Selenium rectifiers

[Findm-Keepm]

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,

[bandersen]

Originally Posted by Findm-Keepm

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.

[Bill Cahill]

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

[bob91343]

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.

[Kamakiri]

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.

[bandersen]

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.

[Bill Cahill]

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

[Kamakiri]

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

[Kamakiri]

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.

[Kamakiri]

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=_bUxH...ature=youtu.be

[jr_tech]

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

[Kamakiri]

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

[jr_tech]

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

[Kamakiri]

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.

[jr_tech]

Originally Posted by Kamakiri

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.

jr