MC60 treadmill speed control for Belt Grinder

Thread Starter

Aardvark63

Joined Jan 6, 2024
6
Hello folks, kinda basic at electronics, but still have all my fingers. Attached the circuit i found for my 2.5 hp 130v motor wired to MC60 128957 board from the donor treadmill, i can get it to run like a scalded cat, but potentiometer will not adjust speed at all, have tried both a 10k and 100k pot, neither is able to adjust speed. Both I have also swapped H & L inputs, still nothin but a full speed 7100rpm and a lot of sparks. Checked thru all the threads here I could find, but nothing sparked any clues for me. Anyone give me a hand on what i expect is an easy answer?
 

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MisterBill2

Joined Jan 23, 2018
19,374
The very first thing will be to verify that the connections to the speed control pot are correct. I see a cobbled connection scheme that could easily have an open circuit that would command full speed. AND we should get a view of the connections at the PCB, because guessing is not a good way to go. Verify the connections before attacking parts. YES, a shorted bridge rectifier could be the problem, but first verify that the control signal is correct.
 

MisterBill2

Joined Jan 23, 2018
19,374
Take a good look at the wiring of that control, as seen in the pictures. THAT is why I suggested what I suggested.
Checking the SCR for a short circuit can easily be done with the power off and the motor disconnected.
 

MaxHeadRoom

Joined Jul 18, 2013
28,820
The first image in the OP explains the connection is correct.
With these, the pot has to be taken to zero before the motor will run, a safety feature to prevent switching on at full RPM capability.
I have fixed several of these and they are relatively easy to fix with common problems,
There is 2 SCR's and three rectifiers in the bridge circuit.
 

Thread Starter

Aardvark63

Joined Jan 6, 2024
6
Thanks for all the input, i agree it was a tangled mess. I cleaned up all the wires, ensured H on the pot in proper spot on the 10k pot, and switch and everything ran properly, no taped connections, solder or connector only. Turned on, motor ran to full and i took pot lock to lock, but still sparking and fuse blew (20a shop outlet). This indicates a board problem? Going to go back thru the links above to see if i missed anything. Appreciate the feedback!
 

Thread Starter

Aardvark63

Joined Jan 6, 2024
6
Saw something in a thread, R42 clipped from manufacturer. Was pulling the hot melt from board edge that locks the board to frame, and under the glue found the resistor clipped. Maybe my issue?
 

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MaxHeadRoom

Joined Jul 18, 2013
28,820
Not always, there was a modification such as that for different options/motors fitted.
There is also a resistor that can be clipped to provide starting at any pot position.
 

MaxHeadRoom

Joined Jul 18, 2013
28,820
First remove the triac opto, see if it results in no output. It is a common source of the problem.
When refitting or replacing uses a DIP socket.
If the problem persists with opto removed, check all 5 devices in the Triac/diode bridge.
You can use a lamp in place of the motor in order to take it easy on it.
 
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