I have a Zenith H724 AM/FM radio with a selenium rectifier in the power supply. I needed to know what type of diode I should replace it with, mainly for safety issues.
When one of these rectifiers goes out, is the smell a sort of "sweet" odor as opposed to a burning rubber smell? I was testing the radio out and after about 5 minutes of running fine, I heard a muted pop and saw a little bit of smoke and that's when I noticed the smell. I was on my way to work, so I didn't have time to check it in detail.

Hi Brian

Glad you think it's a sweet odor. I consider it more like rotten eggs.

You can replace a selenium rectifier with 1N4004 - 1N4007 silicon diode. These are rated 400 volts PIV to 1000 volts PIV @ 1 amp. Only cost a few cents.

Add a 47 ohm resistor in series with the silicon diode. It will drop a little voltage and act as a fuse it something shorts. Be sure to get polarity correct. The band end of a silicon diode connects to the same place as + on a selenium rectifier.

Norm

:I have a Zenith H724 AM/FM radio with a selenium rectifier in the power supply. I needed to know what type of diode I should replace it with, mainly for safety issues.
:When one of these rectifiers goes out, is the smell a sort of "sweet" odor as opposed to a burning rubber smell? I was testing the radio out and after about 5 minutes of running fine, I heard a muted pop and saw a little bit of smoke and that's when I noticed the smell. I was on my way to work, so I didn't have time to check it in detail.

If it's supposed to be a rotten egg smell then it wasn't the rectifier that blew. Maybe it was a resistor or cap that blew. In the couple of seconds that it took me to turn it off and unplug it, the radio still played, but with a lot of distortion and static. It was almost crystal clear before. I'll find out more when I get home. Does it matter which end of the diode I connect the resistor to? Do I need to use a certain wattage? I think I have some 47ohm .5 watt resistors in my parts cabinet

:Hi Brian
:
: Glad you think it's a sweet odor. I consider it more like rotten eggs.
:
: You can replace a selenium rectifier with 1N4004 - 1N4007 silicon diode. These are rated 400 volts PIV to 1000 volts PIV @ 1 amp. Only cost a few cents.
:
: Add a 47 ohm resistor in series with the silicon diode. It will drop a little voltage and act as a fuse it something shorts. Be sure to get polarity correct. The band end of a silicon diode connects to the same place as + on a selenium rectifier.
:
:Norm
:
::I have a Zenith H724 AM/FM radio with a selenium rectifier in the power supply. I needed to know what type of diode I should replace it with, mainly for safety issues.
::When one of these rectifiers goes out, is the smell a sort of "sweet" odor as opposed to a burning rubber smell? I was testing the radio out and after about 5 minutes of running fine, I heard a muted pop and saw a little bit of smoke and that's when I noticed the smell. I was on my way to work, so I didn't have time to check it in detail.

Having worked in a TV repair shop in the 60s, I am rather familiar with the smell of a smoked selenium stack. I wouldn't characterize it as rotten eggs... more like badly burned garlic with just a little rotten egg.
No matter how you describe it... it just ain't pleasant.
It's also toxic if you get a lungful of it.
Rich
**********************************************

:If it's supposed to be a rotten egg smell then it wasn't the rectifier that blew. Maybe it was a resistor or cap that blew. In the couple of seconds that it took me to turn it off and unplug it, the radio still played, but with a lot of distortion and static. It was almost crystal clear before. I'll find out more when I get home. Does it matter which end of the diode I connect the resistor to? Do I need to use a certain wattage? I think I have some 47ohm .5 watt resistors in my parts cabinet
:
::Hi Brian
::
:: Glad you think it's a sweet odor. I consider it more like rotten eggs.
::
:: You can replace a selenium rectifier with 1N4004 - 1N4007 silicon diode. These are rated 400 volts PIV to 1000 volts PIV @ 1 amp. Only cost a few cents.
::
:: Add a 47 ohm resistor in series with the silicon diode. It will drop a little voltage and act as a fuse it something shorts. Be sure to get polarity correct. The band end of a silicon diode connects to the same place as + on a selenium rectifier.
::
::Norm
::
:::I have a Zenith H724 AM/FM radio with a selenium rectifier in the power supply. I needed to know what type of diode I should replace it with, mainly for safety issues.
:::When one of these rectifiers goes out, is the smell a sort of "sweet" odor as opposed to a burning rubber smell? I was testing the radio out and after about 5 minutes of running fine, I heard a muted pop and saw a little bit of smoke and that's when I noticed the smell. I was on my way to work, so I didn't have time to check it in detail.

I've had seleniums blow before. Still alive. Sweet smells are probably electrolytics. If you haven't replaced any of the originals, one may have blown its seal from reforming. If the radio sat for a long time, the electrolytics may have unformed. Running the radio reforms them, but they get hot when this happens. Old seals let go. If the radio still works perfectly, this is probably what happened. Did you hear a rushing sound? The electrolytic will soon go dry, and the set will hum because of this.

If the selenium didn't blow, and gives you adequate voltage, there is no need to replace it. No safety issue involved. I've seen plenty of Lesley (or Leslie....however you spell it) amplifiers with shorted silicon diodes (thank goodness for fuses on the amplifiers). You have danger no matter which way you go.

Thomas

After reading this plus another post, I think I will leave the rectifier alone. The problem was C24, between either side of the main line and the negative side of the rectifier to ground. It was a black beauty cap (not original) that had exploded. The smoke I saw was probably the wax or paper inside that was burning. The paper did seem a bit charred. I replaced it and the radio works beautifully. I eventually may replace the other paper caps, but I'm reluctant to do so since it works as well as it does.

:I've had seleniums blow before. Still alive. Sweet smells are probably electrolytics. If you haven't replaced any of the originals, one may have blown its seal from reforming. If the radio sat for a long time, the electrolytics may have unformed. Running the radio reforms them, but they get hot when this happens. Old seals let go. If the radio still works perfectly, this is probably what happened. Did you hear a rushing sound? The electrolytic will soon go dry, and the set will hum because of this.
:
:If the selenium didn't blow, and gives you adequate voltage, there is no need to replace it. No safety issue involved. I've seen plenty of Lesley (or Leslie....however you spell it) amplifiers with shorted silicon diodes (thank goodness for fuses on the amplifiers). You have danger no matter which way you go.
:
:Thomas

You're probably almost better off replacing the condensers than the rectifier (which may, in turn, save the rectifier from future distruction). Condensers with even the slightest amount of leakage (save electrolytics), can mess with the sensitive audio and radio circuits (though the following will not destroy the rectifier...only a tone condenser short can do this). Leakage in the millions of ohms will not allow your AVC circuit to function properly, leading to distortion on strong signals. The same leakage in the audio circuit will cause distortion in the audio. If your radio distorts after about 15 or 20 minutes, you have leaky audio condensers. A short in the output tone condenser will overload the power supply and the output transformer.

If your radio performs well, though, perhaps you can leave it alone. I have a couple of pieces of equipment with perfectly functioning original components. My three Zenith portables still have their original electrolytics, which function normally (though many of the paper condensers have been replaced). Low voltage AC-DC circuits are less likely to short out electrolytics, so I don't think you have this to worry about. I've only found shorted electrolytics in AC radios (and they may have simply been 'lytics that needed reforming...I don't know...just replaced them).

At any rate, if you find any of the above problems in your radio, then you know which condensers to replace. You certainly don't have to replace anything at this point, if the radio performs well, but look for the above symptoms, which may not be present when the set is first turned on.

Before using the set on a regular basis, though, make sure that the rectifier does not have any reverse leakage (remove all wires from one side to run this test with your multi-meter...if you have a digital meter with a diode test, use this setting....otherwise use a regular meter with the higher resistance settings). Any reverse leakage will cause excessive hum, and will slowly destroy your electrolytics.

T.

I'll try that test on the rectifier that you suggested to make sure it's ok, although when I have it on, I can not hear any hum other than the very slight hum I would expect on an AC/DC radio. It's been serviced at least once in the past (an old repair sticker on the back) but an inspection probably wouldn't hurt.
Nearly all of my radios have needed at least new electrolytics, but this Zenith is a bit of an oddball in my collection being one of the few that had working electrolytics.

:You're probably almost better off replacing the condensers than the rectifier (which may, in turn, save the rectifier from future distruction). Condensers with even the slightest amount of leakage (save electrolytics), can mess with the sensitive audio and radio circuits (though the following will not destroy the rectifier...only a tone condenser short can do this). Leakage in the millions of ohms will not allow your AVC circuit to function properly, leading to distortion on strong signals. The same leakage in the audio circuit will cause distortion in the audio. If your radio distorts after about 15 or 20 minutes, you have leaky audio condensers. A short in the output tone condenser will overload the power supply and the output transformer.
:
:If your radio performs well, though, perhaps you can leave it alone. I have a couple of pieces of equipment with perfectly functioning original components. My three Zenith portables still have their original electrolytics, which function normally (though many of the paper condensers have been replaced). Low voltage AC-DC circuits are less likely to short out electrolytics, so I don't think you have this to worry about. I've only found shorted electrolytics in AC radios (and they may have simply been 'lytics that needed reforming...I don't know...just replaced them).
:
:At any rate, if you find any of the above problems in your radio, then you know which condensers to replace. You certainly don't have to replace anything at this point, if the radio performs well, but look for the above symptoms, which may not be present when the set is first turned on.
:
:Before using the set on a regular basis, though, make sure that the rectifier does not have any reverse leakage (remove all wires from one side to run this test with your multi-meter...if you have a digital meter with a diode test, use this setting....otherwise use a regular meter with the higher resistance settings). Any reverse leakage will cause excessive hum, and will slowly destroy your electrolytics.
:
:T.

Zeniths are interesting in that way. I guess part of it has to do with where the radio was stored, how often it was used, etcetera. Actually, now that I recall, I have 5 Zeniths, all with good electrolytics. Except for one, all are portables. The other is a Cobramatic....hmmm, make that 6....I had the chassis from a 1951 Cobramatic, and the 'lytics were good in it, too. One of the portables has a selenium. However, I have seen some portables with bad electrolytics, and I've repaired a Zenith consol from 1940, that had bad 'lytics.

I guess that using a radio a lot keeps the electrolytics fresh. However, the heat may distroy them, too. I suppose that a good seal is the key to a good electrolytic, so that it may endure all situations.

Zeniths are not the exception, though I seem to have run across a lot with good electrolytics. I own one RCA 66X13, or something like that (forgot exact number), with its original 'lytics. I own another one in which they went bad. I own an RCA 55X, which had bad 'lytics. I have a Silvertone Radio Recorder from 1940 (forgot #). Its 'lytics are fair, though they needed some help (added others).....no leakage, though.

T.