I found a website with instructions on how to replace a selenium rectifier. I understand how to replace it but I would like some hints on how to figure out what size resistor I should use for this radio. Like, What numbers or parts do I look at on the schematic to calculate the correct resistor?

Here is the website that explains how to replace the rectifier. http://w3hwj.com/index_files/RBSelenium2.pdf

Here is the schematic to the radio I am working on. http://www.nostalgiaair.org/PagesByMode ... 011726.pdf

Thanks for the help.
Kris

I would start with this,
My math skills are weak, and I like simple solutions.

When you operate from the 120 VAC power line, can you remove the 6 volt filament battery and still get the set to operate?

I am trying to determine whether the filament power is derived from the selenium rectifier circuit during AC operation.

If the filament circuit is part of the rectifier load, then the rectifier current will be increased and the compensating resistor will need to be calculated based on that fact. Also, you need to be certain that the filament string voltage (6 V DC) does not get increased when the conversion to silicon is made. A little more B+ voltage may be OK, but you don't want to kill the tube heaters.

Rich

Thanks for pointing me to that paragraph Dave. I have not tried the radio since I recapped it. I was going to try and replace everything that is out of date and then give it a try. The radio hummed immeadiatly when turned on before I did any work to it but it did not get any stations.

Would it be ok to go ahead replace the selenium with a 1n4007 diode & a 100 ohm resistor and see if the radio works? Then take a voltage reading? Then I adjust to make it as close as possible to 6 volts?

Rich, I don't see any batteries inside the radio. I am not sure how to tell if the filament circuit is part of the rectifier load.

Thanks again for the help guys.
Kris

I would do as Dave wrote and replace the selenium with the diode and the 100 ohm resistor, then put the voltmeter on the filament voltage and bring the set up on a variac. That way if the filament voltage came up to normal before you got to line voltage, you'd know that you needed a higher value resistor, and vice versa. In this way you wouldn't have any filament surprises.

Thanks Reece. I will get it replaced tonight and report back to let you guys know when I get it fixed.
Kris

Good idea. That way you can see what's happening with B+ and heater voltage before you exceed the prescribed values.

The fact that your radio works without batteries means that the heater circuit is run off the AC line and I am guessing it is rectified by the selenium rectifier. Can you see any marking on the selenium rectifier? How large are the plates? To handle both B+ and heater, I am thinking it must be rated at least for 100 mA. Not really a critical issue, though. The 1N4007 will easily handle a couple of hundred mA.

Rich

PS: I looked at the schematic, but it's hard to tell what's connected where. I guess I should print it rather than try to read it side-ways?

Printing the schematic really helps.

Yes the filament string is tied into the DC power supply. The spec says that the B+ current is nominal 8.5 mA and the filament string is 50 mA. So a 100 ohm resistor will drop "about" 5.8 Volts, which makes 100 ohms a good place to start. The voltage drop isn't simple to calculate because the rectifier only conducts for a short portion of each cycle of the 60 HZ sine wave and the rectifier current is not sinusoidal or constant.

Good luck!

Rich

PS: Be sure to observe polarity when installing the 1N4007. A reversed diode could zap your filter caps.

Thanks for all the help Rich, I will get some pictures of the selenium rectifier and see if I can find any markings on it. I will post them later tonight.

Kris

If that selenium is good then you dont need to replace it just because several people say to . Plug it in & try it .

Here are some pictures of the selenium rectifier and what I have done to replace it. I used a 150 ohm resistor because it was all I had on hand. I can change it if needed before testing the radio.







I know I am still sloppy with recapping. I am always unprepared and never have the correct values I need. I wanted to do some radio work today so I used what I had on hand. I had to run some caps in parallel to get the correct values I needed.

I did not use sleeving because there is a lot of space under the chassis and I did not think it was needed. I made sure the connections were all tight and not moveable. I am still not sure what leads are most important to cover because of arcing. If there is any leads I should have covered please let me know.

If the radio works like it should after I get it all done I will order the correct values and replace the paralleled caps.

I still have the E-caps to replace and then I should be ready to fire it up. I decided to rebuild the original can capacitor rather than clogging up the underside of the chassis with several E-caps. I will show pictures of my finished chassis soon.

Thanks again for all the help guys.
Kris

The stripe on the diode should face the same way as the + of the rectifier. I don't know if that is what you have there or not. The rest of your work looks fine.

Thanks Dave, I will check and make sure I have it all wired correctly before I power it up. I traced the wires and checked resistors against the schematic when I wired it. The positive of the rectifier was wired to the resistor going to a wafer that is connected to the ac line. The negative of the rectifier was hooked to the 2 brown wires going to the E-caps.

Does the AC connect to the unbanded side of the diode? If so I think I have it right, otherwise I have it backwards.

I have been delayed on this because I messed up the can electrolytic so I am in a bind until I find another can. I will have to replace it with a similar one.

Is there a good source I can buy already pre-stuffed or reproduction electrolytic can capacitors?
I looked on the internet but all I found were really pricey ones for Dynaco amps. I was thinking about buying a NOS capacitor on Ebay but I haven't gone that route yet.

Kris

Sounds like you have it right. If you really want to stuff the can, you can, but it may be cost effective to buy the single caps, and a terminal strip, and place them under the chassis.

I have had cans made, and the last one ran around 23.00, for a 4 section, I think for a Strato-world.

It was a couple years ago, so, it may be higher now.

And forget about those NOS ones, most are as old as the ones in your set, so can fail, quickly.

With all due respect, it looks like you've got plenty of room under there to mount the electrolytics. Why bother trying to rebuild the one on top? Even with a crowded chassis, I can always find room, myself. Seems like a big hassle for no real benefit.

I usually use trial-and-error for the dropping resistor. Do the recap job FIRST including the electrolytics, and then try different values of resistors with the diode and measure your voltage. 5 or 10-watt wirewound resistors are cheap enough at Radio Shack that I just keep a bunch of different values on hand, from 10 up to 150 ohm. Usually somewhere around 75 ohm is what you'll end up with. Start high, with a 150, and work your way down so you don't over-voltage the radio. You may need to combine more than one resistor to get it "just right". For me, once I find a resistance that delivers between 115 and 120 volts, I call it good enough.

Jeff

I am still working on this radio. I almost have the electrolytics mounted under the chassis. It is a very tight squeeze because it has 4 E-caps but I am making it work. I am replacing the can cap I messed up with one I just pulled from a parts chassis. They are not the same but I don't think it matters. I should have the chassis done by tomorrow. Wish me luck, I have been having a hard time getting things to work lately.
Kris

I replaced all of the capacitors and tried the radio through my variac. Nothing happened until I got to almost full power and then I heard a faint hissing sound coming from the 150 ohm resistor connected to the diode. I felt it and it was to hot to touch and smelled. The radio did nothing not one sound came from the speaker.

What could make the resistor hot, the tubes not light and radio not work?

Hopefully I just need to change the resistor but it is possible I wired it wrong because I was having a really tough time reading the schematic but I was pretty sure I had it all figured out.

Thanks for the help in advance.

Kris

Hi, I can't tell what the big green paper thing is, but if it's a cap, it should go away. Also, how large ifs the 150 ohm one that you added? I think it should be about the same wattage as the 68 ohm, 2 watts or better.

Also, none of the caps go to the chassis, this chassis has a floating ground. I can't tell from your pictures if that may be what has happened.

Remove the 150 ohm resistor and then re-install the selenium. Measure the dc voltage on the output of the selenium. If the voltage is right then you are good to go.

Only time a selenium rectifier must be changed is if it is visibly bad, has low output voltage or you are working on a TV.

Thanks for the help guys.

Dave, The big green paper thing is the 150 ohm resistor I added to the diode, it is what got really hot. I did not tie anything to the chassis but I did move a few of the ground wires to a different ground tie to make wiring the E-caps easier.

Tube Radio,
I put the selenium back in and tried the radio. Now nothing is getting hot that I can see but the radio still does not work and the tubes do not light up. When I tested it before recapping it the radio hummed immeadiatly so I must have messed something up.

Using my meter to test the DC voltage from the selenium I measured 101 volts. I used the negative leads of the E-caps for the ground and the + of the rectifier to do the measurement.

Edit:
I think I might have a bad 1A5 output tube. My tube tester does not have this tube on the chart so I can't test it. I used my ohm meter and could not get a reading between pins 1 & 8. I don't want to put a new tube in until I know I won't ruin it. I went through and tested all of the resistors. A few of the resistors are high but I don't think they are the problem. I checked all of the caps and connections against the schematic and they all look good. The volume switch turns on ok but when you turn it off or down it feels scratchy or something.


Thanks again for the help.
Kris