Help repairing a DC power supply

[Jim Starbuck]

Any electronics guys out there that could help with this?

This is a home built DC transformer/ throttle that I bought second hand many years ago. It has a corded handheld control for speed and a toggle switch for direction.
It works beautifully for testing motors and locos before installing decoders. I also used it for just running some old engines that never will get decoders but still fun to watch run around the test loop.

The problem is the (ex)wife's 100 lb retriever puppy decided to chew the cord into oblivion.
I can easily strip back some of the cord and re solder if I had a clue where to connect the wires. The 4 coiled cord wires are all the same color. Everything inside the power supply is fine but all the wires were pulled out of the handheld.
Hoping someone can look at this and recognize how the electronics are put together and offer some advice.
Thanks

[peteski]

All the wires in the coiled cord are the same color?!  That is really dumb!!  (I know - not your fault). 
Looks like the design uses a relay and a SPST switch to control direction.  Interesting...

There are two 3-lead components in that circuit. One has a heat sink and the other is next to the clear cover relay.  How can you read what is written on their cases and post it here?

[Jim Starbuck]

The component with the heat sink:
8728
TIP 120

The component next to the relay:
19FK
LM340T12
7812 P+

[Jim Starbuck]

Peteski,

I could take the board off the risers and photograph the underside if you need me to.

[John]

probably 3 of the wires come off the switch ..  with the center wire being the one that eventually goes to the potentiometer ..

[peteski]

John, the switch is a SPST switch and it only has 2 terminals. 

Yes, even though the white printing on the top of the board shows where the copper traces are under the board, seeing the underside will be very helpful. Also, while you have the circuit board out, take another closeup photo from the top and from all 4 sides (at about 45 degree angle to show the board surface and the components).

The TIP120 is a power transistor and the 7812 is a 12V regulator.  You mist have had this thing for a while - I think that 8728 is the date code ( 28th week of year 1987!).
The wiring is a bit unusual (especially the way the transformer's secondary winding is wired up).  It is rated at 28V 4A and I suspect only half of the center-tap winding is used (14V 4A).

If you can, please select "Inline full-size image" in the pulldown to the right of the file name - that way the photos will show up full size (the way you posted before each thumbnail has to be clicked on to show a full size photo - that to me is annoying). 

[Jim Starbuck]

Hope these are better pics. I tried to show the location of each of the 4 coiled wires. One is connected at the back of the regulator and another is at the end of the relay socket. The other two are amongst the resistors.
The pic of the backside has the heat sink at the bottom for reference.

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[peteski]

Excellent! I think that should be enough to solve the puzzle. Give me some time to do some reverse engineering. 

EDIT: Actually, I need some clarification.  On the TIP120 (looking at it from the front, where you can see the writing on it), I see that the left lead connects to a resistor. Where is the center and the right lead connected?

Also, could you gently lift up and slightly separate the bundle of red and blue wires around the relay and take more photos, so I can get a better picture where those all connect?

[peteski]

Figured it out. That is the type of puzzle I like.  Switching puzzles - not so much. 

Here is a diagram to show you which wire goes where.

I also noticed that the white wires at the circuit board were marked with black marker (2 and 3 marks) but since the handheld side does not have most of the wires, that wasn't helpful.

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[Jim Starbuck]

Thanks a million Peteski!
  to the rescue

[peteski]

LOL, you're welcome Skytop 47!  Make sure to post whether I was correct or if something smoked....



Just kidding - it should work fine. 

[Jim Starbuck]

Peteski,

I wired everything back together tonight.
We're close but not quite there yet.
Checked and rechecked continuity and all is connected per the diagram.
The toggle operates the relay properly and reverses polarity but the output is off.
With the pot turned all the way down, output still reads 11.43 vdc. With the pot turned all the way up, output reads 16.12 vdc.

Thanks,
Jim

[peteski]

Hmmm... that doesn't make sense to me.

If the relay works that means I have B and D correct. B is the "raw" positive DC voltage (16V) and the red wire in the handheld also connects it to one side of the pot (which is on the max throttle setting side).  Then I have the ground (or negative) A hooked up to the opposite end of the pot (min throttle setting). Then the center pot lead (the wiper) is hooked up to C, which is the control (base) of the TIP 120 transistor (which is what controls the output voltage).  So the pot's wiper can be anything from zero to 16V (depending on the position of the knob).  That should work.  When you are testing the voltage do you have any load (like a DC locomotive or a 12V light bulb) placed on the output of the throttle?  It might give wrong readings when there is no load on the output.

Sounds like you have a multimeter handy.  Can you measure the voltage between A and C (both in the handheld and on the main circuit board) while you turn the throttle knob?  The voltage should read from close to zero volts (at minimum speed setting) to 16V at max speed setting.

I could have you reverse A and C, but that doesn't make sense (and if you turned the knob to maximum that would burn up the pot, because that would create short through the pot between ground and 16V).

The resistor color codes are hard to see in the photos. Can you tell me what are the color stripes on all the resistors?  Also, on the bottom of the pot, is that "5K B"?

[Jim Starbuck]

I'm going to replace the pot before we do much fiddling with the wiring. Your explanation makes perfect sense and I'm sure is correct.
I assumed the pot was OK but I think it was damaged by the dog. As I rotate the knob which used to be very smooth it now intermittently "zips" and doesn't feel right. Also when the meter is on the output and I turn the knob, it gives wonky readings instead of a nice steady count up or down.
Just a guess, I think the knob was forced past the end stops and the wiper is damaged.
Does that sound feasible?

Oh yeah..I did connect it to my test loop with a train set loco on it. Power on and the loco takes off predictably at a pretty good clip. Haha I've got full throttle and fuller throttle.

[peteski]

That pot is pretty sturdy so I'm surprised that the dog damaged it. But sure, I guess it is possible.

And of course, if that pot is damaged, that will mess up the throttle voltage control.  I'm curious - do a quick test: Use a test jumper (if you have one) or just a screwdriver blade and short out terminals A and C on the pot. Make sure the knob is set to the minimum speed before you do that). That should make the throttle output go to the minimum setting (off).

Also, did you make sure that wire A is not open somewhere else in the curled cord?  If that wire is open then the throttle will never turn off (but the direction relay will still work).

If you do get a replacement, get one with linear taper (that is the "B" shown on the back of the pot's case).  The other common taper is logarithmic "A" (which stands for Audio taper).  That one has a non-linear resistance vs. she shaft rotation angle and doesn't work well as a speed control.