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A question about Selenium rectifiers ! - Printable Version

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A question about Selenium rectifiers ! - Lee Petrie - 04-13-2012

Hi out their in Philco-land !
A man wanted to bring his Philco 49-1611 over to have me take a look at it ! When over-viewing the schematic first, before he made a 2 hour trip in one direction, it appears the radio has Selenium rectifiers , which normally go bad, or so I'm told ! How will changing them out with "equal value" diodes, from our local Radio Shack store effect the voltages in the set , I was told b+ will be higher, but how much higher ? I don't want to experiment with dropping resisters , in a set 2 hours away, for the next service round, if the set fails again ! It's not fair to anyone, maybe best leave it alone, and not repair his radio !
Thanks for your help, Lee Petrie Joilet, IL.

RE: A question about Selenium rectifiers ! - codefox1 - 04-13-2012

(04-13-2012, 11:29 AM)Lee Petrie Wrote:   Hi out their in Philco-land !
A man wanted to bring his Philco 49-1611 over to have me take a look at it ! When over-viewing the schematic first, before he made a 2 hour trip in one direction, it appears the radio has Selenium rectifiers , which normally go bad, or so I'm told ! How will changing them out with "equal value" diodes, from our local Radio Shack store effect the voltages in the set , I was told b+ will be higher, but how much higher ? I don't want to experiment with dropping resisters , in a set 2 hours away, for the next service round, if the set fails again ! It's not fair to anyone, maybe best leave it alone, and not repair his radio !
Thanks for your help, Lee Petrie Joilet, IL.

Take a look here
http://w3hwj.com/index_files/RBSelenium2.pdf

You'd need the chassis and speaker, and do a recap job. It'd be on your bench for a month unless you have a lot of experience and a fully stocked stash of parts.

Enjoy.

RE: A question about Selenium rectifiers ! - ipwizard - 04-13-2012

The above link is a good one for explaining how to replace the SR's. From my experience you will need to experiment with a dropping resistor of 5 - 10w to get the B+ close to what it needs to be. But once its done it will probably work forever.



RE: A question about Selenium rectifiers ! - Lee Petrie - 04-13-2012

Thanks to you both, for your replies ! If it was my personal radio, I would have no problem experimentimg , but this radio belonging to someone else, 2 hours away, I don't have that "warm fuzzy feeling " inside, for a first Selenium change out attempt ! I'm tring to find someone closer, hopefully with some " selenium experience" , to repair the man's radio !
Lee Petrie, Joliet, IL.

RE: A question about Selenium rectifiers ! - Arran - 04-13-2012

Is this a portable, an AC/DC, or an AC set? If its' a portable you will need to be careful about the size of the dropping resistor, if the volatge is too high it will blow the tube filaments, too low and the converor tube may stop working.
Regards
Arran

RE: A question about Selenium rectifiers ! - Arran - 04-13-2012

I found the schematic on Nolstalgia Air, it's a series string transformerless set, you can add a dropping resistor if you want but the value isn't as critical, some people add them and some do not, normally the drop across a good selenium rectifier is about 4 volts or so whereas a silicon one is .7 volts so it likely won't be that much higher. What normally happens is that the internal resistance of selenium rectifier increases with age to the point that it can't pass enough current to operate anything, it sin't common for them to short out and smoke unless they are overloaded in some way. There is already a voltage divider resistor in there I see, which looks like a candohm type, this should be checked for open sections. If the selenium rectifiers are defective I would replace them with diodes and try it out, if the voltage is considerably higher you can add a resistor ahead of the voltage divider. Another way to replicate the drop across a selenium rectifier is to connect two or three diodes in series, basically this is what a selenium rectifier amounts to is a stack of diodes in series, but made out of dry metal plates instead of a silicon crystal.
Regards
Arran