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  #46  
Old 11-20-2013, 01:54 PM
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Kamakiri Kamakiri is offline
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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.
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  #47  
Old 11-20-2013, 02:11 PM
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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.
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  #48  
Old 11-20-2013, 03:59 PM
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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.

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  #49  
Old 11-20-2013, 04:05 PM
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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.
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  #50  
Old 11-20-2013, 04:32 PM
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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.
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  #51  
Old 11-20-2013, 06:57 PM
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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.
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  #52  
Old 11-20-2013, 08:22 PM
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Quote:
Originally Posted by Kamakiri View Post
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...
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Last edited by Findm-Keepm; 11-20-2013 at 08:25 PM. Reason: whom, not who..
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  #53  
Old 11-20-2013, 10:53 PM
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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.
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  #54  
Old 11-21-2013, 12:31 AM
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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.
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  #55  
Old 11-21-2013, 05:37 AM
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This is the exact resistor I used:

http://www.mouser.com/ProductDetail/...M0vXW2IQ%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.
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  #56  
Old 11-21-2013, 05:41 AM
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Quote:
Originally Posted by bandersen View Post
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.
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  #57  
Old 11-21-2013, 05:44 AM
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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,
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  #58  
Old 11-21-2013, 03:05 PM
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Quote:
Originally Posted by Findm-Keepm View Post
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.
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  #59  
Old 11-22-2013, 11:20 AM
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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.....
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  #60  
Old 11-22-2013, 12:29 PM
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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.
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Last edited by bandersen; 11-22-2013 at 12:34 PM.
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