Restoring/Modifying Old Electro-Matic Rectifier

Thread Starter

Nicholas K. Heinrich

Joined Feb 25, 2012
102
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.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
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.
 

Thread Starter

Nicholas K. Heinrich

Joined Feb 25, 2012
102
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.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
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.
 

Thread Starter

Nicholas K. Heinrich

Joined Feb 25, 2012
102
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.
 

Thread Starter

Nicholas K. Heinrich

Joined Feb 25, 2012
102
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.
 

alfacliff

Joined Dec 13, 2013
2,458
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.
 

Thread Starter

Nicholas K. Heinrich

Joined Feb 25, 2012
102
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.
 

MaxHeadRoom

Joined Jul 18, 2013
20,911
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.
 

alfacliff

Joined Dec 13, 2013
2,458
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.
 
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