aaa

Have you looked at using modern diodes? I've heard plenty of stories about seleniums failing and stinking up the joint. I do have a handful of NOS and used seleniums but would have to check for the ratings.

Yeah, the smell will get into your furniture as well.

aaa

I find that the 1N4007 works for nearly all cases like this. You will also need a resistor in series with the diode, maybe 47 Ohms 2 Watts. 1N4007 diodes cost a few pennies each so buy a bunch for future use.

aaa

will that work for the .200a and .130 a?

1N4007 diodes can handle a full amp, so they should be more than enough to replace either of those.

one 1n4007 for one selenium rectifier , one for the other , its pretty easy to do and more reliable since the nos ones would fail after a while anyway.

mike

aaa

A resistor is needed. See my post above.

Also not a bad idea to put a .01uF 600V film cap in parallel with the diode to reduce the increased switching noise inherent in modern diodes. It is called a snubber cap when used in this application IIRC.

We sure could use a good tutorial on replacing these. I'm diving into a Halolight that is going to need them, and I've never done it before. Even a simple diagram would work :)

To begin with while you have a working selenium in place, measure the voltage drop across it, the current running through it, and the shape of the rectified signal. Use the current, and voltage drop to figure out what resistor to put there..... And the scope pic of the wave after it, when you put a cap across it, compare the new picture to the old one..... Call it engineering a solution.

In this case, this set probably hasn't seen a wall receptacle since the Nixon administration, if not before that.

I have to do a full recap on it, so let's assume for the moment that I can't do that :yes:

A couple things I do, if replacing selenium with silicon, I would leave a term strip setup for a B+ dropping (and fuse for that matter if it does not have one), then use a variac and start at about 90v see how the set behaves (this is assuming its working). if everything working check the B+ at the output of the rectifier to the sams check the current and get a ballpark figure for the dropping resistor using ohms laws, prob between 50 and 200 ohms. check your line voltage and fine tune the resistor to the correct B+

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.php?attachmentid=180220&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/STMicroelectronics/STTH310/?qs=sGAEpiMZZMtbRapU8LlZDyV9kcfoNEfr%2fp8QPCBfkcQ% 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..

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.php?attachmentid=180220&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/STMicroelectronics/STTH310/?qs=sGAEpiMZZMtbRapU8LlZDyV9kcfoNEfr%2fp8QPCBfkcQ% 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?

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,

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.php?attachmentid=180220&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/STMicroelectronics/STTH310/?qs=sGAEpiMZZMtbRapU8LlZDyV9kcfoNEfr%2fp8QPCBfkcQ% 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?

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?

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 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,

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 (http://www.tubebooks.org/Books/srhbst.pdf) - 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,

This set doesn't have a power transformer. It's series string.... I still have the concern for the wattage. I didn't realize you were saying baxically the same thing as I. But, I still doubt it will be as low as 30 ohms..In any case, there is the surge factor... I prefer to start high, and, work my way down.. Perhaps ten watts is plenty, but, Tim is afraid to even try that. He is afraid he will dammage the set. I tried telling him that wouldn't happen, the worst that could happen is the resistor prematurely fail.. I can understand why he refuses to spend sixty dollars on twenty watt resistors, but, I tried even telling him to put resistors in parallel to get the desired wattage...
Bill Cahill

Even if it is a series string, the filaments do not come into play - they are a separate leg/circuit. The total load coming from the doubler cannot exceed the ampere rating of the original seleniums, which Tim stated was about 320mA.

Second, the voltage drop across the selenium is rarely above 5V, and with the 7/10s of a volt that a silicon diode drops, you only have to drop the additional 4.3V or so. You aren't dropping tens or hundreds of extra volts, so the resistance and wattage are going to be low.

I stand by the calculations - two previous posters also weighed in, saying 25 watts is overkill - and I too have never used more than a ten watt resistor. In any case, it is imperative that the set be fused, and adding a B+ fuse would also be recommended.

60 bucks for a 20 watt resistor(s)? Where? Three 5 watt resistors could be had for a buck-thirty plus shipping.

I too can sense Tim's confusion -and a good reference for replacing seleniums is nowhere on the web. It's all Ohms Law and Kirchoffs Law, but no one on the web applies either to a selenium replacement scenario. I guess someday they'll all be replaced, making this thread obsolete.

Cheers,

The set is fused, so no worries there. The 60 bucks was for buying a dozen 25 watt resistors to experiment with, at $4-$6 each. Bill and I started our conversation on this a couple days before I started posting in this thread.

I really appreciate everyone weighing in on this thread, and will be ordering parts today. I'd rather do this job completely and correctly, and I'm thankful for the guidance here :)

Option B would be to take 7 modern diodes and string them in series as 7*0.7 = 4.9 which should be close enough to 5 to not matter much, and possibly cheaper than a dropping resistor.

Option B would be to take 7 modern diodes and string them in series as 7*0.7 = 4.9 which should be close enough to 5 to not matter much, and possibly cheaper than a dropping resistor.

He's got two selenium rectifiers in a voltage doubler configuration - best not to use series diodes in his application but they are fine in a straight half-wave configuration.

Of note, I had fun experimenting with an old GE Selenium today - put 40V from one of my Lambda supplies on the anode, and hung a 120 ohm 10 watt resistor off the cathode to ground - got 353 mA through the selenium, and had a volt and a half of loss through the selenium. My guess is they get leaky, and drop less across them as they age. I've got a coffee can full of Sarkes (ST logo) seleniums in the shop at home, and will try one or two later tonight. I think my Sorensen DCR40-40 is up to the task - just gotta get the right load without exceeding the limits of the selenium rectifier, as my shop at home is quite small and I don't want/need a stinkfest again. I had a Silvertone portable give up it's flyback in there last May, and it still smells of burnt flyback.

Cheers,

Brian, if you'd be so kind, please review with me where I'm at, and the next step in the process.....when doing something that I've never done, I like to be very clear on what I'm doing.

Here's where I'm at right now.....

I have in my possession, the following parts (in quantities more than I need I assume):

(4) 1N5408 diodes, rated at 3 amps.

(8) 39 ohm 5 watt resistors

As shown, I've removed the selenium rectifiers, and installed two terminal strips, marked with the respective polarities. The underneath shot is just showing my work.

The selenium rectifiers that were in the set had a part number of 13-84630-2, and were rated 450 mA. Replaced at one time, I assume, with a heavier duty part.

Now, why do I have a feeling you will burn up that resistor????

Now, why do I have a feeling you will burn up that resistor????

Bill, I've got a lot of hours in working on this set and I'm trying to do something that I've never done before.

That said, mind elaborating? :smoke:

Tim,

Attached is a simplified schematic of what you are doing - I drew it from memory - if you want a marked up copy of the Sams, I can scan that tomorrow, noon-ish. The Sams is out in my shop, and I am in for the evening.

As you can see, the silicon 1n5408's are simply installed where the selenium was removed - cathode (banded end) to the "+" connection, and anode to the "-" connection. Off of the cathode of the output (rightmost) rectifier, we add in a series resistance. At 39 ohms/5W, you are good for up to about 12V of voltage drop across the resistor. I LOVE those RH-series Dale resistors you've chosen, as they make for easy mounting, and the chassis can sink some of the heat. Some purists want only axial or old ceramic resistors in their sets.


Sams (IIRC) says the B+ should be about 195V for your set.

If the B+ is low, (lower by 5-8 volts or more) add some Parallel resistance to bring it up. I don't think that will be the case, as 17 to 75 ohms is my experience for most series resistors in tube sets.

If the B+ is high, (higher than 5 volts or more) add some Series Resistance. Doing so will not only drop more voltage, it will also decrease the power dissipated by the (each) resistor.

Your "fusistor" is the 4.7 ohm 15 Watt resistor under the chassis in one of your photos.

Hope this helps - I'm up for another hour or so - I have to take my daughter to school in the morning, so an early evening for me.

Cheers,

I re-read what I wrote above, and I wasn't clear that each selenium rectifier is replaced by one 1N5408.

Cheers,

I LOVE those RH-series Dale resistors you've chosen, as they make for easy mounting, and the chassis can sink some of the heat. Some purists want only axial or old ceramic resistors in their sets.

That to me was an obvious choice in this case. That resistor replaced a resistor that used to be in the lower right corner of the second picture. There was so much heat there that it ended up burning the terminals right off the phenolic strip, and the old resistor was just dangling. I also upped the rating on it, and used a 25 watt instead of a 15 IIRC.

I fully believe in over-building a circuit, especially when it's apparent that the way it was made just wasn't good enough over the long haul :)

I'll be doing this job today, after all the housework I just ran out of steam last night....

At the risk of sounding like an idiot, B+ is checked across the first filter cap, yes?

B+ is measured at the point I marked on the schematic. B+ refers to the main DC supply in most sets - it's a carry over from the old battery days when they had A and B batteries - A Batteries were for filaments and low voltage bias, and B batteries (45V, 67.5V and 90V) were for the plate and other high voltage needs.

I marked up the schematic for you - hope it helps. Start with a 39 ohm 5W resistor and measure your B+, and follow my previous advice if it is high or low.

The Sams should carry you forth from here.

Cheers,

Now, why do I have a feeling you will burn up that resistor????

Those metal cased types don't burn they explode....Don't ask me how I know.

Took me about a half hours' worth of racking my brain to trace everything out.....turns out that on this, M2 is on the left, and M1 is on the right.....but I guess it depends on how you're looking at the chassis.

Just one last question. I want to put the dropping resistor in between the filter choke and the resistor junction, and not between the diodes and the choke? If that's the case, I might want to mount that remotely with another Dale resistor when it's finished, as that's a REALLY tight junction.

Took me about a half hours' worth of racking my brain to trace everything out.....turns out that on this, M2 is on the left, and M1 is on the right.....but I guess it depends on how you're looking at the chassis.

Just one last question. I want to put the dropping resistor in between the filter choke and the resistor junction, and not between the diodes and the choke? If that's the case, I might want to mount that remotely with another Dale resistor when it's finished, as that's a REALLY tight junction.

Before or after the choke is fine, it's all one series DC path. C2A and C2B, along with the choke form a Pi Filter, and additional DC resistance shouldn't be a problem. Technically, you could even raise the value of R91, the fusistor, but best to leave that alone.

Cheers,

Got 235V on the B+, so by the Sams it's under by 20V. I only allowed it to run for about a minute, assuming that would be enough time to get the set up to operating voltage.

Okay, now this is interesting.

Powered the set up again with another 39 ohm resistor in parallel, voltage is jumping between the 240 and maybe 260 range. Full raster, everything's looking good, but I think I'm smelling something.

Resistor is starting to burn. Shut down the set.

added Series resistor burning? Is the voltage stable before the resistor?, at the cathode of M2? Powering up via variac, or cold start from wall?

B+ might be okay without the added series resistor - jumper it with a wire and bring it up on a variac. Monitor B+ at 120V, and post results - that'll give us a starting point from here.

Cheers,

At the stated current in the sams, the resistor should drop about 12 or 13 volts, so if you are 20 volts low, I'd lose the resistor:

E=I*R
E=320ma x 39
E= 12.48V, and you are 20 low, so something is odd.

I don't agree. You must have an extra resistor to handle the surge. Further, I've told Tim this before, and, I'll state it again.
A tv with a 5 watt resistor? Balderdash!!!
I've see ten watt resistors in simple table radios. I've never seen a five watt loading resistor in a tv/ You need at least twenty watts.
Bill Cahill

Just to clarify, I did a soft start on a variac, but not too slowly....I took about 20 seconds to bring it up to voltage. Initially, the reading was 305 volts, and dropped to 235 on the first try, with the one resistor in place. The voltage reading was tested at the B+ point that was on the schematic on the previous page in this thread.

I then shut it down, put in a second resistor in place, then again with a quick variac power up, the reading briefly stabilized at around 253 volts. I noticed that I had a full raster, snow, and static in the audio....exactly the way it should be. Observed it only for a few seconds, then went back to the meter and noticed that it started jumping all over the board, ranging from 240-260 volts. When I caught a light odor of something, I shut the set down immediately.

This is a picture of the resistor, along with brand new ones from comparison. Obviously I caught it in the very beginning stage. The picture doesn't show it, but there is a very light browning on one spot.

I clipped the lead of the resistor to check it afterwards. It's reading at 39.5 ohms.

Just to clarify, I did a soft start on a variac, but not too slowly....I took about 20 seconds to bring it up to voltage. Initially, the reading was 305 volts, and dropped to 235 on the first try, with the one resistor in place. The voltage reading was tested at the B+ point that was on the schematic on the previous page in this thread.

I then shut it down, put in a second resistor in place, then again with a quick variac power up, the reading briefly stabilized at around 253 volts. I noticed that I had a full raster, snow, and static in the audio....exactly the way it should be. Observed it only for a few seconds, then went back to the meter and noticed that it started jumping all over the board, ranging from 240-260 volts. When I caught a light odor of something, I shut the set down immediately.

This is a picture of the resistor, along with brand new ones from comparison. Obviously I caught it in the very beginning stage. The picture doesn't show it, but there is a very light browning on one spot.

I clipped the lead of the resistor to check it afterwards. It's reading at 39.5 ohms.

Are those wirewounds? The look like film to me. What's the series or part number? The wattage?

With one resistor, your B+ is low. This is indicative of too much series resistance. 20 Volts low by the Sams, but the original B+ with the seleniums was what?

You've changed the seleniums to silicon, check. Added series resistance, albeit too much, check. With 39 ohms, you are too low by the Sams, so you need to decrease the resistance, and thus the voltage drop across the resistor, resulting in more B+. Simple - just replace the existing resistor with one of a smaller value, or parallel another beside it. With lower voltage drop, the wattage dissipated will also decrease. If you are still overheating, recheck everything with the wiring you've added.

As to the wattage, and the other comments - BS. I've got several working sets with simple 10 -75 ohm 5 watt resistors working just fine. My Penncrest in the den has a 270V B+, and has had all 4 of it's seleniums replaced, and a 47 ohm 5W resistor added to bring it down.

Take a look at R90 on the original Sams - it's dropping some 40 volts across it, and it's rated at 5 watts. You rate the resistor based on Ohms law and Kirchoff's law, not something you pull out of a hat. Take a look at w3hwj.com/index_files/RBSelenium2.pdf‎ - I'm not the author, but it backs up what I'm putting out, and look! -> page 3 of 6 shows a dropping resistor....of 5 watts.


The voltage drop across the original seleniums is all that has to be accounted for. It's not a loading resistor of any sort, just a series dropping resistor. R92, the filament dropping resistor performs a similar function, dropping 30V (120 line minus the 90V at pin 2 of the 12DQ6) in the 600mA series string of the filaments. P=IxE, so .6A x 30V = 18W, and it's sized at 20W, a standard value. Solid theory, put into practice...

You can obviously listen to whom you choose...

Right and P is also I2 X R. So if B+ is drawing 320mA, then you'll have (0.32 * 0.32) * 39 = 4 watts out of that resistor so a 5W should be OK, but maybe 7W for a little overhead. Or use a smaller 5W resistor like 33 or 27 ohms.

The fly in the ointment is that this isn't pure DC. Therefore you cannot be accurate with all these calculations. This is a perfect use for a true rms meter.

There is a lot of ripple that doesn't register on the dc meters but still causes heat.

The error may not be great but don't cut it too closely or reliability will suffer.

This is the exact resistor I used:

http://www.mouser.com/ProductDetail/Vishay/AC05000003909JAC00/?qs=%2fha2pyFadug9rqcTfPQXM6P7IDAp1t%2fJFzMJWHebmL Fx3kM0vXW2IQ%3d%3d

The set was working, and working well during the brief test. Before I went about this replacement, I had probably an hour or so on the set with the original seleniums in place.

With the second dropping resistor in place, I'd be operating at 18.5 ohms. The voltage with 18.5 ohms was right at 253V, which looks good to me, as the Sams says 255V.

Right and P is also I2 X R. So if B+ is drawing 320mA, then you'll have (0.32 * 0.32) * 39 = 4 watts out of that resistor so a 5W should be OK, but maybe 7W for a little overhead. Or use a smaller 5W resistor like 33 or 27 ohms.

He's 20V low, and the 39 ohms is dropping that. My original calculations called for a ~13 ohm resistor. (see post 27, page 2) Take three 39 ohm resistors, parallel them, and place them in the circuit. That'll give you the proper (lower) voltage drop, which should give you the proper B+, and plenty of overhead on the wattage. .33A X 13 = 4.29W, and you've got (3) 5 watters in parallel.

And Tim, if you need more resistors, lemme know. I've got my dad's old stock of resistors, and yes tons of diodes too. Save your money on parts and buy sets....


Cheers,

He's 20V low, and the 39 ohms is dropping that. My original calculations called for a ~13 ohm resistor. (see post 27, page 2) Take three 39 ohm resistors, parallel them, and place them in the circuit. That'll give you the proper (lower) voltage drop, which should give you the proper B+, and plenty of overhead on the wattage. .33A X 13 = 4.29W, and you've got (3) 5 watters in parallel.

Minor correction - it's current squared times resistance or (0.33 * 0.33) * 13 or 1.5 watts like you calculated earlier. So only 0.5 watts per 39 ohm resistor.

I don't know where you guyes are getting your fancy calculations, but, you both are way off.... I will state for a fact. Every one tube record player I have run into has had a five watt resistor on selenium.
Every simple five tube radio, five, to ten watts.
I have NEVER seen any BW tv with anything less than ten watts, and, sometimes as high as twenty five watt resistors on the seleniums.
Further, Tim made it clear it was burning out five watt wirewound resistors. And, you think a half watt resistor is going to drive an entire tv???
Wake up, guys.. Your fancy calculations are really amiss.....
Bill Cahill

It's called Ohms law and I said three in parallel with each 13 ohm resistor dissipating 1/2 watt - not a single 1/2 watt resistor.

I know what ohm's law is. But, I don't care. That just isn't enough wattge. Period.

Just to clarify, I did a soft start on a variac, but not too slowly....I took about 20 seconds to bring it up to voltage. Initially, the reading was 305 volts, and dropped to 235 on the first try, with the one resistor in place. The voltage reading was tested at the B+ point that was on the schematic on the previous page in this thread.

I then shut it down, put in a second resistor in place, then again with a quick variac power up, the reading briefly stabilized at around 253 volts. I noticed that I had a full raster, snow, and static in the audio....exactly the way it should be. Observed it only for a few seconds, then went back to the meter and noticed that it started jumping all over the board, ranging from 240-260 volts. When I caught a light odor of something, I shut the set down immediately.



I'm still trying to understand this sequence... it appears that with one 39 ohm resistor in series with the diodes to the B+ test point, there was no problem other than the B+ voltage being slightly too low? The resistor did not overheat ... correct?
Then you put a second 39 ohm resistor in parallel with the first to increase the B+, which it did, but then the voltage started jumping around and one resistor started smoking? Was the first resistor still ok? Putting the two identical resistors in parallel would have reduced the voltage drop across the resistor(s) and, therefore reduced the power dissipated by the resistors(s) as well as splitting that power between the two. Neither should have overheated. Perhaps the second resistor was defective?
If it were mine, I would try again with a fresh resistor and monitor the current in series with the resistor pair.
jr

The first resistor is what smoked, the second was fine. Strange, but true...unless the first resistor started to crispify at the first test, and I didn't notice.

At this point, Findm-Keepm's advice had got me this far, as far as the values of the resistors, and precise directions on how to hook things up and arrive at the proper specs. Bill Cahill's advice of a 25 watt resistor to do this job right will only cost me about $5, for a good Dale chassis mount resistor with a heat sink.

That said, everyone here seems to agree that a 25 watt resistor here will certainly not hurt anything, at worst it's crazy overkill....at best, it's the proper part....so that's the way I'm going to go. I'll check back into this thread in a few days when the Mouser order arrives.

I certainly do appreciate the input and education I've been given thus far :)

Finally got the resistor in from Mouser. The only bad thing is that I must not have left that prior resistor in circuit long enough.....the chassis voltage with this one in line dropped to 243V. Should be at 255 :(

I do have to say, the 18 ohm RH series Dale resistor I put in is getting HOT. Handling the set fine, but it sure is warm.

Picture isn't fantastic, reception wise, but the tuner in this set isn't the greatest and I'm using clip leads for an antenna picking up signal from my agile modulator. I'm going to button it up for the moment and move it to cable and see where we're at. The chassis voltage is off by -12V, meaning that I'll have to probably order yet another resistor, but I want to let it play out a little where it is and see how it looks. The picture filled out nicely as opposed to where it was before.

Here's where I'm at right now. Seems to have some slight blooming, which I assume is due to the low voltage on the chassis.

http://www.youtube.com/watch?v=_bUxHpMFG5w&feature=youtu.be

IMHO you should prepare for smoke! Something is wrong, the set should not be drawing enough current to get an 18 ohm 25 watt resistor *really* hot!

Calculator:
http://www.crownaudio.com/ohms-law.htm

If you plug in 18 ohms and, say 15 watts (my estimate of what it might take to get a 25 watt resistor fairly hot), you will find a voltage drop of about 16.4 volts (too much drop) and a current of about 900 ma! Since the original rectifiers were only 340 ma or so, it would appear that the set is drawing more current than it should,
just my 2 cents,
jr

If that's the case, what the heck would cause that in a TV that by and large is running as it should? :dunno:

I don't know, perhaps an infant failure of one of the new electrolytics. Can you measure the current in series with the resistor?

Am I correct in assuming that the resistor is indeed, bolted to the chassis? If not, it could reach a high temperature at less than its rating.

jr

No, at the time, I had the resistor put in with clip leads. I didn't want to hard mount it to find out I needed a different value.

No, at the time, I had the resistor put in with clip leads. I didn't want to hard mount it to find out I needed a different value.

OK, I don't know what the "free air" dissipation of the resistor might be, possibly it could get pretty hot if dissipating, say only 5 watts... where does that take us? Plugging in 5 watts and 18 ohms into the equation yields about 9.5 volts drop (but you say you are 12 volts too low) and about 500 ma current (but the orig rectifiers were only 350 ma or so). IMHO, something downstream is drawing more current than the original design called for.:scratch2:

jr

Add to that the blooming. There was no blooming of the picture when the seleniums were in place. Also now, if you look really hard at the video, you can see a very light line going down the center of the screen, which isn't really visible on some stations. Is that horizontal foldover?

So then we're left with the question, is the blooming caused by the low B+, or is the same component causing the blooming and low B+, dragging the chassis voltage down, and sucking current?

First thing I'm going to do is test the diodes. Not sure if another diode in series would help matters much, as I got 3 watt diodes. Next thing, I'll re-test the tubes. Beyond that, I just don't know.....

If you connected a doubler cap backwards that could do it.

I would first try to measure the current in series with the resistor, to see what the set is drawing off of the B+ line... that should answer the question; "is the set drawing too much current or is the power supply not working correctly?".

jr

I agree checking the current is one of the best ways to find out what's happening. But, the blooming, along with slightly short width, which he told me about, and, the white line in middle, which sounds like slight horizontal fold over, obviously, hv is low. I'm not sure if that is caused by low voltage, or, something wrong in hv section. In fact, if the screen resistor on the horizontal output, or, even the cathode resistor on same, if it uses one, which often they do, that would cause that section to draw more current, causing the power supply to drag down.
Bill Cahill

Went to measure the current draw, but then realized that the ammeter function on my meter isn't working. Have to go pick one up today or tomorrow.

I would first try to measure the current in series with the resistor, to see what the set is drawing off of the B+ line... that should answer the question; "is the set drawing too much current or is the power supply not working correctly?".

jr

Current in series with the resistor is 280 mA.....well within spec.

Current in series with the resistor is 280 mA.....well within spec.

Is the resistor still getting really hot? How many volts is it dropping?

jr

Temperature probe on my meter stabilized at around 197 degrees F, but was still climbing very very slowly when I shut it down.

How about current between the B+ terminal (250 volts?) and the rest of the set? Thinking that there could a lot of ripple current on the input to the filter cap that is contributing to the resistor heating (are the filter caps the correct value?) but perhaps the "fee air" dissipation of the 25 Watt resistor is quite low, and 197 degrees is "normal".
somewhat stumped,
jr

Filter caps are at a minimum at least what was in the set, and are all the correct values.

Well, at last reading it was 243V, or 12V low. I haven't changed anything since, and the set's seen barely 15 minutes run time since.

Compare the two videos....the right side, which wasn't filled out, now is. However, the picture's somewhat bloomed:

Before the selenium replacement:

http://www.youtube.com/watch?v=RPSNHOG0TIU

After:

http://www.youtube.com/watch?v=_bUxHpMFG5w

The only thing that's changed between the two videos is the addition of the diodes and resistors, replacing the selenium rectifiers.

Now the only thing that I can think of, from a purely mechanical standpoint (which may be off, but makes sense to me) is that the heat sinks on the selenium rectifiers were massive. In this case, we're asking a much physically smaller component to do the job of two that were much larger. To me, it almost makes sense that the resistor would get really hot just because of that. But I don't know, because I've never done this job before :dunno:

Hoping some others will chime in with thoughts as well.

How about current between the B+ terminal (250 volts?) and the rest of the set? Thinking that there could a lot of ripple current on the input to the filter cap that is contributing to the resistor heating (are the filter caps the correct value?) but perhaps the "fee air" dissipation of the 25 Watt resistor is quite low, and 197 degrees is "normal".
somewhat stumped,
jr

I just looked up the spec of the resistor, and it seems that it has a temp range of up to 250C. Since 197 F is about 92C, I think this is just normal.

And, since I only read 280 mA on the current draw, I know I'm not overdriving the resistor risking burnout.

Think I'll hard mount it to the chassis and try watching some television for a bit and see what happens.

Think I'll hard mount it to the chassis and try watching some television for a bit and see what happens.

I suspect that it will be pretty cool with proper mounting... Hope the tv remains stable. Still a question remains about the low voltage and blooming. :scratch2:

Good luck,
jr

Got up at an ungodly hour this morning :) , mounted the resistor, and watched the set for about an hour. I was able to dial in the picture with the vertical height and width controls. Everything is stable, and the set's running fine.

Realize that your 280 mA reading isn't accurate, as it's a pulsating DC, which most milliammeters won't correctly measure. Having said that, it's probably not that far off.

Realize that your 280 mA reading isn't accurate, as it's a pulsating DC, which most milliammeters won't correctly measure. Having said that, it's probably not that far off.

But there is a fair amount of filtering (150uF and a choke) before the resistor and meter, shouldn't the ripple be pretty small at that point?
A better measurement would be to place the current meter in the B+ source lines (255V, 215V and 145V), one at a time and add the results.

jr

The ripple current goes through the rectifier of course, so an ammeter placed there reads the raw rectified current, not filtered at all. Measuring the current downstream simply removes the AC component and isn't a true indicator of the resistor current. (I am assuming the resistor is where it belongs, in series with the diode and not after any filtering.)