[Boatanchors] selenium rectifier... replacement strategies

[Ed Tanton]

Glen is exactly right. I don't like to modify my gear, but make a special 
exception for selenium rectifiers. There is a very good reason to do this 
anywhere it is practicable: that awful smell when a selenium rectifier goes 
up in smoke is selenium sulfide. Besides stinking to high heaven, it is a 
poisonous gas.

The one time back in the early 60s when I toasted one, the gas didn't kill 
me, but maybe that will explain a lot about my behavior to some of you? In 
any event, the replacement needs careful monitoring. Here's why.

I found a small 1/2 wave selenium cell in my B&W 380-B T/R Switch, as I was 
looking into it for the 1st time after purchase. It uses-as I recall-a 6BH6 
as the cutoff tube. Once I had 'formed' the electrolytics, I measured the 
voltage right at the capacitor. It may have used a dual section, with a 
resistor in the middle as a cheapo choke, I don't remember. (It's that gas 
business, I'm sure.)

IF it has the resistor, all you have to do is measure the voltage at the 
output of the filtering, and then measure the voltage across that resistor. 
That'll allow you to calculate what the current is. (I'd measure the actual 
resistance after you make certain the circuit is off and 'drained' of any 
residual electrons!) If there isn't a series resistor you can check like 
that, just disconnect the wire at the filter output and measure the current.

Now change out the selenium rectifier, and send it to me. I have a modest 
collection of 'em, and will pay the postage. Put in an appropriate silicon 
diode-a 1N4007 or something like it, depending on the current, etc. Get at 
least one lead of your voltmeter attached-such as the ground wire) and hold 
the other (POS) lead at that previously measured filter output point. Turn 
the thing on, and as quickly as you can, get a stable voltage reading, and 
turn it off.

The reason for the hurry is the marginal voltage tolerances likely to be 
present in the capacitors of your device-under-test (DUT). Once you know 
the voltage increase, you already have measured the current, and you can 
calculate both the required resistance, and power rating of that resistor. 
If it figures to be 1W, I'd use 2W, etc. It's not a bad idea to replace the 
filter capacitor(s) while you're in there-and you can use anything up to 50 
or even 100% more capacitance, if you just HAVE to-BUT if you do, do it 
BEFORE you measure the voltages. Your changes may increase the values. (Try 
to use ~ the same value. A 20% increase shouldn't affect anything.) And 
increase the voltage rating at least 20% if you can. It never hurts.

OK... so you changed the rectifier out, and added the calculated resistor. 
Measure the voltage again. Despite all this, my output voltage was over 20V 
higher than the original. I could probably have left it at that, but being 
persnickety and an obsessive-compulsive, I altered the resistor values 
until they were within 5V (out of 125 VDC/whatever it was.)

I then changed out all the cheapo 0.01's B&W used for AC line bypasses with 
REAL 250VAC-rated disc-caps, and added a V150LA20 or 3 for surge 
protection. The final two things I did was to exchange the RF coupling cap 
(a 0.01 or 0.001) for a small 5 or10kV monolithic version I had... since 
the '380-B was only rated for "medium power" I figured that couldn't hurt 
either. My last change was to replace the old SO-239s with nice, new ones. 
Had to drill out the rivets used on them, but you cannot tell the nice new 
stainless hardware wasn't original-unless you know better.

It may sound like a lot of trouble, but it isn't really. Don't just pop 
that silicon rectifier in place of the selenium. You may get away with the 
new, higher (count on it) voltage, and you may not. All this stuff was 
easily obtained, inexpensively. New power transformers are not.


73 Ed Tanton N4XY <n4xy@earthlink.net>

Ed Tanton N4XY
189 Pioneer Trail
Marietta, GA 30068-3466

website: http://www.n4xy.com

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