LT40HD Power Feed no speed control

[Elec-Tech]

Hi all.
I have a problem with an old LT40 power feed control, here's the back ground:
1. Fwd motion was erratic and went from slow to slower as speed control adjusted to faster.
After checking connections and cabling to the speed control unit, I discovered that -ve supply was non existent and 12 v +ve was good. I replaced them both anyway with required csa wiring as well as cleaned the terminals.
I also checked the potentiometer and found the ohms readings increased in certain spots when adjusting toward the speed increase, obviously worn on the most used settings, so I replaced with another 1k ohm pot.
I also replaced the two caps on the pcb as these were con-vexed and looked in bad order.(although they did still operate on a cap test with my dvm)
I also removed the diode and tested that, and it passed.
I re-assembled and tested, feed now works great, no erratic behavior, but alas I cannot control the speed, fast only!
Any ideas on what I should check next? I've studied the wiring diagram and everything seems to be in order.
Thank you in advance.
Jeff

[VictorH]

I just replaced my potentiometer and that fixed my erratic forward progress.  It sounds like the new one you installed may be bad - have you tried the old one to see if you can adjust even if it is erratic just to see if that's the problem?

btw Welcome to the forum!!  What year is your unit mine is a 1995

[uler3161]

Sounds an awful lot like what just happened to the guy we sold my dads mill to. We found out both circuit boards were messed up. I swapped him the boards out of my mill so that he could saw. I couldn't see spending the money on new circuit boards (About $350) when I could just replace the components on the boards.

I was able to fix the mosfet board with a couple new diodes. The pwm/feed board got all components replaced, but I think I must have purchased an incorrect component. There is a TIP145 darlington transistor and I bought a TIP147 because somewhere I saw it was listed as a replacement number. The pwm chip seems to be producing the correct signal as I turn the pot.  However, the output from the darlington output jumps from 0 to around 5v when the pot gets to around 20%, then continues on like I'd expect.

If the LT40 you are working on is the same vintage, you might do the same tests I did. One would be to test voltage on the green wire coming out of the pwm board. That tests the one side of the darlington. The other side of the darlington is connected to the pwm chip. So if you check voltage on that as well, I think you should be able to compare both voltages as the pot is turned. I believe the green wire should gradually gain in voltage, but I think the pwm chip gradually decreases in voltage as I recall.

[Elec-Tech]

Cheers for the replies, I will get back down the yard tomorrow, and try your suggestions.
I've also read that the speed is controlled by pulse width modulation; is it possible to replace the existing pcb with an after market P.W.M unit rated at 12 volt dc @ 30 amp continuous load?
I believe the mill is a 1989 vintage, but I will confirm.

[Magicman]

Welcome to the Forestry Forum, Elec-Tech.   

[uler3161]

Cheers for the replies, I will get back down the yard tomorrow, and try your suggestions.
I've also read that the speed is controlled by pulse width modulation; is it possible to replace the existing pcb with an after market P.W.M unit rated at 12 volt dc @ 30 amp continuous load?
I believe the mill is a 1989 vintage, but I will confirm.

Same year as mine, so probably the same exact setup.

I'm not an expert at electronics, but I'm not sure you have to worry about the 30 amp load. I think that goes through a different component and the pwm board just controls it. I think originally they just a transistor and there was a later upgrade to a mosfet board, which is what I have.

I thought about maybe adapting an Arduino to it since it can generate pwm. But I'm not sure what other components I'd need.

[backwoods sawyer]

Before I upgraded to the Accuset 2 I went thru several mosfet boards. After taking a close look at it I added an 1/8 piece of aluminum in the gap to trasfer the heat away as the gap creats a heat chamber, never changed another mosfet.