RCA 630TS Resistance Measurement - Videokarma.org TV - Video

Jonathan · #1 11-09-2006, 12:12 AM

Guys,

I started work on my 630TS B+ to ground short. I completely removed the work I had done and began to rewire things convinced I had something wired wrong. From B+ to ground the resistance was 10 ohms. From B+ to -100V was also 10 ohms. I had -100 shorted to ground somehow. But here is my new problem. From the one side of the field coil it connects directly to B+ from the rectifier tube. At this B+ point, two 40uF capacitors in parallel is connected to it and -100V. Now with the new rewiring, I chose to use an 80uF so not as much space would be needed. The -100V point is the center tap of the B+ winding on the transformer. So I use this center tap connection to connect everything to the -100V point. Now for the problem.

The B+ has the 80uF capacitor on it to the center tap. The one end of the field coil is connected, but the other end is not. Nothing past the other end of the field coil is connected. What I need is the resistance measurement from the center tap of the B+ winding on the power transformer (which is also the -100V point) and B+. I measure 10 ohms and it doesn't seem right. Both ends of the B+ winding measure 18 ohms. Can someone tell me what the resistance should be from -100V to B+?

Thanks.

Jonathan

Chimes · #2 11-09-2006, 03:04 PM

Disconnect the B+ from the pin 8 of the 5U4 rectifiers then measure from B+ to center tap and to ground. Measure from pin 8 to center tap and to ground.

John Folsom · #3 11-09-2006, 03:26 PM

Jonathan,

The resistance from B+ to ground should be approximately the sum of the bleeder resistors between B+ and ground, which is about 8k ohms. Likewise, from B- to ground should be about 500 ohms, and from B+ to B- about 8.5K. 10 ohms is WAY to small!

First, remove the 5U4's, and measure from the filament pins to ground (or -100V, it does not much matter, you are looking for thousands of ohms verses 10 ohms). If you still get 10 ohms, you need to start unhooking wires in the B+ B- distribution to fault isolate the short. An obvious place to start would be to unhook the low side of the field coil, and measure from the 5U4 filaments to ground. If you still have 10 ohms, the fault is either in the field coil, the electrolytic(s), or the secondary circuit of the transformer. And if not, the fault is downstream of the field coil. Continue to unhook portions of the B+ and B- circuitry to isolate the fault. Just follow you nose, and you will find the fault.

Jonathan · #4 11-09-2006, 05:08 PM

Thanks guys, you were a great help. I measured B- (or -100V) to ground and gave me around 500 ohms like it should. I measured B+ (from the rectifier filament to ground and it measured infinite, because I don't have the rest of B+ connected to anything yet. Total resistance between the two ends of the B+ winding is around 23 ohms.

Everything seems normal there just as you guys said. I measured the filament resistance of both rectifier tubes. One reads 0.3 ohms. No filament to plate shorts as both plates read infinite from the filament pins. The other rectifier reads an open filament. Obviously this one is bad. Is the other one good with those resistance measurements?

Thanks.

Jonathan

John Folsom · #5 11-09-2006, 07:01 PM

The best check is to use a tube tester on the 5U4, but if the filament is ont open, it is likely good.

*Bill Cahill* · #6 11-09-2006, 09:22 PM

In those sets the B+ Resistors tend to short to chassis when powered up. Disconnect all connections to the bank, and,check eat point of resistor to chassis. Anything less than infinate, you have a shorted resistor. Since most of these resistors are connected together internally, you will need to replace all of them. Bill

Jonathan · #7 11-10-2006, 03:24 PM

Guys,
I'll put in a good 5U4 and see how it goes.

Thanks.

Jonathan

Tubejunke · #8 11-11-2006, 01:23 AM

I am starting to see that the power supply was the weak link in the old RCA's. I have two with that basic chassis and both had major power supply problems. My TC-127 transformer problem is not over yet. Bill's comments about the B+ resistor being a weak spot made me wonder about my set. In my case though I have disconnected the secondary leads and still have a howling trans which should rule out the B+ resistor being the culprit. Nothing is hooked up but the primary circuit. Minus A.C. line caps.

Jonathan · #9 11-16-2006, 07:56 PM

Well, I rewired it, no shorts. It's connected exactly as the schematic. The AC switch and volume pot, as well as other pots I cleaned with some Ecoline Contact Cleaner. They work perfectly now. Stuff cost $22 for a 12oz. can and has a warning "For professional or industrial use only." When it says that you know it's good stuff.

Anyway, all your resistance measurements help greatly. With everything rewired, B+ to ground is 6k ohms, B- to ground is 580 ohms. Just as it should be. I've heard the horror stories about the bleeder resistors but I was lucky to get a set with all the bleeder resistors with tolerance. I know Phil had issues with his bleeder resistors and know that rebuilding that section can be a royal pain.

Now my next step is to let the contact cleaner dry (it evaporates fast, but just to make sure), pull a good rectifier tube out of my Air King chassis and give it a try. I removed all H output and oscillator tubes so I'll be testing it for sound and let it play a while with sound. I'll keep you guys posted.

Thanks once again guys.

Jonathan

Jonathan · #10 11-16-2006, 10:23 PM

Well, I plugged it in and I got smoke from the power transformer. I had my meter attached to the B+ and +275V points. B+ was about 380V, the +275V point off the bleeder resistor was about 340V. I don't leave it on long enough for anything to come out of the speaker. Right now I'm stumped because nothing gets hot, nothing else burns expect for the transformer. The voltages seem right. Other than a bad transformer, what else could be wrong?

Thanks.

Jonathan

Tubejunke · #11 11-17-2006, 10:14 PM

Hey Jonathan, you and I seem to be in the same boat. I found that if I worried too much about trying to verify my problem through resistance readings then I was driving myself crazy. I don't know if you have read my similar thread about problems on my TC-127 but my conclusion was that if I was getting smoke from my transformer with everything on the secondary side disconnected then I should start looking for a new transformer. Its pretty cut and dry. With me I just did not want to find out that the 57 year old transformer was my problem.

As I said earlier in this thread, I think that the power supply was the weak point on these early RCA sets. My smoking transformer may not be an original RCA part to begin with. It bears a 970972-1-3/138012 stamping where as my older 8T-243 has a painted on RCA logo and part #. That transformer works even after handling the load of DC filters that were gone enough to flash open the 5u4 filament. So maybe my bad trans has already been replaced through the years. Who knows?

The good news is I found a Merit replacement and should receive it next week. Wish me luck!!

blue_lateral · #12 11-18-2006, 01:07 AM

Quote:

Originally Posted by Jonathan

Well, I plugged it in and I got smoke from the power transformer. I had my meter attached to the B+ and +275V points. B+ was about 380V, the +275V point off the bleeder resistor was about 340V. I don't leave it on long enough for anything to come out of the speaker. Right now I'm stumped because nothing gets hot, nothing else burns expect for the transformer. The voltages seem right. Other than a bad transformer, what else could be wrong?

Thanks.

Jonathan

It does sound like it's toast, but if you have nothing to lose, disconnect all the wires, yes unsolder them all completely, then hook power to it. A dim bulb tester would be a good idea.

There have been cases where people thought a power transformer was shot, only to find out there was leakage through the rectifier tube socket or something. Remember that one of the filament windings is probably grounded. There have also been things like shorted dial light sockets...

Finding out it's ok would be a nice surprise. As Chad suggested in some other thread, even if it's bad you still might fix it by taking the end bells off, and re -insulating the leads inside with heatshrink tubing.

Good luck,

John

Jonathan · #13 11-19-2006, 06:56 PM

I removed both rectifier tubes and plugged it in. After being on for 3 minutes, there is absolutely no smoke. Nothing getting warm. I measured the voltage on the B+ winding, it's 760VAC, unloaded, of course, and 380VAC at the center tap. Again, no smoke. I'm almost 100% certain I have everything wired the way it should be. But when the rectifiers are in I get the proper voltages, don't I? Around 380VDC at B+ and about 340VDC at the 275V point off the bleeder resistor. Think it's the rectifiers, the power transformer, or again, do I have something wired incorrectly?

Thanks.

Jonathan

Jonathan · #14 11-19-2006, 09:04 PM

May I also add, on my transformer, the B+ winding is 18 ohms, this sound right? The schematic says 28 ohms, think it should read this low?

Thanks.

Jonathan

Jonathan · #15 11-20-2006, 01:36 AM

Well, I disconnected every wire (labeling everything of course) and removed the transformer. It's quite amazing how light the chassis feels without the power transformer. I measure the B+ winding and it reads now 22 ohms. Still seems a bit low, you guys think it is? If the trafo really is zapped, what is the part number for the merit/thordarson replacement?

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