SOLVED - Otari MTR-90 mk2 - a few things, mainly head shields - need schematics

[Emilblanca]

Indeed, I just soldered the red and black wires on the new chinese motor to where the red canon motor terminals attaches to on the small PCB. and then I think I just zip tied it to the side of the amazon motor. a little crude, but it saves it getting lopped off again. the amazon motor hole fittings perfectly lined up to the original frame holes, iirc.

And yeah I wondered why my canon motor was so weak too but i came to the conclusion that the brushes were just so old and worn, and no amount of ****ery was gonna resurrect it. So I started at looking at other motors and at first It seemed like all new motors were gonna be too strong but with playing around with resistor values I finally got the force under control.

I think i set up a DC bench supply next to the tape mahcine to monitor the motors current draw etc plugged in/injected 24v to test it. And yeah watch out for wrong motor polarity.
When I first set it up the motor was wwaaaaaayyy too strong, so definetely begin with caution. the shields will smack into the upper part of the headblock if the resistor value is too small. its fine, it wont damage anything but you dont want it like that for too long.

oh and i forgot to mention, if you roll the whole unit away from the wall you should be able to access the relays and headshield resistor without taking that rack off, they are right at the bottom of the rear next to the lower big fan that cools the PSU, its a different section to the upper pull out thing thats just for the motor drive transistors pretty much

 

[JeffKall]

Indeed, I just soldered the red and black wires on the new chinese motor to where the red canon motor terminals attaches to on the small PCB. and then I think I just zip tied it to the side of the amazon motor. a little crude, but it saves it getting lopped off again. the amazon motor hole fittings perfectly lined up to the original frame holes, iirc.

And yeah I wondered why my canon motor was so weak too but i came to the conclusion that the brushes were just so old and worn, and no amount of ****ery was gonna resurrect it. So I started at looking at other motors and at first It seemed like all new motors were gonna be too strong but with playing around with resistor values I finally got the force under control.

I think i set up a DC bench supply next to the tape mahcine to monitor the motors current draw etc plugged in/injected 24v to test it. And yeah watch out for wrong motor polarity.
When I first set it up the motor was wwaaaaaayyy too strong, so definetely begin with caution. the shields will smack into the upper part of the headblock if the resistor value is too small. its fine, it wont damage anything but you dont want it like that for too long.

oh and i forgot to mention, if you roll the whole unit away from the wall you should be able to access the relays and headshield resistor without taking that rack off, they are right at the bottom of the rear next to the lower big fan that cools the PSU, its a different section to the upper pull out thing thats just for the motor drive transistors pretty much

All excellent advice, @Emilblanca !

I took your advice and ordered a set of 20, 40, 60 and 80R resistors in 3 and 5W varieties …

Thank you for walking me through this !

Best Regards

 

[Emilblanca]

well you should be pretty covered then. 3W should be enough theoretically but yeah if you have 5 just throw a 5 in

 

[JeffKall]

Thank you, @Emilblanca !
I will post my results when my resistor shipment arrives and I get a day to take my time and follow your advice on dialing in the proper resistor value (I am inclined to start at 60R … and then follow your suggestion about ultimately bumping up to the 80R) …

Best Regards !

 

[JeffKall]

Well, @Emilblanca …
I finally got at my MTR90 mkIII today … and like you, I found R1 on my PB-7DDC board totally fried !

I alligator clipped a 20R 3W in place of the original 1/2W … and my original shields motor worked flawlessly !

The fried original measured open (not surprisingly) … but both my brother (who is a low voltage tech) and an audio engineering tech suggested replacing the original resistor with the same 20R 1/2W value …

They both explained that R1 is meant to act as a “fuse” to protect the motor in the case of a sensor or relay failure …

Here was the audio engineering tech’s explanation:

“I would hesitate to go with a higher wattage. It's my guess that the motor shorting out is what caused the extra current that burned the resistor. In a way the resistor acted like a fuse. Without that fuse effect, unknown other stuff might have fried.”

After they both explained it … I agreed and ordered some 20R 1/2W replacement resistors …

I will do the final install when they get here !

Best Regards,

Jeff

 

[Emilblanca]

I was pretty sure the original was a 2Watt not a 1/2 watt? I might be wrong. good result on that thought

 

[JeffKall]

I was pretty sure the original was a 2Watt not a 1/2 watt? I might be wrong. good result on that thought

Here is a photo of the lower portion of the schematic for the MTR90IIIa Mother Card …

R1 is visible in the lower left quadrant …

While it’s possible that it was a 2W in the MTR90II … I don’t believe there was much in the way of change between the MkII and the MkIII PB-7DDx mother cards of the machines (although there are marked differences in other areas of the MkII and MkIII) …

I hope you were able to find a set of schematics for your MkII …

Jeff

 

[Emilblanca]

i just had a look at my schematic and yep its 1/2 W there too, I wonder why i had it in my head it was 2W. maybe I should change mine back also. So you didnt have to use the new motor in the end?