RCA 630TS Resistance Measurement

[blue_lateral]

If it works fine disconnected, it's probably ok. I'll bet the problem is hidden in the chassis in some non-obvious place.

For the trans to be bad, as far as I know, it would have to fail in one of three ways:

(1) One of the windings goes open. This would be obvious because one winding wouldn't work.

(2) One of the windings has a shorted turn. This causes a whole bunch of energy to be unloaded into that one shorted turn. If you disconnect everything like you did, and apply power to the primary, it should burn up just as bad with no load on it as it did loaded. A multimeter wont test for this, because one shorted turn wont change the resistance of a winding much.

(3) One of the windings has a short to ground. All of them should be insulated from the frame.

Of the above options, number three is just about the only one left. That, or the transformer is not bad.

If you have a dim bulb tester lash-up, you could check it with that. With everything laying on an insulating surface, one side of the power line would go *through* the light bulb to the transformer frame. The other side of the power line goes to *one* wire from any winding. When energized, the bulb should not light. Try with one wire from each winding. Obviously, don't touch anything while it's energized.

If the bulb never lights, it passes the test. If it fails the test, you still might be able to take the end bells off and fix it.

If your rectifier socket was shorted from a filament pin to ground, things would only heat up with the rectifier in the socket, because only then would the filament pins be hot with B+

When B+ comes up, the rectifier filament and winding floats at B+. If the winding is shorted to the transformer frame, that is a short from b+ to ground.

What comes next? The 80uf capacitor? Could it be backwards? What comes after that? A choke or a speaker field? Could it be shorted to ground?

I don't have a schematic for this set in front of me, but usually one of the filament windings (for the other tubes) is grounded on one side. If the other one gets grounded by a defective tube socket or lamp socket or something, you would get smoke.

If it passes the funky dim-bulb leakage test, and you're gonna put it back in, I would hook it up a little at a time. First just the HV winding, and not much will happen. Then add the rectifier filament winding. This will bring up B+, and things will probably go to hell. If they do, see if disconnecting the lead to the 80uf and B+ bus fixes it. If it's ok, proceed to a filament winding (which will put some load on B+), and so on.

By the way, if the excessive current draw is further down the B+ chain, it should be easy to find. RCA usually gives the voltages of several B+ taps. Find the first one that is way low and look there.

Good luck,

John

[Jonathan]

Thanks John. I found some solder splashes, but I'm not really sure they were shorting anything. I'll look very carefully and install the transformer back again. I'll look for the shorts to ground in the transformer, sockets, and field coil.

Thanks for the tips, they are a big help.

Jonathan

[Tubejunke]

Originally Posted by Jonathan

Tubejunkie,

My power transformer has a similar number. 940157 274/1SUB5 719
The filament of one of my 5U4's was burned open too. When there is no load on the secondary B+ winding (rectifiers out) it doesn't smoke at all, so I just don't know.

Thanks.

Jonathan

Jonathan, first I must state that although both of our sets are of similar vintage and design they are very different. They are comparable enough for us to trade notes until we both have our RCA's working well. Luckily the power supply is one of the easier circuits to understand. Again, I was worrying myself trying to get resistance measurements while in denial of the real need for a transformer.

I disconnected everything and still had a smoker. Cut and dry.. Your case Jonathan is much different. You say that you only have a problem when you add the rectifiers. So you know that the transformer is not the problem. You have a dead short somewhere after the 5U4's. Cut and dry. To anyone else I would say you probably have shorted B+ filters due to age. You have already changed out your caps if I read right. So that eliminates the filters, short of the replacement caps being bad.

I think I know what is wrong with your set Jonathan! If yours is anything like mine the negative side of all the power supply filters does NOT go to chassis/ground. One of the first mistakes I ALMOST made with an RCA was to wire in a replacement filter with negative to ground. This would have been one BIG short. I don't have a schematic for your set, or my set for that matter. I just know this from hands on experience with an 8T-243 and my beloved TC-127. Both are wired like this. The whole filter section is isolated. Check your wiring. Its a VERY easy mistake to make assuming that the - side goes to chassis.

I got my transformer and have some of my own questions but to keep our issues straight I am going to put them on my own thread.

I hope this helps get the RCA going.

Tube out........

[Jonathan]

Welp guys, I have an update. I fixed the wires on the power transformer making sure everything was insulated properly. I wire it back up the right way (made sure of it). I plug it in and nothing happens until I heard a pop. C225A (250uF, 50V)(on the RCA schematic) had exploded and let out all it's smoke. It looked like a smoke bomb, it was pretty neat. The failure was on the negative end of the capacitor. I never understood this capacitor anyway. It's positive end is connected directly to the filters after the field coil, the +280V point on the RCA schematic. It's negative end is connected to the negative end of C225B (1000uF 100V). At that point it also connects to the +275V point on the schematic. I never understood how DC current goes through it or if it's part of a voltage doubler. Can anyone here explain it's purpose a bit more?

Thanks.

Jonathan

[blue_lateral]

I just went and looked at the schematic. Here it is BTW, thanks to a thread Phil Nelson posted a while back:

http://antiqueradio.org/art/temp/RCA630TSSchematic2.gif
http://antiqueradio.org/art/temp/RCA630TSSchematic1.gif

I don't know exactly why that capacitor is there, but my best guess is that it's bypassing various trash from the Horizontal and Vertical scanning, and keeping it out of the rest of the set. Nothing looks like a voltage doubler to me.

It looks like in normal use, that cap would have 5vdc across it. However if the Horiz or Vertical centering pots opened up, most of the *unloaded* voltage of the power supply, maybe 350vdc would appear across that poor little 50vdc cap. That might have got it pissed. If it was the vertical centering pot that opened up, c225b might not be too happy either.

All the best,

John