Selenium Rectifier Failure Modes?

[Todd, KA1KAQ]

I've got a piece of gear from the late 50s/early 60s here that is exhibiting some odd (to me) symptoms with its rectifiers. They are selenium, 2 stacks of 3 rectifiers. The piece of equipment is in near-new condition, which means nothing for old components, I know.

When I plug it in, it works fine. No hum in the speaker, no smoke, filter cap can is cool to the touch. It works fine for maybe ten minutes or so. Then it starts to smell like a resistor or something is burning up. Pulled the cabinet off and traced the smell to the selenium stacks. They look fine, of course, as does everything around them.

I know the cure is to replace them with 4 diodes, and the manual even includes this on the errata page. But I'm curious. The few bad selenium rectifiers I've dealt with either shorted or opened up, usually the former with resulting stinkfest. This one doesn't appear to be shorting out as it takes 10+ minutes to display any symptoms. Or is it? Maybe one of the six units is shorting after it warms up a bit?

Just strikes me as kinda weird. Then again - selenium stacks are newer technology than I'm usually dealing with in the rectifier department. Xenon, MV, 5Z3s etc are what I'm most familiar with.

Anyone else come across this gradual breakdown before?

[Tom WA3KLR]

Nasty things, get rid of them.

From Wikipedia -

"Selenium rectifiers had a shorter lifespan than desired. During catastrophic failure they produced significant quantities of malodorous and highly toxic fumes that let the repair technician know what the problem was. By far the most common failure mode was a progressive increase in forward resistance, increasing forward voltage drop and reducing the rectifier's efficiency. During the 1960s they began to be superseded by silicon rectifiers which exhibited lower forward voltage drop, lower cost, and higher reliability.[1] They are still manufactured for exact replacement purposes but are not designed into new equipment except for some automotive battery chargers.[2]"

[Tom WA3KLR]

From about.com Chemistry -

"Selenium exists in several forms, but is usually prepared with an amorphous or crystalline structure. Amorphous selenium is either red (powder form) or black (vitreous form). Crystalline monoclinic selenium is deep red; crystalline hexagonal selenium, the most stable variety, is gray with a metallic luster. Elemental selenium is fairly nontoxic and is considered an essential trace element for proper nutrition. However, hydrogen selenide (H2Se) and other selenium compounds are extremely toxic, resembling arsenic in their physiological reactions."

[w3jn]

Sometimes they start exhibiting increased resistance as they open up, that's probably what's happening here Todd.

I agree with Tom.  Get rid of 'em before they gas ya.

[WQ9E]

I have been fortunate never to have one go up in stinky smoke but many that demonstrated the failure mode Todd noticed, a slow increase in resistance as they warm up. 

The only selenium units that rarely cause problems are those used for very low voltage but all the rest are definitely best replaced.  A friend left a tube type Zenith TO playing while he went outside to walk the dog and when he came back the house was filled with eau de selenium perfume; unfortunately his wife was hosting a bridal shower later that afternoon and there were reports of severe indoor thunderstorms that day

[KM1H]

I experienced my first selenium stinkfest in the families phonograph in the 50's and have been on a mission to remove them from existance ever since.

The only thing that smells worse is the NYC subway at rush hour and Woodstock in 69

[WA1GFZ]

Stinkonium rectifiers are useless. You may get lucky and they open up. Then just solder I diode in parallel with them if you really need to keep it looking stock. My parents bought me a cool electronic kit with a 6J5 and 2 6SJ7s.
There were a number of projects. I blew the rectifier when a resistor lead was too close to it. I remember my Dad getting ripped off by a TV repair shop for a new one.

[WU2D]

"produced significant quantities of malodorous and highly toxic fumes" Frank's Pickled rectum friers?

[W4AAB]

When I was in Radio-TV Repair in Tech College, we had a BIG overhead fan in the top of the building to pull the fumes out when one of the selenium rectifiers went bad. Seems like the focus rectifier was also made of selenium.Poison gas, you bet!!

[Todd, KA1KAQ]

TNX guys. I knew these rectifiers were/are bad news and had already planned to replace it. Really. The fact that the manual has an errata sheet included explaining which diodes to use is a big clue, too. I was mainly curious to understand the mechanism at work since all the failures I've dealt with were very evident.

I think Johnny and Rodger came up with what's going on. I've had the radio operating for half an hour or more with nothing more than the slight odor being emitted. No need to risk a meltdown, though.

And while it won't change the outcome (selenium getting replaced by silicon), it at least satisfies my need to know what and why. Hopefully it can help someone else who might experience the same type of symptom. In my case, the selenium stacks are hidden in a small enclosure and surrounded by a stiff, gray boxboard type material. Doesn't appear to be asbestos, more likely a cover to keep crap out of the rectifier fins or maybe a protective insulator.

[Steve - K4HX]

But if you change out those rectifiers, the radio won't be original any longer!   

[WD5JKO]

  They can get worse, I think

 As a kid I fished out an old battery charger from a local swamp while looking for Daphnia to feed to my tropical fish. It was a complete battery charger for 6v car batteries with you guessed, selenium rectifiers. I took it home, and hosed it off. After that it sat in the garage for a few years awaiting that first use. My brother bought a boat, and old Chris Craft with a 6v electrical system. He needed to charge that 6V lead acid battery, so I quickly pulled out my battery charger. It had meters on it too, and they were sealed from the swamp water.

  So to make a long story short, we got what as I recall was green smoke combined with swamp smell all in the confines of a small enclosed garage.
Jim
WD5JKO

[flintstone mop]

Is there a voltage drop with Seleniums?? Similar to a tube rectifier??

I thought I read that it was more voltage drop than tube.
Fred

[WD5JKO]

Is there a voltage drop with Seleniums?? Similar to a tube rectifier??

I thought I read that it was more voltage drop than tube.
Fred

Fred, You might have seen Selenium rectifiers in the old style VOM. They have almost no threshold voltage, and are very good in an AC voltmeter to do the rectification. Now when you pass current through them, they are much like a one way resistor where the resistance is determined by size of the rectifier. They could be scaled to pass hundreds of amps.

Horrible devices for most applications today.

Jim
WD5JKO

[N0WEK]

I've got a new in the box big one here for a paper weight. It's about 10 inches square. I have no idea of the ratings.

Pretty though!

[Todd, KA1KAQ]

They do look neat, very industrial or Sci-Fi in appearance. Easy enough to add some diodes while leaving the original in place physically.

But if you change out those rectifiers, the radio won't be original any longer!   

True, but it will actually work! Whaddaya want me to do, send it out for powder coating, add a pilot light, and claim an expert restoration?   

[KM1H]

Is there a voltage drop with Seleniums?? Similar to a tube rectifier??

I thought I read that it was more voltage drop than tube.
Fred

About 1.5 to 2.5V per cell in a typical radio B+. Less than a tube but more than SS. As they age the drop increases.

[WB2EMS]

When I was in Radio-TV Repair in Tech College, we had a BIG overhead fan in the top of the building to pull the fumes out when one of the selenium rectifiers went bad. Seems like the focus rectifier was also made of selenium.Poison gas, you bet!!

When I worked at Harris RF Communications in the 70's, one of my fellow engineers, who had been there for a while told of being on board a submarine testing a new, very powerful, SONAR system that had a serious number of selenium rectifiers as part of it installed in the forward torpedo room. Something went very wrong (final tube arc over I think he said) and the selenium rectifiers started failing and outgassing en masse. The doors to the torpedo room were sealed to keep from contaminating the rest of the vessel, with him inside, and frantic calls were made to the control room to surface ASAP! He said it was a long couple of minutes but they went up at a steep deck angle and with lots of air blowing and arrived on the surface with a big splash, then immediately opened up all the hatches and ventilation systems for the area. Said it was fairly exciting.

Probably arrived somewhat like this from how he described it.
http://www.youtube.com/watch?v=ZMQDWDxHeXU

He never liked selenium rectifiers after that. Or submarines.

[aafradio]

They do look neat, very industrial or Sci-Fi in appearance. Easy enough to add some diodes while leaving the original in place physically.

But if you change out those rectifiers, the radio won't be original any longer!   

True, but it will actually work! Whaddaya want me to do, send it out for powder coating, add a pilot light, and claim an expert restoration?   

No, you should dismantle each cell and bore out the center.  Then you simply reassemble with solid state diodes in the bore.  No one will ever be able to detect the change....

[Todd, KA1KAQ]

Ha! If only!

Or, maybe if I had your machine shop and skills, sir. 

[W7SOE]

Selenium Rectifiers in the movies:

http://www.antiqueradio.org/tvshow.htm

Rich

[k4kyv]

Interesting article in the Feb 9th issue of Radioworld (the broadcast rag) about old technology rectifiers for broadcast transmitters. They mention mercury arc rectifiers, high vacuum and MV rectifiers, that copper oxide rectifiers were once used for voltages less than 3 KV, but with mixed success, and that seleniums were never known to have been used for the HV plate supplies for broadcast transmitters.

Of course, hams once used "slop jar" rectifiers, a borax solution electrolyte and metal electrodes, zinc and/or aluminium I believe, contained in glass jars.

The mercury arc rectifiers were said to each contain more than a pound of mercury, and they had been known to occasionally explode during operation, and naturally they sometimes got dropped and broken. I saw one at a flea market space last year at Dayton.  The old timer vendor who brought it just to show off as a curiosity was surprised and delighted  that I knew what it was.

According to  the Radioworld article, station KOB in LasCruces, NM used a series string of motor-generators for the plate supply for their 10 KW transmitter, with the frames isolated and mounted on insulators above ground.  The engineer kept a wooden yardstick handy, and routinely he periodically whacked the generator brushes to make sure they were seated properly.  One day the humidity was unusually high in normally arid NM, enough to make the concrete floor and wooden yardstick become damp. He tapped the brushes with his yard stick, and received an instantly fatal jolt from the 20 KV plate supply through his damp yard stick, and down through his shoes to the wet concrete.