BEI encoder replacement

Here is how I fixed my BEI encoder with a bad incandescent lamp

Taken from my web page



While disassembling the Y axis I noted there was a BEI incremental encoder
mounted between the ball screw and the servo motor. I removed it and tested
it. There was no output from channel A, B or the index. I called BEI tech
support and relayed what I had found. They asked me how much current it
drew. It was 70 MA. They told me that was too low and I would either have to
buy a new one for $400 or send it in for repairs and that repairs could cost
up to $200 if repairable. After thinking about it for a while I decided that
I could easily replace it with a US digital encoder for a lot less. But I
wanted to do a few more checks on the BEI encoder. I check quite a few items
and then realized that it must be the light source in the encoder. I cut one
of the leads and sure enough the encoder light source was open. I assumed
it was an IR LED. I removed it and noted that this little light was only
.125D and about .187L. I looked at the circuitry and was surprised to see no
dropping resistor or the LED. It was wired directly to the +5 terminal and
ground. Upon closer examination I could see that there was a portion of a
filament inside the miniature bulb. It was an Incandescent Lamp!.

I went through my catalogs and found that Chicago Miniature makes hundreds
of different incandescent miniature lamps some of which were 5 volts. I then
was able to find that Mouser electronics carried this replacement bulb for
$1.60 . I needed to order some other parts for various orders I had so I
went ahead and ordered it. When it arrived I installed it and powered it up.
I could see the light on and when I connected my scope I could see that all
the channels were working properly. These little lamps come with different
filaments so you can get them with 10,000 -40,000 hrs of life. Failure
analysis of the Y axis was probably caused by the encoder lamp burning out.
This most likely caused the Y axis to run uncontrollably into the column
causing the Y axis servo amp to burnout, blow the fuse and fractured the
ball nut flange.



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dmauch@...

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