Information and pictures on Otari MX 5050 BII and Mark III
speed switch and capstan motor problems
Here is a response I sent via email regarding speed and capstan motor problems relating to the Otari MX 5050 BII or Mark III series machines.  I am including the text of my past emails below and pictures. Perhaps in the future I will re-write this in the form of instructions.  The one thing to note is to be careful in removing the plastic button caps from the switch assembly.  They have a non hardening mastic that was used to hold them on.  They will pull off but if you do not hold the shaft from below while removing the caps you will bend the switch case and damage the switch.  I was able to hold the shaft with a small needle nose pliers while pulling the cap off.  Just be careful and you will avoid damage, they do come off.

This article covers two issues one with the capstan and the other with the speed switch.
Here is the basic question concerning the speed switch that I have recived from several people

Question.... I was wondering if anybody knows the cure to this odd behavior...If I push the speed button from lo to hi, the capstan stops. After some time (15 sec up to a few minutes) you can hear relays clicking and then eventually the capstan will finally start moving at the hi speed. This only happens when going from lo to hi... it never happens going back to lo speed.I have trolled the net a long time and the best I can find was on gearslutz... about cleaning the internal speed range switch (3.25ips & 7.5ips range OR 7.5 & 15ips range). I cleaned it w/ deoxit but does not help.Once it does move it will do so w/o issue, but if you cycle the speed button, it happens all over again.Any ideas... cap, resistor, IC, relay???Thanks!

I have had a similar problem on two different BII machines. I think you may find your problem in one of two places or perhaps both. The hi/lo speed selection switch or in the variable speed control knob in the center of the machine.
In both cases my problem was traced to the speed selection switch and I think you will find it there. I found this a very confusing diagnosis because it was so odd that you could push the switch in and only after a few seconds the motor would respond to the change. The explanation that I have is that there is a deposit buildup on the switch contacts that when engaged for a short time the resistance breaks down and the contact is made. I made this discovery because the contact eventually became so poor that there would be no response at all and the motor would stop. After much manulipation of the switch button it would usually start again, but only after a few seconds. I tried to improve the switch with contact cleaner with no success. I finally removed the switch from the board and made a positive diagnosis with a continuity tester that the switch was indeed bad. The remedy that I used, was to swap the switch for the power one because the power sw only uses two of the six contacts.

I have also had another problem with two BII machines where one, of a set of three, inductor coils in the capstan motor comes loose from it's housing, gets jammed in the motor and the motor fails. I did both of these repairs on a friends machine and I sent him the story along with pictures. I will send that email to you with the pictures and text and I think it will help you out.

In another instance, I had a machine with a slow turning capstan that remedied itself by exercising the variable control in and out presumably again improving a poor contact.
I hope you find your problem. Please feel free to ask me any questions and let me know how it worked out.

This is a note I wrote to the fellow on whose deck I discovered the problem .....
WE ARE HAPPY IN OTARI LAND TONIGHT! It is quite a long story but I will try to make it short and I can tell you the details in person (or phone). It turns out that there were multiple problems. First the problem with the motor trying to run all the time was simple, the guide stop was inadvertently moved during ABL's work...quick fix. Now for the gory details the capstan motor it turns out had the same problem that my motor had on the MKIII. There is an assembly of three tiny electro magnetic coils that are cemented into a plastic housing and over time the cement let loose on one causing it to hit the motor and break the fragile iron core. Since I have not received the other motor I decided to remove one of the good coils from my parts motor and repair your motor. There were two reasons for this, first it would allow me to get on with the project, and, since this is now two out of two MKIII capstan motors that have had this problem I thought it would be wise to re-cement all the coils again. This was a delicate procedure but it worked. I now had a working motor but it still failed to run sometimes on the low speed. Now this is the long part but suffice it to say that after some time of jiggling wires switches and the like I began to sense that it was a problem in the speed switch itself. This was not clear however because often much manipulation of the switch did not result in a start at low speed and yet without touching it sometimes the motor would just jump to life and take off. I finally decided to remove the switch board assembly and use contact cleaner but this produced no better results nor any clearer indication of the exact source of the problem. I then decided to actually de-solder the switch from the board test it with a continuity tester and if I still could not get a definitive answer to swap it with one of the other switches that were identical to it on the board. Well, once out the problem did show up on the tester and there was an obvious intermittent contact problem on one pole. So now what to do? I know that you said you have been able to get parts from Otari but if this individual switch would be available might be a question (it likely would be the complete circuit board assembly) Seeing that these were two pole double throw switches and the one for the power was only using one of the poles in only one direction it was a simple matter to swap the two switches and use two of the remaining good contacts on the former speed switch for power. It would only figure however that the circuit board locations for the bad two connections on the speed switch would of course line up with the same 2 out of six that the power switch used, so jumper wires were added and we had a complete repair. WHEEEW!
THE STORY in pictures!
Click on the picture to see a slide show of the repairs.