Okay, so here's where I'm at with my selenium dilemma....

As posted in another thread, I'm replacing .320a selenium rectifiers, the rating chart is here:

http://www.videokarma.org/attachment...0&d=1380216772

These are the diodes I intend to use, 3 amp 1000 v. Since diodes are cheap, might as well step up the rating a bit, just for good measure:

http://www.mouser.com/ProductDetail/...fp8QPCBfkcQ%3d

Since this set isn't working, I'd prefer to just get a resistor value and replace it. It's been recommended to me that I get a 25 watt resistor as opposed to a 2 watt, because of how hot they run. Does that sound right?

And what value resistor would I run?

A selenium rectifier "Let Go" in an otherwise PRISTINE Scott 800-B...OMG...Stunk to High Heaven ! Laid a greasy pall of smoke over the chrome I'd so meticulously cleaned to a Fare-Thee-Well earlier.. Trust me, you DON'T wanna Selenium to go "Tango Uniform" on you..

Well you know the measured current - what is the drop (in volts) across the selenium rectifier? With that, you'd have two of the three variables you need, and could plug them in ohms law for an answer.

Years ago, we made a selenium "drop box" - it was a rotary switch and 23 silicon diodes in series, with the common selecting a junction in the series string of 23 diodes. We set it for the highest forward drop, monitored the B+, and started down with the switch. When B+ was in, we stopped, multiplied the number of diodes that were in series times .7, and arrived at a voltage drop. A 1 ohm resistor in the box (in series with the diodes) provided a unity arrangement to measure current in the circuit (320mV across the resistor=320mA of current in the circuit). A simple R=E/I and we had our drop resistor value. I wish I still had the box, or the money to build another. The 23 position rotary switch we used is now some 81 dollars, so it ain't cheap to build. I've seen others that use a series rheostat from Ohmite (60 ohms, rated at 750mA max current) to do the same we did with diodes.

Measure the DC voltage drop across the selenium rectifier (Vforward) and you'll have E - the Sams gave you I (.320A), and you need "R". Simple Ohms law.

Cheers,

Just noticed your choice of diode - I'd go with the slower (and CHEAPER) 1N5408 diode. You don't need ultrafast diodes to replace a selenium rectifier.

Cheers,

I follow where you're headed, but I'm trying to get a replacement before I try to power the set up if possible.

I've got a couple more parts to order from Mouser that crept up on me when I started delving further into the chassis, and I figured I'd just get them all at the same time. If that's not possible, then so be it, I'll just reassemble the thing and check for voltage drop then :)

I think a 25 Watt rating is gross overkill. I have never needed more than a Watt or two for the resistor.

I don't recall the forward drop of a selenium rectifier but if you know it, then you know the extra voltage you will have to drop in the resistor. Ohm's law is your friend.

However, since the current isn't steady or even sinusoidal, you will have some error, often inconsequential. If that bothers you, adjust the resistance to get the desired B+.

I agree that it will be 100 ohms up or down like 75ohms or 125 ohms etc , 5 or 10 watt resistors make it a safe bet , 25 watts is overkill , I just start with 100 ohms and read the voltage and take it from there if I should increase ohms or decrease.

mike

Set is now powered up and functioning, so now I can check and replace the seleniums.

To do so as I understand it, black lead on the AC side, red lead on the positive side (basically one on each side of the rectifier), watch for fluctuations on power up, set meter for DC voltage. Sound correct?

If you are measuring voltage drop across the selenium rectifier, yes. With a DMM, polarity really doesn't matter. What is your B+ measuring right now, with the selenium? Note it now, so you can be sure to match it with whatever dropping resistor you choose.

The last selenium I replaced (in a drug-store tube checker), had 2.9 volts across it. I replaced it with 5 series connected 1N4007 diodes, and it is still plugging along. Just to show that not all selenium replacements require a resistor. My selenium was a Sarkes-Tarzian, rated at 400mA, so the 1 Amp diodes were overkill, but matched the B+ within 2 volts, good enough for me.

Cheers,

Here are the figures that I've got from the seleniums. There are two seleniums side by side. These figures are measuring one side of each rectifier:

Left one: 108 VDC, 115 VAC

Right one: 104 VDC, 116 VAC

If the one you did showed 2.9 volts across it, am I reading something wrong somehow?

I've been talking with someone off the board that's told me that voltage drop can't be measured because of the AC leakage, and that I should start with a 150 ohm 25 watt resistor, and change values of said resistor to see how the set performs in each instance.

If that's true, it's hard for me to wrap my brain around the fact that I have a working television and a set of schematics, but that I still have to come to a proper part replacement purely by experimentation. I've got a lot of time and parts into this set, I just don't want to make a costly mistake.

Take a look at the Sarkes Tarzian Selenium rectifier databook - most seleniums have about a 5V drop across them, and a bit lower when leaky.

Assuming a 5V drop, your 320ma (from an earlier post), and you have your two variables needed to get started. A silicon diode (1n4004/5/6/7 series) drops approximately .7V, so you need to drop the other 4.3V.

For your series resistor, R=E/I, so 4.3, divided by your current 320mA = 13.5 ohms, roughly. Power is as easy as P=I X E, so 320mA X 4.3V = 1.5 Watts, better to upsize it to a standard wattage. I'd go with a wirewound, as most filament resistors of yore are wirewound:

http://www.talonix.com/images/RESPFIL.jpg

I'm surprised there isn't more info (reliable, that is) about replacing seleniums on the web. We used to replace them all the time when I worked for my dad - even attached a new schematic for the next tech, so he'd know what he was dealing with. My least favorite swap was in a Nutone intercom for a dentist - the selenium was buried under a multi-station switch, with little room, and harly any room to mount the resistor. We went with a chassis mount, and used a resistor only a watt above the calculation. Solid state intercom, but it had a selenium rectifier - go figure!

Cheers,

I'd go with a couple of paralleled 39 ohm resistors to start, and if the B+ is low, add another 39 ohm across the first two, bringing you to 13 ohms.

39 is a nice standard value, so you can get some 39 ohm, 3 or 5 watt resistors easily. Is the set fused, or does it have a breaker?

Cheers,

First, I'd go in the higher resistance direction for safety sake. You can always lower it. Don't forget there is going to be a very nasty surge during warm up.
I also find it hard to believe that two watts is high enough rating for that much current draw. I've seen five watt tesistors in ac dc radios have a melt down, and, they draw alot less current than a tv set does..
I lean in the direction of saying resistor should be at least 10-20 watts wire wound. Twenty five is even better.
I also find that it's best to start high, and, work my way down, rather than starting low, and, possibly blowing electrolytics.
Bill Cahill

Starting with a higher resistance is exactly as I described. B+ low? Add the third resistor in parallel. Too bulky? Replace with one resistor, valued at the final resistance obtained by paralleling.

I've never used any B+ dropping resistor greater than 10 watts. We aren't powering filaments - just the B+, and you want the resistors to be sized right, as you can easily make the power tranny the weakest link with high wattage resistors and beefier silicon diodes. Resistors are cheap, and power transformers are not. Hence my request for fusing info - make sure all is fused, as it gives you an added safety margin.

Unless Ohms Law has changed, and Kirchoff's rescinded, I see no need to up the wattages when all variables are known. If you've recapped the set, and have faith in the B+ filter caps, go with the lesser/calculated wattage, with some wiggle, but sizing the resistor to almost 20X the wattage is not needed nor recommended. Remember, the original seleniums were rated at 320mA max, and that is the worst case/largest surge the originals could handle.

I've done selenium replacements in all types of equipment - the only gotcha is some sequential equipment, where relays time out and add loads. I had that with an industrial stitching machine (German made, for mattress manufacturing) - the thing had seleniums about the size of a lunch box, and supplied about 3 and a half amps in the first stage of the equipment start-up, and added about 2 amps at full run. Common bridge rectifier rated at 8 amps, a 1.2 ohm, 10 watt resistor, and they were back in business, and the machine worked fine until it was replaced in the 1990s.

Cheers,