Power transformer replacement??

[Chad Hauris]

The way I would do this is to remove the transformer from the chassis, put a 60 watt lamp in series with the primary. Connect to 110 volt ac. No secondary leads connected to anything. Bulb glows bright=short. Remove end bells, check for bad insulation. Bulb glows diim=short somewhere in the chassis wiring.
There can be carbon tracks on a tube socket or some other elusive issue causing a short. It's a waste of time to keep trying to troubleshoot it without completly removing the loads from it.

I don't get into the whole how much is it worth thing but you will often have to spend more to repair something than you could sell it for. That only matters if you're going to sell it, though.

[David Roper]

The criteria for collectable sets is pretty much confined to those with round tubes, especially for black & white sets. Post 1951 for black & white and maybe not until after 1960 or so for color is when sets go from truly collectable to merely interesting.

[Chimes]

Originally Posted by Tubejunke

I used the 200v. I seriously doubt that the momentary spike would damage anything. Nothing got hot

I doubt the meg-ohmeter will put out enough current for anything to get hot.

A few minutes ago I took some readings. Here are some examples. These checks are to ground. Pin 4/5U4=1,300 ohms, pin 6=1294 ohms,

This would be the hv windings to center tap. I am assuming the center tap is grounded to chassis. Since both measure within a few ohms of each other they are probably OK.

the lead that goes to the switch has 91,000 ohms.

That should not cause enough current draw to make the line cord heat up.

I get 1.8 ohms across the ac line with the filters disconnected.

That seems a bit low. What filters are disconnected? I would do what chad suggest. Disconnect all secondary leads and see if the problem persists. If so, then there are probably shorted turns in the primary.

Bill H.

[wa2ise]

" I get 1.8 ohms across the ac line with the filters disconnected."

That seems a bit low. What filters are disconnected? I would do what chad suggest. Disconnect all secondary leads and see if the problem persists. If so, then there are probably shorted turns in the primary.

1.8 ohms isn't outside reason. I just measured the primary of my RCA TV set (it works quite well, other than a weak CRT that wants a brightner to work well) from 1957 and got 2 ohms (using a DVM on ohmmeter mode across the power plug pins with the set's power switch turned "on", which presents to me the transformer primary without having to take the back off the set). The impedance of the primary winding of the power transformer at 60Hz is what limits the current to a lot less than a pure 2 ohm resistor would draw.

[Tubejunke]

Originally Posted by Chimes

What filters are disconnected? I would do what chad suggest. Disconnect all secondary leads and see if the problem persists. If so, then there are probably shorted turns in the primary.Bill H.

ALL DC electrolytics, the two AC line filters, along with everything else are disconnected from circuit. I admit that as I stated before I simply pulled the 5u4, which automatically takes 3 or 4 transformer leads out of circuit. On one pin you have to remove the smaller wire that goes to the filters. I checked for pins shorted to ground. No problems. I also left a lead going to the on/off/volume potentiometer-switch. I checked the switch for short to ground. No problem. There is a lead going to one side of the AC line connection. That is not shorted.

Someone made the point that the transformer will likely end up coming out anyway so why not remove it from the chassis. Good point. I guess I took some "short" cuts while waiting to see if I can secure a direct replacement. Sometimes I take stuff apart and get sidetracked or disgusted and it sits in pieces way to long. Or the pieces may get lost. I shouldnt be able to lose a transformer though.

Again I have a good trans in my old 8t-243 but evidently that set is more "collectable" and it is kind of dumb to scrap a good set to fix another. Also although the chassis are identical there are different part #'s on the two transformers. I don't need to confuse the situation any more with thoughts of swapping.

Thanks to everyone for trying to protect me from hastily going and purchasing a transformer to find out that it was never the problem. Last I still think that there should simply be a resistance measurement that would tell the tale of a large short. Transformer checking has always confused me a bit because you usually don't have a set measurement that you are looking for on the particular transformer. Sometimes the winding resistance is listed on the schematic. The 8t schematic is all I have and it is not listed.

[Chad Hauris]

You may find another set that has a bad CRT or cabinet that could serve as a parts set...I haven't found the task of removing a power transfomer from a chassis to be too bad. It's pretty non-destructive so you wouldn't be destroying another chassis to remove the transformer.
I have not used resistance measurements very much as a troubleshooting tool except in cases of components completely removed from the circuit.

[John Folsom]

Resistance measurements are almost never any good to fault isolate a problem in a power transformer, unless it is an open circuit in a winding.

In the case of a short, usually what happens is adjacent turns in one of the layers in a windings short together. This only reduces the overall resistance of that winding by a fraction of an ohm, which is undetectable. But the effect on the transformer is dramatic. All the power in the transformer is delivered into that one shorted turn. That shorted turn overheats and causes the symptoms you describe. But a short EXTERNAL to the transformer will cause much the same results. You need to unhook ALL the secondary windings of the transformer. For the B+ circuits, this can be accomplished by simply removing the rectifier tube(s). For the filaments, you would need to unsolder at leas one of the wires on each of the filament secondaries.

Once you have done this, apply 115VAC power to the primary, and if the transformer still groans and gets hot, it has an internal short.

[Chimes]

Originally Posted by Tubejunke

ALL DC electrolytics, the two AC line filters, along with everything else are disconnected from circuit.

No need to disconnect the electrolytics, pulling the 5U4 will isolate them.

Someone made the point that the transformer will likely end up coming out anyway so why not remove it from the chassis. Good point.

You shouldn't need to pull the transformer to find the problem. Do as John suggests, that will prove it one way or the other. If it proves bad, then you can remove it.

[Tubejunke]

Great explanation of what happens when a short exists in a transformer John Folsom. Thanks!!

About the DC electrolytics, I guess I said it wrong. I meant that by pulling the LV rectifier I had eliminated the electolytics as a possible cause of the heavy load. Still thanks Chimes for backing up the no need to remove idea.

I think I have a line on a NEW Merit replacement. Wish me luck...........

[Tubejunke]

Well I have the Merit replacement. I am going to need some help from someone who can pinpoint where each secondary leg will go. Reason being the replacement has a different color code. If I had a GOOD schematic I could look at where the various voltages are applied. For now I have no correct schematic. I have a Sams for a similar RCA and it does not show the voltages. The Merit replacement is quite a bit smaller as the end bells are not as tall, but it looks like it mounts up the same. Its a Merit P-0370 and the supplier insured me that it was a direct/drop in replacement. I hope he was right!

Even the way the color code breakdown on the box is confusing. I think it has something to do with the center tap. You can plainly see that black is 117 VAC, Green is 6.3v, yellow is 5v @3A, brown is 6.3v @2.7a. The two reds and the red/yellow say 350-350 vac @ 225 m.a.d.c.

If anyone can help pinpoint my connections and/or verify that this transformer is correct it would be greatly apreciated. Also some explanation of the red and red/yellow wires would be nice. Does this have to do with a "center tap"?

Thanks folks!!

[John Folsom]

Is your set a KCS-38 chassis?

I assume you mean the two reds are the secondary and the red/yellow is the center tap of the secondary. A center tap is just an wire connected to the mid-point in a winding. For this set, it is the return for the -120V supply. An ohmmeter can verify this, and check all the other windings and verify all the windings are isolated from each other.

The original transformer has a 6.3V 1.2A winding for the damper tube, a 6.3V 9A winding for all other filaments, except the rectifier, which uses a 5V 3A winding.

So I would assume yellow is for your rectifier, brown is for your damper, and green is for the other filaments, though you did not say what its current rating was. It better be close to 9A. If it is something less than 9A, then you can add up all the tube currents from data-sheets to calculate the actual current requirement to see if you green winding provides enough.

Good luck

[Tubejunke]

Hi John, thanks for the info. I think this thing will work out and most of what you said was close to what I had in mind. Yes the green lead is rated at 10A. Now my unit has the Brown damper lead reated at 2.7A. Will thet present a problem? As far as the "two reds" go I mean I have 2 reds and a single red/yellow stripe. I have two of every color.

I measure 1.7 ohms between the blacks, .2 between the greens, .2 between the yellows, 65.2 between the reds, and 34.1/31.2 from either red to a red/yellow.

The chassis is a KCS-34b.

Do you have a schematic for this particular chassis? I already know other models that look the same have diffferent value components. I need to find out exactly where to solder every wire. The damper, rectifier, and other filaments should be simple enough. All of the red or red/yellow still confuses me. I'm asking for quidance because I want to be sure on this and not cause more problems.

Maybe these readings and more info will help you decipher some of this stuff. Efforts greatly apreciated!!!!

Thanks

[blue_lateral]

There should be 3 wires on the b+ winding, and the different one will be the center tap. This is the red,red,red/yel probably. The red-yel will be the center tap.

So, just using the way the old one was hooked up as a guide, hook it up. What I mean by this is that the old one must have also had 2 leads the same and one different. Use this as a guide.

Another way: Hook the two wires that are the same color to the rectifier plates, and the different one where the old third wire was.

John

[polaraman]

PM me your address. I am certain that I have a Spare SAMS 93-9 for your set.

polaraman

[John Folsom]

Tiubejunke,

What blue_lateral said. The two red wires go to your rectifier plates, the red/hello0w to the -120V bus return (where your old transformer denter tap wire went).

I can scan you jpeg images of the RCA schematic next week, but you shoud take polaraman up on his offer to send you a Sams for your set.

Let me know next week if you want me to scan the RCA schematic.

Good luck!