Help Mechanical speed control

[Wikman8]

Hi everyone,

Need some help. I have recently found my old RC cars at my parents and have fully refurbished the first one from early -90s. The car is a electric raider outlaw.

After replacement of mechanical parts and a new 7.2V 3000mah battery it was ready to start.

But after plugging in the battery that fully charged gives about 8.5V the resistor connected to the speed control started to smoke and got super hot.

Can someone tell me the resistors function in the speed control and what can be my problem? Is it the high current? as far as I remember my old battery back in the days gave +8V fully charged.

The mechanical speed control is the type that’s is in black plastic and have a resistor about 30mm long with 3 connections and a heat sink that has round holes over it.

[coxbros1]

The resister absorbs the voltage on medium and low so it doesnt make it to the motor, no current to the resistor on high. Not sure why it smoked, 8ish volts should have been ok, possibly damaged from age

[BattleTrak]

It did this when the speed controller was in the neutral position or when you tried to go forward? Is the resistor cracked or otherwise damaged? Does the motor and driveline spin freely?

The resistor cuts down the voltage for low and medium speeds, high speed gets full voltage from the battery and doesn’t go through the resistor. It will get hot, especially if you’re driving around slow..maybe there was some old oil and dust on it..

[Wikman8]

Thank you for the feedback. I assumed it was the voltage absorber during lower speeds. The reason for three cables are for both forward and backwards or what’s the reason?

I got some confidence now at least for another go. Last time when I had this experience I had the car with power on for about 1-2 minutes with out any high speed driving only a couple of checks that the car was moving on my way outdoor. But after this I measured it and it still has the specified resistance.

Can be oil or something on the resistor as you say. Will try again, ordered a new resistor in case it is damaged or will burn up.

[threesheds]

The resistors will get hot with current across them that's why they're big ceramic ones usually.
Are they on a heatsink? Some motors draw more current than others. Another reply might tell you how the number of turns in a motor would affect current drawn. I'd go over to a cheap ESC these days which takes up less room than the servo, speed control and resistors.

[Biggs]

Even a good vintage ESC can be had pretty cheaply these days. On my old Raider I ran a Futaba MC112B. I just picked up a vintage one that looks like it was never used, for like $25. The nice thing about this ESC, is it mounts just like a servo, usually where the servo for the mechanical controller went.