Zenith H724 Selenium Rectifier

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Zenith H724 Selenium Rectifier

Post by MrBill on Thu Jun 05, 2014 1:39 pm

Ok since I have things sorted out on the 12H090 and the speaker is off for reconing I pulled out this Zenith H724 chassis 7H02Z found that R21 was open which is in series with all the tube heaters. I replaced all out of range resisitors and all capacitors. The set plays very well. It does have a selenium rectifier which I have read horror stories about. Should I replace it with a rectifier diode? I have a IN5404 which is rated for 400v and 3 amps. The highest DC voltage in this set is 140v. What do you guys suggest?

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Re: Zenith H724 Selenium Rectifier

Post by Resistance is Futile on Thu Jun 05, 2014 4:34 pm

Nope not as long as it works. Save your time (Remember--if it isn't, broke don't fix it--)
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Re: Zenith H724 Selenium Rectifier

Post by Bill Cahill on Thu Jun 05, 2014 8:28 pm

That isn't enough protection. You need a 4007, which is rated at two amps, one thousand inverse peak volts. It's not the dc voltage as much as the ac spikes to worry about. They can't be read with a meter, but, they are there.

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Re: Zenith H724 Selenium Rectifier

Post by Dr. Radio on Thu Jun 05, 2014 8:37 pm

Whether to replace or not is always a debate. The Zenith radios that use these seem to be quite reliable. Any TV that has a selenium should be replaced ASAP.

If you feel better about it (I wouldn't blame you) replace with a diode AND a 5 watt resistor of the appropriate value that gives the correct voltage drop to mimic the original equipment based on the schematic.


As an aside, I hope you have performed voltage checks to see if the circuitry is functioning within its parameters based on schematic data---resistors don't just go open, there may be current issues down stream.
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Re: Zenith H724 Selenium Rectifier

Post by MrBill on Thu Jun 05, 2014 10:55 pm

I checked all voltages according to the schematic and found everything correct right after I warmed it up and had it working. (you guys are training me well) What I did find was a shorted out arc place on the speaker frame which I believe was the cause as the speaker is very close to the selenium rectifer and someone was clumsy with a screwdriver.

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Re: Zenith H724 Selenium Rectifier

Post by Wildcat445 on Thu Jun 05, 2014 11:13 pm

I agree with leaving it alone, if B+ is within specs. I have several Zenith radios of that vintage still running their selenium rectifiers. Just to eliminate any possibility of even a partial short in B+, you could measure the resistance from the cathode of the rectifier to B-, probably one side of the power switch. As long as you see several hundred thousand ohms, you can be reasonably safe in assuming there is no substantial short dropping B+. Good luck.

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