Title says it all.

I've got a portable with selenium rectifiers that starts out good, and then after ten minutes, the set starts losing power, signal, deflection, etc.

I know they should be replaced with some 4007's, but I want to make sure I got this power loss problem fixed first before popping in new diodes.

Seleniums get weak (increased forward voltage drop) with age. Does your symptom seem to be thermal in nature? If you use a fan or cooling spray to keep the seleniums cool, does the set work OK?

Seleniums were often used in a voltage doubler arrangement, and the 2 capacitors in the doubler can also cause problems if they get leaky or lose capacity as they warm up.

Replacing both the rectifiers and the doubler caps is the real fix.

Selenium rectifiers are a problem waiting to happen. They deteriorate with time, generally, in all aspects. The symptoms are reduced output, increased ripple, and greater heat loss as well as that characteristic odor.

Not to say that retrofitting with silicon will solve your problems, but it does eliminate one possible source.

The set has all new capacitors, so that's ruled out. Not sure if issue is thermal... since it only takes ten minutes or less to start having problems.

I will be putting in new diodes anyway... I was just curious if these things got weak before crapping out. Thanks for your input! :yes:

Uh-oh, better not fire up anything else before replacing seleniums, you could have the smell of another rat's nest to deal with.

Man, that one will never get old, at least not to me. :D

Seleniums also develop reverse leakage. This adds to the heating already caused by excessive forward drop.

Selenium rectifiers are a problem waiting to happen. They deteriorate with time, generally, in all aspects. The symptoms are reduced output, increased ripple, and greater heat loss as well as that characteristic odor.

Not to say that retrofitting with silicon will solve your problems, but it does eliminate one possible source.
That's why it always amazed me that the first color sets came out with selenium rectifiers in a voltage doubler configuration.
All except Admiral and Philco. Then a few of the others went to back to 5U4"s.
Then everyone but RCA, quit building color sets.:D

I've heard that the mechanical connection between selenium rectifier plates through washers etc. can oxidize over time and reduce their output. Be interesting to hook a DVM to the B+ and watch the voltage from cold startup.

If it's true about corroded contacts, perhaps tightening the center bolt or even disassembling the unit and putting it back together might improve operation.

If it's true about corroded contacts, perhaps tightening the center bolt or even disassembling the unit and putting it back together might improve operation.

Easier and safer to upgrade to a 1N4007 at that point....

Of course that's true, but the OP seems reluctant to modernize just yet.

...perhaps tightening the center bolt or even disassembling the unit and putting it back together might improve operation.

Nah man... I'm not going that route. That would be like playing with fire... not worth it. The set is getting new diodes... and probably toss in a CL-90 as well for good measure. I just wanted to know if these old rectifiers actually got weak.

We should have a cross reference from seleninum to silicon
Rectifiers with a new layout per TV set with new schematic
Showing changeover to a new setup

You'll know when a selenium goes bad. Rotten eggs, big time! :yuck: