Lionel KW Transformer Rebuild

If you are really confident that you have identified the U wire, then you're in excellent shape, because the U connection(s) are referenced to all the other wires.  Using the output panel designations as a guide, take your digital meter and take it one other wire at a time.

Plug in the transformer and keep stray tools, kids and cats away for a few minutes.

Put the meter scale at "more than" 25 volts AC.  Fasten one meter lead securely to the known U wire.  Even though we think you know that you have the U wire, first verify that assumption. Place both handles (A and B) in their "off" positions. Take the other meter lead and probe the other wires. The A and B wires will have zero or negligible voltage. Label them with a little masking tape as "maybe A" and "maybe B". Then use the meter lead to probe some more. You can find the two fixed voltages (with respect to U) and label them.

A 6 volt reading will give you the "C" post, and a 20 volt reading will give you the "D" post. Prove that by then using the meter to go between the suspected C and D posts. You should get 14 volts. Label both of these.

Go back and connect one side of the meter to "U"

While probing either "maybe A" or "maybe B" have an assistant slowly move one of the handles. Watch the meter. If you see a variable voltage, guess what? You have identified one of them. Then verify the other moving handle.

All done!

Here is an excellent KW rebuilding page, Tranz4mr’s KW Page

The attached pdf has a pictorial wiring diagram.

Hi Susan and Arthur,     Thanks very much for those great instructions Arthur and wow Susan - that pdf file is just what I needed to understand this transformer.  I can't wait to try these tips.  Pretty sure I'll get this transformer up and running in no time.   Thanks again.   Bob

Hi again - I wanted to get back to Arthur and Susan and to others that might have this problem in the future.  I was able to get most of my problems solved. I did have one of the conductors correctly identified as the common (U) which attached over the "A" connector.  Tested that connection and had the variable output from approximately zero to 12.6 vdc with the A throttle.  Then using my know good U, measured and identified the C terminal at 6.6 vdc,  same for D at 12.6 vdc, and then measuring from C to D at 19.6 vdc.  All went well except when I measured from the known U terminal to the last two conductors.  The first conductor measured zero vdc with no effect moving B throttle.  Switched to the other conductor and measured 12.6 vdc at the zero throttle B position and moving it to full throttle dropped the voltage down to 0.3 vdc.  Just the opposite of what it should be.

Earlier in my testing, I thought I had identified the second U conductor.  Turned out it was the C terminal and when I landed it on the common U copper bus and powered up the KW, it just about instantly gave me a little smoke to tell me to power down.  Lesson learned.  As a much younger person, my brothers and I never used the B throttle and probably had issues with it and forgot about it.  I did replace the A throttle contact bushing as it was totally shot.  Naturally, the B throttle bushing looked brand new so I didn't replace it.  I can get a variable voltage using it but as noted above, it's just backwards.  

I'm only using one train now but would love to have two in the future.  I would think if I have a winding open I'd get no variable voltage from these two conductors.  However, it would be common for both.  Not sure why one conductor gives me a variable voltage just opposite to what I'd expect.  Could the old disk type rectifier be causing this problem?    Thanks to all and I'll continue to read this thread and see what thoughts any of you guys come up with.   Bob

Glad to hear that you're making progress, with minimum smoke. To be clear, you should be using the AC scale on the meter, not the DC scale. You wrote DC, so I'm not sure if you are using the correct scale.

Hi Arthur,   Good catch re: dc voltage.  But, sad to say no, I was on the ac scale.  Most of my hobby work deals with digital stuff and that's all dc so I'm just used to saying vdc.  I was just looking again at the schematic and not sure what's going on.  The "rectifier resistor".  Is that just a short piece of conductor and is it the white conductor wrapped around the disk diode?  Could that be open and if so, would that cause my problem?  One thing I am worried about is when I shorted out the 6.6 vac winding - the warning lamp did not come on and the circuit breaker didn't open.  It's old but that's something that will be addressed.  Thanks for your note and will stay tuned.     Bob

If you install a Zener diode you don't need a resistor wire.  It provides a DC offset independent of the load and the engine won't speed up when you blow the horn.

I would install a 1N3311B 50 W 12V Zener diode in place of the original rectifier disc.  Then get rid of the resistance wire or store it somewhere.

John

@Craftech posted:

If you install a Zener diode you don't need a resistor wire.  It provides a DC offset independent of the load and the engine won't speed up when you blow the horn.

I would install a 1N3311B 50 W 12V Zener diode in place of the original rectifier disc.  Then get rid of the resistance wire or store it somewhere.

John

What happens if you used the resister wire with the zenor diode. Just replaced power cord on an LW Transformer and observed that I had previously added the zenor but reused the full length resistor wire(wrapped in white insulation). Should I replace that with a sufficient length copper wire?

@EJN posted:

What happens if you used the resister wire with the zenor diode. Just replaced power cord on an LW Transformer and observed that I had previously added the zenor but reused the full length resistor wire(wrapped in white insulation). Should I replace that with a sufficient length copper wire?

You shouldn't need a resistor wire at all if you installed a Zener Diode.

John

Hi Guys - Thanks for your comments re: replacing the disk rectifier with either a zener diode or conventional silicon diode.  Not sure why you're recommending a zener which I always remembered to use if I want to maintain a certain voltage. Why would a voltage consideration be important here versus using a conventional diode say at 2 amps or 6 amps as someone mentioned?

My latest revelation came just a few minutes ago when I looked at  the pdf that Susan told me about. Page 3 of the manual.  Let me say first that most of my connections had fallen off so I was just taking my best guess as where they were connected.  From the drawing, there are 7 conductors going to the terminal plate.  I had 6.  The reason being there were two conductors soldered together.  I assumed these were factory soldered.  One conductor is a heavier gauge covered with black sheathing going to the transformer winding under the A throttle. The other conductor, a smaller gauge connects to the input (center conductor) for the lamp. Based on the schematic, these two conductors should go to the C terminal.  Mine, however, go to my U terminal which is somehow working for the A throttle and correct voltages at C and D.  Strange!

I would like to verify one thing for me to start with.  Are there 7 conductors coming to the terminal plate or 6 with two of them soldered together.  AND, most important - Is there only one conductor landed at the two U points with the copper buss?  Sorry for the long note but I want to be as precise as possible.     Bob

@Corvette Rob posted:

The KW diagrams are available on the web, this one here should answer all of your questions.

Hello and thanks to all of you that helped me.  Those schematics and manual did the trick.  My KW is up and working fine now.  I do have a follow up question if anyone reads this.  Does anyone know of a vendor who sells parts for this transformer?  I'd like to replace some of them that look worn to me such as the arm assemblies and shafts.  Thanks again.    Bob

Try Jeff @TrainTender.  Welcome to The Train Tender - Internet Reference (ttender.com)

Good guy and he shows a good number of parts for the KW. If he doesn't have it he can point you in the right direction.

Eric

Thanks for the information Eric.  I'll send him a note.

@Corvette Rob posted:

Hi Guys - Thanks for your comments re: replacing the disk rectifier with either a zener diode or conventional silicon diode.  Not sure why you're recommending a zener which I always remembered to use if I want to maintain a certain voltage. Why would a voltage consideration be important here versus using a conventional diode say at 2 amps or 6 amps as someone mentioned?

Hi Bob  

I just found this thread. I also have a KW that works except for the whistle which I assume is the old rectifier disk. Did you ever find out whether the replacement should be a zener diode or a silicone diode? Do you know what the pros and cons to each method are? If the silicone diode is used I think the resistor wire is required and it is not required with the zener diode?

While I am in there I am planning on replacing the carbon rollers and putting in a new modern circuit breaker.

Thanks  

Eddie

I just went back and reread some of these posts. The resistor wire is not required with the zener diode.

I do have a question for everyone. If I went with a silicone diode then I could easily wire a DPDT switch that could be used to flip the polarity of the diode. This would allow the sound action to be the whistle or bell depending on that switch setting.

Thoughts anyone ?

Thanks

Eddie

@Eddie in FL posted:

If I went with a silicone diode then I could easily wire a DPDT switch that could be used to flip the polarity of the diode.

This is a solid plan. Go with it.

I am going to order the parts and give it a try. I will let everyone know how it goes.

@Eddie in FL posted:

I just went back and reread some of these posts. The resistor wire is not required with the zener diode.

I do have a question for everyone. If I went with a silicone diode then I could easily wire a DPDT switch that could be used to flip the polarity of the diode. This would allow the sound action to be the whistle or bell depending on that switch setting.

Thoughts anyone ?

Thanks

Eddie

Mine is a silicone diode with a sound activation button.  I ended up replacing the flimsy Lionel button with a home built one from Susan's website that also includes block protection via a PTC:

https://slsprr.net/features/SoundBlkProtect.htm

It works really well.

Note:  If you use either button all center rail power has to go through the button in series before reaching the track.  And you will have to hold down the button for at least 2 seconds which will slow down the locomotive engine.  I don't know any way around this in Conventional.

John

Hi Eddie,   I'm planning on installing the Zener diode and removing the resistor wire.  There is an upcoming train show in Bradenton this coming weekend - Feb 19 and 20.  Hope to talk to someone there who's tried this and get their findings.  I'll report back if I find anything.  Looking forward to your findings as well.  Bob