Restoring/Modifying Old Electro-Matic Rectifier

Discussion in 'The Projects Forum' started by Nicholas K. Heinrich, Nov 1, 2015.

  1. Nicholas K. Heinrich

    Thread Starter Member

    Feb 25, 2012
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    I am trying to restore this old Electro-Matic rectifier. It is rated for 250w at 120/240 VAC input and 120/240 VDC output. It contains a multi tapped transformer, two fuse blocks, and two selenium bridge rectifiers. I am intending on cleaning and repainting the unit, replacing the selenium rectifiers with modern silicon ones, and adding some flickering blue/purple/green leds to give it the mercury arc rectifier look. The old selenium rectifiers will be stripped and repainted or clearcoated, and will be reinstalled for looks, just not wired in. The transformer is quite beefy for only being rated for 250w, and I'm thinking the ratings are reflective of the rectifiers themselves more than the transformer. Since I'm replacing the selenium rectifiers with silicon ones, I would like to take advantage of any extra headroom the transformer has, within safe limits of course. The only information on the transformer itself is the letter "R" on one side and the number "1221" on the other. I would guess it weighs about 20lbs. Also I have found no documentation online for this product whatsoever, and any additional information would be much appreciated. I will attach pictures of the unit.
     
  2. Nicholas K. Heinrich

    Thread Starter Member

    Feb 25, 2012
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    Here are the pictures.
     
  3. Nicholas K. Heinrich

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    Feb 25, 2012
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    Forgot two
     
  4. MaxHeadRoom

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    Jul 18, 2013
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    The silicon replacements will be more efficient and result in slightly higher voltage, but in this case it is probably not a problem.
    What is the core dimentions, this will give an indication of Va.
    Max.
     
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  5. Nicholas K. Heinrich

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    Feb 25, 2012
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    Core is 6" X 5" X 2"
     
  6. MaxHeadRoom

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    Jul 18, 2013
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    I have one exactly the same core size, 750va.
    But of course the wire gauge has to support it.
    Max.
     
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  7. Nicholas K. Heinrich

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    Feb 25, 2012
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    From what I can tell, all the wires feeding it seem to be either 14 or 16, as does what little bit of the winding I can see.
     
  8. MaxHeadRoom

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    14g is good for 15amps, 16g 10a.
    Max.
     
  9. Nicholas K. Heinrich

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    Feb 25, 2012
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    So even at 120v, it could likely support 750VA. I'll probably run no more than 500 VA, just to be nice to it. I got it from work, and I'm sure it's had its fair share of abuse. Considering the place I worked for ran 30A through a 3A rated switch, then proceeded to ask me why it failed... Any idea as to what type of paint would have been used to coat the rectifiers? Whatever it is the only effective things so far has been non chlorinated brake cleaner.
     
  10. MaxHeadRoom

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    The paint appears to be some kind of red oxide, maybe Conformal coating, electrical protective which also comes in red.
    It comes in an aerosol can also from any electrical supplier.
    The area of the plates decide the current rating and the number the max. voltage.
    If not reusing them you could take the clamping nut off and the plates separate individually for repainting.
    I assume by your avatar you are a railway buff?
    Max.
     
  11. Nicholas K. Heinrich

    Thread Starter Member

    Feb 25, 2012
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    Repainting isn't so much of a problem, the problem has been removing the old. It's some seriously though stuff! I took one apart, and I'm going to try to strip the pieces at work tomorrow. I am somewhat of a railway buff. I happen to volunteer on the Pere Marquette 1225, the locomotive used to make the polar express movie. The locomotive in my avatar is a twin produced for another railroad that we had come up to be a substitute while 1225 was in overhaul.
     
  12. MaxHeadRoom

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    These are a couple of retro-fits we did for CPR that required dynamic braking for the Rockies.
    Came from use in Ohio.
    Max.
     
  13. Nicholas K. Heinrich

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    Feb 25, 2012
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    Wow, absolutely beautiful, now if only they still looked like that.
     
  14. MaxHeadRoom

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    This was only about 7 or 8 years ago, this was after the work was done and they had a paint job.
    Max.
     
  15. Nicholas K. Heinrich

    Thread Starter Member

    Feb 25, 2012
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    Good to see them still in service then! They don't make them like they used to do they? Anyway thanks for all the help! I will post more pictures as I continue to clean and repaint things. Eventually I'll get it pretty and operational.
     
  16. alfacliff

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    Dec 13, 2013
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    silicon rectifyers have quite different charistics than selenium. the voltage drop to current curves are quite a bit different. silicon has a fairly constant foltage drop, but selenium has squite a bit of drop, dependant on the current.
     
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  17. Nicholas K. Heinrich

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    Feb 25, 2012
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    I read that while looking them up, although I don't mind the extra voltage, and the transformer has taps for 110v, 115v, and 120v, so I can probably compensate. Since it's just a general purpose rectifier, and there is no specifc application that would be voltage sensitive, it should suit whatever need arises.
     
  18. MaxHeadRoom

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    If it is just a general purpose supply, there will be no difference, except slightly higher voltage as I previously mentioned, silicon rectifier's were often used to replace tube rectifiers, in some cases a low value series resistor was used in conjunction.
    Max.
     
  19. alfacliff

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    Dec 13, 2013
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    and when silcon rectifiers replace tube rectifiers, the plate voltage goes up especialy when first turned on tube rectifiers warm up slowly compared to silicon rectifiers, the higher voltage tends to damage the filter caps. the resistors only help hold down the voltage when the other tubes warm up and draw current.
     
  20. Nicholas K. Heinrich

    Thread Starter Member

    Feb 25, 2012
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    What I really wish I had was a mercury arc rectifier. They are absolutely beautiful in my opinion.
     
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