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 Help, please, with a selenium rectifier replacement question

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thetexan



Posts: 4
Join date: 2010-10-17

PostSubject: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 1:44 am

I am restoring a kg650 Knight rf generator. It has an old selenium rectifier. I have two diode rectifiers on hand and they are.....

a.....Rectifier; 3A IF; 1.2 V (Max.); 200V; 200A Ifms; 500 uA (Max.) IR; Axial Leaded

and a

Rectifier; 3 A (Max.) @ 105C IFAV@Tcase; 50 to 1000V VRSM; 200A IFRM.

please look at the image at

http://www.davidskidmore.com/kg650rfrectifier.jpg to see the schematic and the diodes.

Can I use either one of these to replace the selenium rectifier? In the parts list of the kg650 the part is listed as a selenium rectifier, 50MA, part number 620007.

If not can you please suggest any current component that will replace it.

thank you,

tex
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simplex1040



Posts: 40
Join date: 2010-10-12

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 2:03 am

Use the 1000 volt one and you will need a voltage dropping resistor of prob around 150 ohms at 2 watts may not need that high wattage.
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easyrider8



Posts: 12
Join date: 2010-10-18

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 2:11 am

If your voltages are good after replacing the electrolytics and the paper capacitors leave the selenium in there.

Dave
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classicaudiorepair



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Join date: 2010-10-20
Location: 3401-F Adams Ave. San Diego CA 92116

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 6:29 am

From time to time a selenium rectifier will break down and spew toxic selenium. Thus, not only will the circuit fail - you will have a toxic chemical cleanup task.

Replace the rectifier. A 1-amp, 800 - 1000 PIV diode will work fine. Note the voltage on the schematic.

Now, experiment with various values of series dropping resistors (150 ohm 2 watt has been suggested, so use this as a starting value) in order to bring the supply voltage down to the value given in the schematic.

Fred
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thetexan



Posts: 4
Join date: 2010-10-17

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 1:34 pm

Let me draw you attention to the schematic again....

http://www.davidskidmore.com/kg650rfrectifier.jpg

Voltages arent labeled but you can deduce some of them such as filament and plate voltages for the tubes as listed in their detail sheets.

As for a dropping resister...isnt that the function of R1, the 2.7K resister in line with the main supply line, and in series with the rectifier? Apparently 2.7K was sufficient for the selenium rectifier (which doesnt itself drop any voltage) so wouldnt R1 serve the same purpose with a replacement diode?

With a 120vac voltage at the rectifier (I measured it) and 2.7K that gives you 45MA at the rectifier which matches the rectifier rating of 50MA. So is the added resister necessary?

tex
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Lenoj



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Join date: 2010-10-20

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 1:44 pm

actually selenium rectifiers do drop voltage, about a volt per section (fin) and needed several sections in series to handle much voltage as the Peak reverse voltage per section was only about 30 volts or so. Those rated for 120 volts were 5 or 6 sections and dropped about 7 volts.
Silicone diodes drop only about 0.6 volts. Thus the need for the added series resistor to bring the voltages back down to where they were with the less efficient selenium rectifier. You could approximate the required resistance by dividing 7 volts by the current through the resistor and then proceed by subbing various resistors or just sub resistors.... :-)
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easyrider8



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PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 2:28 pm

classicaudiorepair wrote:
From time to time a selenium rectifier will break down and spew toxic selenium. Thus, not only will the circuit fail - you will have a toxic chemical cleanup task.

Fred

Looks like someone has been reading fairy tales. I suggest you do some research and see what they actually spew, it is not selenium. Can you find one instance where there was a toxic chemical cleanup necessary from a failed selenium?

Dave
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philsoldradios



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Join date: 2010-10-19

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 3:09 pm

They fail, spewing or not. There's a reason why they became obsolete decades ago.

Who enjoys fixing a radio twice? The new diode costs less than a buck. Install it and forget about it.

Phil
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Rich, W3HWJ



Posts: 10
Join date: 2010-10-17

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 4:30 pm

I don't wish to debate, but offer this to the Texan if he wants to do a selenium replacement.

http://www.w3hwj.com/index_files/RBSelenium2.pdf

I did do quite a bit of research for this article and had it reviewed by two EE power supply engineers.

Rich


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dschulma



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PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 5:15 pm

Great article Rich, thanks for the link. Wish I'd have had it about one month ago!
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dberman51



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PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 8:20 pm

Good article -- I would have had only one quibble: Selenium rectifiers also required series resistors to limit inrush current, if they were directly connected to the line. 100-120 ohm was the minimum recommended. If transformer-powered, no series resistor was necessary as the transformer inductance would limit inrush.

Personally I don't sweat the seven volts. Selenium out, silicon in.

-David
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Rich, W3HWJ



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PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 8:27 pm

David:

Good comments. You are correct about the series resistor.

I usually don't worry about the voltage drop difference, unless something in the rest of the circuit is voltage sensitive. You will encounter this in the Zenith Trans-Oceanics, where the filament voltage is derived from the B+ line.

Rich

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classicaudiorepair



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Location: 3401-F Adams Ave. San Diego CA 92116

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 8:37 pm

Easy Rider,

Please don't attack my comments when you yourself haven't done YOUR homework.

http://yarchive.net/electr/selenium_rectifiers.html

Also, this is from Wiki --

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis.[22] This 400 microgram (µg) Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992.[23] The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries.[24] The Chinese people who suffered from selenium toxicity ingested selenium by eating corn grown in extremely selenium-rich stony coal (carbonaceous shale). This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature.[25] A dose of selenium as small as 5 milligram (5000 µg) per day can be lethal for many humans.[26]

Fred Longworth, MBA, CHMM

By the way, CHMM stands for "Certified Hazardous Materials Manager"
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smeezekitty



Posts: 49
Join date: 2010-10-13
Location: Washington/USA

PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 8:47 pm

classicaudiorepair wrote:
Easy Rider,

Please don't attack my comments when you yourself haven't done YOUR homework.

http://yarchive.net/electr/selenium_rectifiers.html

Also, this is from Wiki --

Although selenium is an essential trace element, it is toxic if taken in excess. Exceeding the Tolerable Upper Intake Level of 400 micrograms per day can lead to selenosis.[22] This 400 microgram (µg) Tolerable Upper Intake Level is based primarily on a 1986 study of five Chinese patients who exhibited overt signs of selenosis and a follow up study on the same five people in 1992.[23] The 1992 study actually found the maximum safe dietary Se intake to be approximately 800 micrograms per day (15 micrograms per kilogram body weight), but suggested 400 micrograms per day to not only avoid toxicity, but also to avoid creating an imbalance of nutrients in the diet and to account for data from other countries.[24] The Chinese people who suffered from selenium toxicity ingested selenium by eating corn grown in extremely selenium-rich stony coal (carbonaceous shale). This coal was shown to have selenium content as high as 9.1%, the highest concentration in coal ever recorded in literature.[25] A dose of selenium as small as 5 milligram (5000 µg) per day can be lethal for many humans.[26]

Fred Longworth, MBA, CHMM

By the way, CHMM stands for "Certified Hazardous Materials Manager"
And its imposable to get that big of dose from rectifier failure.
I have to agree to leave it unless theres a reason to change it.
The more original parts the better (except capacitors of course).
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classicaudiorepair



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PostSubject: Re: Help, please, with a selenium rectifier replacement question   Fri Oct 22, 2010 8:54 pm

Regarding dropping resistors for voltage trimming when silicon diodes are used to replace selenium rectifiers:

As a repair expert, I always try to duplicate the original operating voltages as close as possible. This tends to make the set more able to work "to spec" than if the voltages vary up or down. After all, the engineers who designed the gear were highly skilled professionals, not weekend hobbiests.

For example, in high-fidelity Fisher tube receivers (e.g. Fisher 500) a selenium rectifier is used for the negative bias supply and to generate a DC current to run the filaments for the low-level preamp 12AX7's. A small difference in the voltage generated by this power supply can produce significant differences in how "hot" the preamp tubes are running and how the output tubes are biased. To a good ear, these differences are audible!

Thus, in the above-cited application, the voltages and currents generated in the negative bias supply are much more critical than in some simpler, lower-fidelity applications, such as garden-variety table radios. I would NEVER just slap silicon rectifiers in the bias supply for an audiophile client.

Fred Longworth

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