Dove right in! Help with RCA 21-D-8628

This would be hilarious if after cracking this thing open I find that it works perfectly and I just had a great learning experience. And then I have to flatten and recreate the box that I broke open!

[JohnCT]

Originally Posted by BandDirector

What is B+, and, what do I need to worry about?

In your schematic, you'll see the 5U4 rectifier tube.



That, along with the filtering capacitors, is what turns the AC voltage from the transformer secondary to the DC voltage that the TV circuits need. The 281V point on pin 2 of the 5U4 is called B+ voltage.

The term itself is a throwback to the very early radios which were powered by batteries. Radios had "A" batteries, "B" batteries, and "C" batteries.

Why it's important is that any excessive load or short on the secondary of the transformer can cause it to fail. Why your transformer failed is something we don't know right now - it could have been a bad 5U4 or a short or loading down of the 281V B+ line. It also could have just failed on it's own by developing a shorted turn or there could have been a prolonged load on it.

Monitoring the current in the B+ line and monitoring the current on the AC input side is important if we install a new transformer and want to be sure that there is no other problem in the TV chassis that would overheat or otherwise damage the new transformer.

John

Originally Posted by JohnCT

In your schematic, you'll see the 5U4 rectifier tube.



That, along with the filtering capacitors, is what turns the AC voltage from the transformer secondary to the DC voltage that the TV circuits need. The 281V point on pin 2 of the 5U4 is called B+ voltage.

The term itself is a throwback to the very early radios which were powered by batteries. Radios had "A" batteries, "B" batteries, and "C" batteries.

Why it's important is that any excessive load or short on the secondary of the transformer can cause it to fail. Why your transformer failed is something we don't know right now - it could have been a bad 5U4 or a short or loading down of the 281V B+ line. It also could have just failed on it's own by developing a shorted turn or there could have been a prolonged load on it.

I am having trouble with black and red/black lead, when I run ac to them they hum and cause smoke. If I run hot to the red only, I seem to get more accurate readings without hum. It seems there is a short between the primary winding? What else causes a hum with just black and and red/black to black and red on incoming a/c?

[old_coot88]

You shouldn't be running AC to anything but black & red/black, nowhere else. That's the primary of the xfmr. Do this -

Remove all the tubes except the high voltage rectifier (probably 1B3)*. This removes all loads from the secondaries of the power xfmr. Now run AC to the primary (black & red/black) again. If the xfmr still smokes, it's shot.

*The 1B3's filament runs off the flyback.

Originally Posted by old_coot88

You shouldn't be running AC to anything but black & red/black, nowhere else. That's the primary of the xfmr. Do this -

Remove all the tubes except the high voltage rectifier (probably 1B3)*. This removes all loads from the secondaries of the power xfmr. Now run AC to the primary (black & red/black) again. If the xfmr still smokes, it's shot.

*The 1B3's filament runs off the flyback.

It's shot. Smokeup and sizzle with no load and completely disconnected.

[old_coot88]

99.999% certain it's shot. But just to be 100.00% certain, do this:

On the off chance there's a hard short on the heater (filament) rail, disconnect the brown & brown/black wires so they 'float' free.

On the off chance there's a hard short in the 5U4 socket, disconnect the red & red/green wires* from the socket so they float free. (pins 4 and 6 respectively.)

Apply power to the primary as before. If the xfmr still smokes, 100.00% it's shot.

*Careful. There's normally around 800 - 900 VAC between these two.

[fixmeplease]

Originally Posted by BandDirector

I don't know what the cut, loop, and crimp technique is; can you elaborate?

As to the transformer, that would be tremendous! I was thinking about rebuilding it; and, it may be worth it in the end just for the experience. I don't expect it to work once I am done with it...and since I cracked the original shell I'll have to have it machined somewhere. Not impossible.

I love the speakers, I attached my GE stereo to it and it was probably a bit overloaded on those RCA speakers, but the clarity is incredible. I'm tempted to just leave it connected to some L/C/R leads just to have Panoramic monaural sound coming from my stereo.

My transformer wont work. Voltages are off. But there were some in 1959 that will work so they made similar ones for a few years. The Thordarson one may be easier to find.

The cut loop thing I may have typed/worded/titled wrong. Somewhere on this forum there is a picture in an RCA repair book of how to do it as they recommended it. But for a quick description you crush the capacitor, peel it all apart, and keep as long of a lead as you can, clean it, make a 180 bend, put a 180 bend on new capacitor, squeeze them together and solder. All of this while not causing stress on the board. Its not fun but there are worse jobs. There are quite a few of them if you replace them all as I did. it takes patience which i have. Some people dont. i wouldnt recommend it to anyone with anger issues, lol

Originally Posted by old_coot88

99.999% certain it's shot. But just to be 100.00% certain, do this:

On the off chance there's a hard short on the heater (filament) rail, disconnect the brown & brown/black wires so they 'float' free.

On the off chance there's a hard short in the 5U4 socket, disconnect the red & red/green wires* from the socket so they float free. (pins 4 and 6 respectively.)

Apply power to the primary as before. If the xfmr still smokes, 100.00% it's shot.

*Careful. There's normally around 800 - 900 VAC between these two.

It has been disconnected completely for this last test, only power to red/blk and black. It's probably just shorted. Maybe I can I rewind the entire transformer.

http://imgur.com/gallery/GM9GuOi

Here is a view of the transformer winding. "Jim, he's dead!"

[JohnCT]

Originally Posted by BandDirector

http://imgur.com/gallery/GM9GuOi

Here is a view of the transformer winding. "Jim, he's dead!"

Of all the transformers in an old TV, the power transformer is easiest to sub from a donor. You may have to fudge some parts but if you can't find an exact replacement, you can rube up one without a lot of trouble.

John

That is what I was thinking, I just don't know how you all can figure out the other voltages when I only see 280 and 260.

Otherwise I can just mock up several and make a stage/box in which to put them.

[maxhifi]

Get a Sams manual for the TV, they usually have transformer specs listed.

[Tim]

I showed the transformer specs from Sams back in post #5 of this thread.

[JohnCT]

Originally Posted by BandDirector

That is what I was thinking, I just don't know how you all can figure out the other voltages when I only see 280 and 260.

There are only two voltages, the B+ supply of 281 and the filament supply. The filament winding is the brown and brown/black is 6.3vac and that won't vary.

The rest of the chassis voltages are based off the 281, which isn't really critical. Most old tube TVs show more B+ than when new because the AC line voltage is a few volts higher generally speaking than it was in the '50s. If your replacement transformer is a bit higher in B+, make sure you restuff your filters with higher voltage caps.

You just want to be sure the replacement transformer's filament winding can handle the current of the tubes in that RCA. More is good, less is bad.

Other than that you want to be sure the donor transformer is used in a 5U4 rectifier TV as opposed to a selenium or silicon rectifier chassis, so try to find a transformer that will put out about 280 -300 volts off the 5U4.

If you find a donor chassis, look at the schematic and it will give you a great idea it will work, or, if you're unsure, post the donor information here.

John

[old_coot88]

The replacement xfmr also has to have the dedicated 5 V filament winding for the 5U4.

That's in addition to the 6.3 V winding.