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 Post subject: Z.T.O. memory jog please
PostPosted: Feb Fri 23, 2018 7:19 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11474
Location: Valley City ND USA
I need to replace the selenium with a diode. Can't remember how much to increase resistance,
or what wattage resistor will suffice. I wrote it all down....Someplace?


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 Post subject: Re: Z.T.O. memory jog please
PostPosted: Feb Fri 23, 2018 9:09 am 
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Location: Oxford, MI
It would depend on your line voltage. Around here my line voltage runs just a hair under 120V, so about 119 on average when the grid is not stressed.

I will typically replace the selenium with a 1N4007 diode and then you have two methods of adding resistance. The common denominator is that I like to have 200 ohms after the diode but before the candohm resistor. Either you can add a 70 ohm or so resistor in series with the existing 130 ohm sandohm or replace the sandohm with a single 200 ohm resistor.

The best method of course would be to have an assortment of resistors and start a bit high. Then measuring the filament string, determine what resistor given your line voltage gives 8.4-9.0V on the filament string.

Power dissipation will be just over one watt, so a 2W metal oxide (minimum) or better yet a 5W wirewound resistor should be used.

Also, the original 3W 130 ohm sandohm was designed to act as a fuse if something failed and opened up, if you replace it with something larger in wattage, adding an inline fuse would be a good idea.

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 Post subject: Re: Z.T.O. memory jog please
PostPosted: Feb Sat 24, 2018 12:39 am 
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Location: Colorado Springs, CO
I use a 100 ohm, 25 watt variable resistor in series with a fixed resistor, of a value that makes sense, to dial in the voltage needed, measure what I have and use that value to go with.

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 Post subject: Re: Z.T.O. memory jog please
PostPosted: Feb Sat 24, 2018 1:01 am 
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Joined: Jun Fri 19, 2009 6:34 pm
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Location: Long Island
It really depends on what the application is, and what kind of selenium rectifier you are replacing. In some cases it is necessary, but in many other cases you don't need to add any resistance at all. Forward drop in metallic rectifiers is not a fixed number like it is for silicon diodes; in seleniums of average radio size it could be as low as a few millivolts or as much as 8-10 volts. The drop depends on how much current is flowing, temperature, and the age of the rectifier.

You mentioned ZTO. If that's shorthand for Zenith Trans Oceanic, then you do have to pay attention to filament voltage, especially if it is one of the models that uses an expensive 1L6 tube. Hcompton79 gives all the details in his post.

For other kinds of radios, test instruments, and devices with selenium rectifiers, I look at the B+ voltage with the new diode installed but no additional resistors added. If it is within normal tolerances with proper line voltage is applied, then I leave well enough alone. Only if the voltage becomes excessive do I add a series resistor to knock it down.

Note that in most cases you will find a small resistor (eg. 22 ohms) in series with the old selenium rectifier. Its purpose was to limit the maximum surge current. Selenium rectifiers were easily damaged by surges. It's a good idea to leave that resistor in place when you install a diode, or install one if there's none present. It will also protect the new silicon diode from surges which might eventually cause it to short out.

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 Post subject: Re: Z.T.O. memory jog please
PostPosted: Feb Sat 24, 2018 7:45 am 
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Joined: Jan Thu 01, 1970 1:00 am
Posts: 11474
Location: Valley City ND USA
Thanks guys. It's a G500 with 1L6. My voltage here at home is plenty high. Customer lives in town. Different power source. I'll experiment and adjust. I posted looking for a starting point. I'm good to go now!


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 Post subject: Re: Z.T.O. memory jog please
PostPosted: Feb Sun 25, 2018 5:55 am 
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Joined: Jan Sun 06, 2008 3:28 am
Posts: 4055
Location: Richmond, VA
Download this H500 Restoration Guide. Chapter 4 has illustrated, step by step instructions on how to determine the right resistor value. The G500 is essentially the same radio. The file is 48mb, so it takes a couple minutes to download.

http://143.95.77.243/~elmphoto/pdf/H500%20Restoration%20Guide.pdf

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