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Author Topic: MX5050 problem - Thanks for having me here!  (Read 4171 times)

Offline danh

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MX5050 problem - Thanks for having me here!
« on: December 06, 2011, 04:59:00 PM »
Hey guys, I've been waiting to be approved for this forum for a while. Thanks for having me aboard.


While I was waiting, I described at length a problem I'm facing with an Otari MX5050 B-II. The full detail w/ pictures etc can be found in this gearslutz thread. Long story short, my capstan spins too fast... about 1.4x too fast at 15 ips. So when I load up a 15ips cal tape and play the 1kHz section, the machine outputs 1.41kHz.


Speed controls (either on the front of unit or back of main PCB) have no effect. They controls DO, however, work. Changing speed settings results in the correct voltages hitting the capstan servo PCB. This led others to suspect the tach feedback coil, tucked at the bottom inside the capstan motor housing. (It looks a lot like a playback head).


So I removed that from circuit, and found that the coil was open. Or at least, there was no continuity between the two poles when using a DMM. This should test like any other coil, no? Anyway, I took the one from my parts machine, removed that from circuit, and again saw no continuity. Unsure of whether these test results meant the coils were working or broken, I installed the replacement pickup, reassembled the motor and installed it.





Unfortunately, normal operation was not restored, though playback now fluctuates in speed (with no modification to controls). One possibility: the 2nd tach pickup is also broken... Anyone have any ideas?


Thanks in advance, and I look forward to being a part of this community.




Daniel J. Hodd
WFUV Radio, New York
http://audiodork.com
http://dhodd.com

Offline docb

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Re: MX5050 problem - Thanks for having me here!
« Reply #1 on: December 06, 2011, 06:18:17 PM »
Probably not my most brilliant advice and it's pretty much after the fact at this point, but since you have a parts queen it might be worthwhile to replace the whole capstan motor rather than just the tach sensor, more or less following the same approach as you did in replacing entire cards with ones from the parts queen. Doesn't rule out that you might have the same bad part on the parts unit, but on the other hand it might get you up and running with a little less component level fiddling.

Sorry about the delayed forum approval. Your account must be one that I saw stuck in the approval folder a few days ago. The "approval" process is simply supposed the be the forum software automatically running a registrant's info against a spammer database, deleting the spammers, and moving the good accounts to "awaiting activation" by the account holder. From the user end one should be able to register and simply activate the account from an email link. Not sure why yours got hung up.
Dan "Doc B." Schmalle
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Managing Director - retired, The Tape Project

Offline danh

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Re: MX5050 problem - Thanks for having me here!
« Reply #2 on: December 06, 2011, 08:41:14 PM »
Thanks for the reply, docb -


I certainly will try swapping the entire motor if it comes to that. I'm encouraged right now because changing out the tach sensor did result in a change in operation - even if its not fully operational. If nothing else at this point, I want to fix this damn thing because of everything I'll learn along the way!


No worries on the delayed approval. I'm glad to be aboard. And considering I have another 4 decrepit 5050s sitting in our storage closet, I'm sure I'll be contributing a lot!


Thanks,
Daniel J. Hodd
WFUV Radio, New York
http://audiodork.com
http://dhodd.com

Offline Listens2tubes

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Re: MX5050 problem - Thanks for having me here!
« Reply #3 on: December 06, 2011, 08:48:55 PM »
No worries on having 5 5050's. Your just getting started.
Neal - Ampex Fineline F-44, 3 - Otari MX5050BII-2, Revox A77 Mk II , Teac A 4010s, 4070, Sony TC102A, Magnecord 1020, Systemdek IIX/Dynavector Karat 17D2MKII, CEC CD3300 CDP, K Works NanoMax DAC, Van Alsine Transcendence Eight+ tube preamp, Dynaco MkIV amp pair, Paradigm Reference Studio 100 v1

Offline ironbut

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Re: MX5050 problem - Thanks for having me here!
« Reply #4 on: December 06, 2011, 09:39:41 PM »
Hey Dan,

Welcome to the forum.
I found your GS thread very informative. Thanks for archiving your efforts!

There have been a few members here that have had speed problems which have been solved with either the internal hi/low speed switch or the pitch control. I see that you've worked on both and have found both to be working correctly.
Just for fun, have you tried to bypass the pitch control with jumpers? It seems to be the source of so many issues and almost nobody uses it so why not just bypass the damn thing and never look at it again.

Also, to "rule out" any tension, roller pressure questions, check for speed changes without a tape threaded. Fool the machine into thinking there's a tape threaded by holding the lifter up with a piece of adhesive tape so the capstan will turn. Watch it to see if the speed changes when you hit play.
If it's difficult to see any speed change, cut out a little disk of thin cardboard, draw lines across the center and punch a hole in the center and push over the end of the capstan shaft. The lines will make it easier to see changes in speed.

Otherwise, I'm with Doc. Since you have other machines which are working, exchanging the entire capstan motor would seem in order. After all the other stuff you've tried, pulling the capstan motor will be a cakewalk! (just be careful not to damage the business end)

Good luck and keep us informed on your progress.
steve koto
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Offline danh

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Re: MX5050 problem - Thanks for having me here!
« Reply #5 on: December 16, 2011, 06:13:05 PM »
Hey guys, just to give you an update - the problem was definitely with the tach sensor. Referring to my earlier post, I did replace it with a coil from one of my parts machines - though it seems I didn't mount it very precisely. You know how I said the speed would fluctuate seemingly at random? Well, turns out it followed this pattern:





This appeared to be the control voltage informing the capstan motor how fast to spin. Since the source of my problem was confirmed and I was itchy to see the machine in full operation, I decided to take your advice and swap the whole motor & servo PCB assembly. Powered up and bam! Worked like a charm.


Then I went on to FULL calibration, using this original manual from Otari, starting with capstan speed. I'm proud to say that after a month and a half the machine is up and running beautifully!


I do have a few questions about reference level which I'm sure are answered elsewhere, so I'll poke around. For the record, here's a link to the lengthy thread on gearslutz which includes tech detail and a bunch of pictures, in case anyone is having the same problem. I'm so glad this is resolved!


Thanks guys,
Dan
Daniel J. Hodd
WFUV Radio, New York
http://audiodork.com
http://dhodd.com