



Enjoy,
Rich P

 

[pustelniakr]

The last step before reassembly, is to lube the motor. I use a light, Teflon lube (like Tri-Flow, available from bicycle specialty shops, as a light spray), applied with a toothpick. One very small drop in each bushing. Be careful not to get ANY lube on the brushes or switch contacts (paperweight time again).

Carefully reinstall the polyethylene washers, in the correct locations and installation sequences. Then, put the armature contact end of the motor into the brush assembly, carefully moving the brushes as needed.

Snap the inner case over the end-cap/brush assembly.

Power up the motor, and verify correct operation and good torque.

With a punch and hammer, carefully restore the original retaining divets (or make new ones) in the inner case to hold the inner motor closed.

Install the silicone shock mounts in the outer case, make sure that the mu-metal shield is fully seated and assemble it.

With a punch and hammer, carefully restore the original retaining divets (or make new ones) in the outer case to hold the outer case closed.

You are done. Congratulations :thumbsup:

Enjoy,
Rich P

Special Notes:

• Sometimes, you may need to go in and carefully repeat the motor rebuild. Sometimes it takes several runs at it, before the rebuild takes. This is especially if you changed the way the brushes ride on the armature contacts, in ANY way. You may need to take off a bit more, from the armature contacts, to make sure that any ridges or high spots are gone. Make sure you get no lube where it can get on the armature contacts. Make sure that you have the little washers where they go, in the right order and on the right side.

• One more thing, if the little belt that goes on the motor pulley is too tight, you will put burn marks at the slots in the armature contacts, which make dead spots (before the rebuild takes). I get that belt from Vintage Electronics, and I usually have to stretch it a bit, so it is not so tight. Be careful, because too much stretch and the belt will break while stretching it.

• When you finish, play several tapes, watching the voltage across the motor. it should be very close to 9vDC. It may take several hours of play for the new motor rebuild to take (brushes wear to good contact). Voltage of less than 8vDC across the motor is an indication that you have a problem with brush contact, centrifugal speed control switch contacts (inside the motor), or some kind of binding in the transport, or the small belt is too tight. You will know when the rebuild has taken, when the voltage across the motor remains up around 9vDC, with slight increase from cool to warmed up. The lower the voltage, the more current is being drawn (higher load, lower torque, etc.). Voltage will generally be good at first, then gradually go bad. Sometimes, I have seen the rebuild take, and seen the voltage jump up and stay good. Sometimes, 2 or 3 FF/REW passes of a tape will help a marginal motor past an initial weakness.

 

[pustelniakr]

Special Note:

Do not attempt to adjust the centrifugal switches, or splay the contact cantilevers. The test/adjustment rig you will need is far too complex/expensive for one of us to construct. You will need a mini-dynamo-meter, so that the motor can be allowed to spin at varying speeds, with a dynamically adjustable clutch, while measuring speed, torque, and speed hysteresis effects, all at the same time. As long as you don't hose up the switches, the factory adjustments should suffice until the motor simply expires. Burnish the contacts, that should be enough. :

Bottom line. Want to play with the centrifugal switches? You're a better man than I, Gunga Din :no:

Rich P

 

[pustelniakr]

Special Note:

If a brush detaches, do not try soldering it back on. Re-attach it with what it was originally attached with: a silver-bearing, highly conductive epoxy. For such occasions I use Chemtronics CW2400 Conductive Epoxy. It is expensive stuff (over $20 for a .5 oz pkg), but it will go a long way. I have used it to re-attach these brushes, fix conductive traces in slide pots, repaired PCB traces (bridging fractures), etc.

Enjoy,
Rich P

 

[pustelniakr]

Special Note:

For those that might want to try substituting some other motor for the one in consideration in this thread...

Just because a motor screws in and rotates in the correct direction, does not make it a suitable substitute for the original. There are many parameters that must be considered: speed, torque, current, voltage, thermal characteristics, current and voltage operating range, etc. Finding a deck with a substitute motor does not qualify the motor for use in this deck. The transport and motor drive circuitry is designed, specifically for the original motor. I do not recommend substitution of these motors without consideration of the required design criteria, which are not available outside the Pioneer design group, and not even to them any longer.

Disregard at your own risk.

Cassette decks are very sensitive to changes. If you know how to modify the drive circuitry to produce the correct take-up torque, and the motor can produce smooth, consistent torque under low voltage/current conditions, you "might" be able to get something out of the deck. The reel motor and drive circuitry is designed to produce take-up torque that is within a very specific range during play, and significantly more torque during fast-forward. If it does not do so, you will have excessive wow & flutter, as well as become hyper-sensitive to pinch roller and capstan imperfections during play. Excessive torque during fast mode and you could exceed what the brakes were designed to deal with and you could wind up with slack loops in the cassette body and tape path, resulting in the deck eating tapes, and tape snapping at the ends.

Cassettes were not originally designed for high-fi. They were designed for dictation machines. Engineers have squeezed amazing performance out of the format. However, that requires most operating characteristics to be taken near to the boundaries of their envelopes, and then for each of those characteristics to be carefully balanced against all the rest. As a result, "high end" decks are sensitive to any deviation from design function, take-up torque being one of those characteristics. Wow, flutter, frequency response, tape handling, etc. are all affected when parts are not operating up to spec, or are not original parts.

If your reel motor has been damaged, what you really need is a motor from a parts unit. It is your machine. Obviously, you may do as you wish. However, unless you can do the work of a design engineer, you will not likely be able to get the deck to perform anywhere near the specs it was designed to produce, if you begin to use differing transport components. Each component has specific operating characteristics contributing to its selection, and specific circuit designs to accommodate those characteristics. Design verification, after modifications, requires very specific tools and equipment, special torque meters being needed in your case.

I have said this frequently. It is relatively easy to get a cassette deck to more or less work, but getting it to perform up to original specifications is not so easy, even with original parts. Your deck, you decide.

Rich P

 

[pustelniakr]

Special Note:

I have had to go in a couple of times on individual motors (a couple of times, even more), when I first started doing them. They take a bit of finesse, but they are do-able. You want to modify the brush to commutator contact points as little as possible, so you have more than tiny contact patches to deal with. Also, you do not want the little belt that connects right to the motor pulley to be too tight. Those from one of my sources tend to run a bit tight (I carefully stretch them, before use and there is a risk of breaking, so be careful). You also do not want to put the motor into regular service right away. A loaded motor will conduct too much current for tiny contact points to handle. You want the brushes to fit nicely to the contacts on the armature. Also, you want to have all the little spacers and washers to go back where they belong, so the motor slop is not excessive, and so the brushes ride in the right path.

If you have only contact "points" (misfit between brushes and contacts), you will arc and mess up contact conductivity near the gaps. Also, the motor spins pretty slowly during play (lower voltage, current and torque), rotational momentum is lower, so if the brushes don't bridge the slots (w/associated low spots), the motor will stop. That is the original problem. Hence, you want to modify the contact patches as little as possible, and you want to run the motor in real good before putting it under significant load (regular service). Brush to contact interface should be across the entire contact faces of the brushes.

Once the motor is done and reinstalled, you depress the tape presence detector button (the associated micro-switch must have less than 1 ohm of on state resistance when actuated), and put the unit into play without a tape in it and let it run for a day or two. The only load will be the belts, pulleys, clutch & reel. In play, motor voltage and current is lower, so you won't arc so badly while running in. If you did well in the rebuild, the rebuild should take and hold. Once you have restored good contact patches, the brush wear will drop to nil, and the brushes should keep their contact paths clean and un-corroded.

Rich P