[Eric]

I have this 5H40 chassis that I've recapped and am ready to put it together but someone has put a diode (checks good) in parallel with the selenium rectifier(bad?) with out disconnecting it. It doesn't strike me as proper why not cut out the selenium and do a proper job? Also shouldn't R21 be increased from the current 130 ohms to account for less of a voltage drop across the diode? Also C31 .047 was blown wide open I replaced it with a safety cap.


http://www.nostalgiaair.org/PagesByModel...025549.pdf

[morzh]

You are right, diode should never be put in parallel. The selenium rect if it is good should be left in there and if it is not should be removed and replaced with a diode.
R21 will probably increase in value; you could search for info around as there are many of such topics discussed on the net where people had to do that.
If the cap was blown, it was a bad cap and this is it. I would put a fuse in series to keep it from happening again. Safety or not, a fuse is always a good idea.

[skyscraper]

Disconnect the selenium rectifier, it shouldn't be doing anything the way it is wired, but they are prone to leaking and failing in very dramatic ways.

You do need another resistor in series, what exactly that should be depends on B+ voltage and current, so a bit of trial and error will get it close enough. On my 7 tube Zenith AM/FM, a 50ohm 10 watt corrected the voltage.

[BrendaAnnD]

I've been replacing those resistors with a 200 ohm 3 watt resistor. I also remove the old selenium and use it's mounting hole to install a 3 lug solder strip to mount the diode on and have a tie point for the resistor.

[Eric]

Thanks all I'm going to remove the selenium, Brenda you helped me out on a Emerson that had a selenium rectifier and it appears some parts of my memory still work as I did remember that I have to increase the resistance on the resistor. I also like to add fuses on the sets it seems like pretty cheap insurance

[Raleigh]

If a radio with a selenium rectifier is working, I usually measure the DC current through the selenium before I replace it. Then I use a resistor that will give me the same current.

The Philco D-655 I'm working on now took an 82 ohm resistor. The resulting DC current when plugged into 123 VAC is about the same as the original when plugged into 117. Power dissipation was .3 watt, so I used a 1 watt resistor. I tried it with 200 ohms, as BrendaAnn suggested, and that worked fine too, but the current was a little low, I thought.

I've sometimes put a couple of layers of heat shrink on the silicon diode leads and then fish those leads through the two terminals of the selenium rectifier. Then I solder the leads onto loops on the ends of the silicon diode leads. That takes the selenium out of the circuit but sort of preserves the appearance. Usually I do what BrendaAnn does and use the selenium mounting hole for a terminal strip.

[Radioroslyn]

Some portables can be a little fussy about the hv because they use resistors to drop the hv and use it for the filaments. If the hv is too low you may find that the osc part of the mixer isn't working cause the filament voltage is too low.
Terry

[morzh]

In fact if the selenium is working but you want the diode instead, you can simply parallel them as a diode effetively takes the selenium out . But then if it is working why replace it, right?

[Raleigh]

Eric, one of our members, RadioDave, I think, posted a link to a Transoceanic enthusiast's site. This page, http://home.comcast.net/~morrised2006/ra...adio11.htm has a link to a pdf file about restoring TO 500 and 600 sets. It makes interesting reading and might help a bit if you're stuck on something.

Go down about 1/2 the page to a link that says "Zenith 500 & 600 Series Trans-Oceanic Restoration Guide." When you click on it it will immediately start downloading the pdf file.

[codefox1]

I can see no reason to bridge an old selenium with a modern diode. Agree the ultimate voltage divider must be adjusted so that the proper load of the filament string is correct.