20 replies to this topic

[Stan]

I have a student slot car club at school (track build posted on the blog), and we recently upgraded to some Professor Motor controllers with variable sensitivity and brake pots. Here is a shot of one of them with one of the cover off...


From time to time, you need to replace the brake pots. And since the students (and me as well) are impatient, sending them off to PM for repair is not practical for us. But it would be the best way. Instead, I'd like to trick out this controller out so that I can run down to the Shack, grab an in-stock inexpensive 25 ohm pot, slam it in, and be back racing within the hour. So here goes! See you next post.

Stan

[Stan]

I want to thank the guys at AZGR who always give me great advice, and especially Mike, who is our electronics guru! However, they did not put me up to this!
Possibly destroying a perfectly good controller is my idea!

Here we go!

Now that we can see the frame, notice the two big silver pots, as well as a few diodes.

First, let's put our ohm meter between the black and red wires to verify what's going on with the pot.

I get an initial resistance reading...


But then, as soon as I fiddle with the brake knob, I get nothing, or an "open."

Oops... that's not good. No brakes!
Let's check some more. Put your meter across the outer two tabs on the pot...

You should get something in the range of 25 ohms- the fixed resistance of the pot. Then, try it across the middle and and right tab (those are the ones soldered to the PM board) and that will read the variable resistance... in my case, nothing!

While we're at it, let's check the diodes to see if they are bad as well. Using the "diode" setting on my meter, I'll see if there is a voltage drop across the diodes. It will occur only in one direction.

All good. Since the meter puts out a small voltage, I couldn't help myself, and just had to put the probes across the LED (also a diode) to make it light up!
I'm going to hit the Shack for a pot, and pick up a couple other items as well... a knob for it, some clear tubing, and a couple nylon washers. See you next post.

Stan

[Stan]

Back again.
Here's a shot of the RS pot next to a PM one...


Not only is the Shack one larger, but the shaft is thicker (1/4") and shorter, too! Man, life is just tough, isn't it?!

The good news is that once the mods are done, replacing the pot in the future will be "cake."
Now is the perfect time to change your mind... pack up the controller, and send it off to Professor Motor for a tune up.

You didn't do that, though, did you? OK, you've been warned!!!
First, let's remove the old pot... er "rheostat." Can I just say "pot" instead of rheostat? Good. Thanks!

I started by clipping off the parts of the tabs I could see on the board, as well as the third unsoldered tab. Makes it easier to remove it.


Don't get confused by turning the board over and over, and start hacking on the trigger pot by mistake.

The pot is soldered to the board in two places. As I started to put the heat to it, I was able to wiggle it enough to clip the middle tab off. Now I only have to really heat one of the tabs to remove it.

Gone!

Now with your iron, you can easily push out the tab pieces.
Taking a trick from my Difalco controller, I'm not going to solder in the new pot directly to the board... but to wires. So grab some heavy wire you have laying around, get the ends tinned, and solder them to the board.

This will make it much easier to install and replace the pot.
I need some coffee. See you in a bit.

Stan

[Stan]

Now, v e r y c a r e f u l l y expand the hole in the board to fit the new pot. Drill it very gently or use a reamer like I did...

With the back of the pot facing you, clip off the left-most tab...

and then the little spacer tab...

And then put a bend in the two remaining tabs...


Now let's flatten the two wires as much as we can against the board for clearance...
and solder them to the tabs on our pot...

Leave the wires just long enough to allow the shaft to be inserted. Take off the nut and washer from the shaft, and put on one of the nylon washers.
Gently wiggle and push the wires so that they do not interfere with the throttle wiper...

Slip on the other nylon washer on the other side of the board, and gently screw on the nut.


Having fun yet?
OK, we have a little more to do in the next post.

Stan

[Stan]

The big metal plate you removed (a heat sink) needs to be fitted for the larger diameter shaft on the RS pot.


This is not, particularly delicate work, so a trip to the drill press will do it.

I think used a 1/2" bit. Clean up any burrs of metal with a small file.
Now, in checking the board, I notice that if I touch where the plate contacts the board, and the new pot shaft with my meter, I get a completed circuit. We do not want this to happen when in use.


So we'll take a small piece of clear tubing,

and slip it onto the pot shaft as an insulator...

Let's install the heat sink...


Almost done. We need to clip a couple of the plastic ribs on the handle to let the new shaft stick out.


Finally, the knobs I bought were a bit tall, and would not grab the shaft well. So I cut off the bottom portion of the plastic...




Clean up the wiper and contact points with some VooDoo juice while we're in here, and put everything back.




Let's go racing!

Stan

[John Miller]

Why didn't you just buy the direct replacement brake pot from PM, don't they sell them?

[Duffy]

Read the first post, John--"Impatient" is the reason cited, but consider that this is a bunch of High School guys learning PROBLEM SOLVING. A very big deal indeed, I think, especially as we're always bemoaning how the younger generation (ANY younger generation, in my long experience) doesn't do handy stuff when they can just consume. Makes this tool-usin' curmudgeon smile, it surely do.

Duffy

[Stan]

Yes, the wise thing to do would be to order some PM replacements, or send off the controller to them. But why not mess with it a bit? Plus, the Shack pots are only $4 right now, vs the $17 + shipping from PM... and the wait...

Stan

[John Miller]

Let us know how well the radio shack pots hold up. I'd be interested in knowing as I'll be needing an inexpensive controller for club/home track purposes as well.

[Vulcan]

Nice work Stan. If I can add ...

The real name of the device is potentiometer, or "pot". Radio Shack has it mislabeled.

Potential (voltage) meter (controlled). Usually a voltage is applied to the outer terminals. The wiper can then be adjusted to give any voltage between the two rails.

Rheostat refers to a variable resistor, which is the configuration in which it is applied here using only two terminals.

Most pots are carbon film and are not intended for carrying current. Inexpensive ones fail under heavy use. Wirewounds are more robust.

[John Streisguth]

uh, actually if you look closely, the package says wire wound. So the labeling is correct..

[Ewicp]

Hey Enzo!
Just wanted to say that this is a great write-up! Your attention to detail, the "completeness" of the whole thing as well as the quality of the writing, well, it makes this a joy to read! (plus, IMHO, you did the surgery correctly)

Eric

[Vulcan]

uh, actually if you look closely, the package says wire wound. So the labeling is correct..

Right but I was referring to the "Rheostat" labeling. Stan made the right choice in getting the less common wirewound.

[Stan]

Just for fun, I ripped open the Professor motor pot...


Here's a blurry shot taken through a magnifying lens... can you see the windings?


Stan

[NSwanberg]

Nice write up and presentation. I think your students are lucky to have you as a teacher. I see it in my classes. Young people are lacking hands on problem solving skills. Life does not come with a reset button. As I look back I can see that slot car racing can teach so much about science, engineering and technology. Especially as we have merged modern day electronics and computer technology with the 60's technology the hobby/sport is based on.

[Stan]

Let us know how well the radio shack pots hold up. I'd be interested in knowing as I'll be needing an inexpensive controller for club/home track purposes as well.

Will do. 14-year-olds can be tough on equipment, as you can well imagine! A fellow AZGR racer uses the Shack pot in his Difalco with no problems. I've got 4 of the PM controllers now, and so I'm hoping there won't need to be as much plugging and unplugging of the alligator clips as before.

One of the classic student moves is sprinting off toward a crashed car with the controller still 'death-gripped' in one hand-- "Sproing!" All three wires yanked off the track, followed by the controller hitting the floor.

One way racers can help infect younger generations with slot car sickness is to offer support to any local school-based programs you may have (or even offer to help start one!). Our AZGR club members are always offering to help my student club, and have donated money, parts, and time.

Stan

[Stan]

A few students came in during their holiday to do some racing yesterday. Four hours went by in a flash! Now that we have four working Professor Motor controllers with adjustable throttle and brake pots... I've lost my slight controller advantage!





Get ready.... GO!

Stan

[Ramcatlarry]

Radio Shucks is NOT the only electrical/electronics supplier. Other such as Digikey, Mouser, Allied, and Newark are all online and you MAY actually find one that fits like the original. The brake pot should be more like 5 ohms...read the numbers on the canister.

[MG Brown]

Thank you for this great how-to. It's enjoyable reading & well illustrated.

JFYI I have used the Digi-key CT2150-ND (CTS Electrocomponents) 5 ohm 5 watt wirewound pot in 1/24 racing with great success. The price is right too if you are doing several controllers.

[phrossbyte]

Do you think this is a mod that can be done to a PM controller with no factory adjustable knobs? I recently purchased a "rental"
PM controller from my track and it is of course not adjustable. There appears to be a jumper wire connecting the two points that you wire the pots too. So my thinking is, I remove the jumper wire and use your method for attaching adjustable knobs to my cheap rental controller.

 

Think that might work?

[team burrito]

Radio Shucks is NOT the only electrical/electronics supplier. Other such as Digikey, Mouser, Allied, and Newark are all online and you MAY actually find one that fits like the original. The brake pot should be more like 5 ohms...read the numbers on the canister.

 

Yeah, but the Shack is just down the street & easier & faster than the rest.