WM Power Feed Circuit Board

Bodger · April 22, 2011, 06:17:01 PM

Has anyone successfully replaced a capacitor on their power feed circuit board? About ten years ago one (there were two) must have gotten hot and dropped off. It's been running fine so I haven't messed with it. Early this week the engine started missing and backfiring when the forward switch was engaged and the speed control increased. Thought it might have been the potentiometer but it does not carry much current. Opened the control box and wiggled the remaining capacitor and it (temporarily) fixed the problem. I pulled the board today and resoldered the connection. Still having the problem. Here is a quote from Sparks back in 2006 "If this mill was built between 1992 and 1995 and it has the original power feed circuit board you could be shuting down the ignition system. The old style board would have one of the caps go bad which caused the board to bleed off current effecting the coil. As you increase the potentiometer the engine would begin to missfire. You would not see it in reverse because it is not controlled by the board. " It could be a short in the 'trace' the copper path on the circuit board but more than likely the capacitor. Sure would like to fix this rather than shell out $350 for a new one.

Magicman · April 22, 2011, 07:44:02 PM

No I haven't, but I absolutely would try. You have nothing to lose and everything to gain.

Bodger · April 23, 2011, 11:48:50 AM

The remaining capacitor is a 470uf63v - like anything else I'm learning more as I go. WM says they don't even have the diagram of this circuit board on their computer so they can not tell me what the other capacitor would be. My mill is a 95 LTHD40 w/24onan. It's the board without the lights. If anybody would be willing to crack open their control box and give me the numbers off their capacitors I will try to find some way to reward you. Thanks

Brucer · April 23, 2011, 08:03:31 PM

If it were me I would try replacing the capacitor -- not much to lose if it doesn't work. I can't help you with the size, but I can give you some pointers.

It's the capacitance that is most important. The voltage is the maximum voltage that a particular cap. can withstand. It's a good idea to give yourself a nice margin on the voltage -- 16 volts at the very least in a 12 volt circuit. A bit more is good.

When you start to get up above 1 uF you are probably dealing with an electrolytic capacitor and these are polarity sensitive. You 470 uF cap is almost certainly electrolytic. The cap. will usually be shaped like a small can and will have a + and or a - sign along once side. The trick is to figure out which pin goes where on the circuit board. A lot of circuit boards will have a small + symbol etched onto the board next to appropriate hole.

If you install an electrolytic cap backwards you'll usually get a nice little "pop" (or a "bang" if it's a bigger one) when you power it up. Remember how it was installed when you put in the replacement ;D.

pineywoods · April 23, 2011, 10:38:47 PM

Bodger, my mill is a 95 also and I know it still has the original board. I'll pull the cover tomorrow and have a look. Most likely, the remaining cap is shorted and will need to be replaced. The actual size in ufd probably isn't that critical, it's probably just a filter to smooth out the pulses going to the feed motor.

Brucer · April 25, 2011, 02:25:30 AM

I don't think they would want to smooth out the pulses. That would defeat the purpose of pulsing the voltage.

Lowering the voltage is a great way to slow down a DC motor, but the torque also goes down in proportion to the voltage. That means at lower speed settings your motor wouldn't have enough "push" to move the carriage.

The idea of Pulse Width Modulation (or PWM) is to send a series of on-off pulses. Each "on" pulse is at maximum voltage so the full motor torque is available. The proportion of time that the pulse is on determines the speed that the motor runs at. Basically the motor rotates in a series of short jerks. Momentum tends to keep things moving smoothly.

On the newer machines you can hear the high-frequency "whine" generated by the PWM circuit.

Bodger · April 25, 2011, 07:39:14 AM

OK, I've heard that whine noise...I figured it was some kind of indicator, like the beep-beep on machinery backing up. When I started to have the problem with the control panel that noise was not there. I will know this morning if I can get the capacitors locally. Gotta get back up and running. Sometimes it's better just to replace and keep working. Thanks for the information on the PWM. That explains why there has to be a circuit board.

Bodger · April 25, 2011, 02:55:04 PM

Alright, I picked up two of the capacitors (@ $.75 ea) and replaced the one I pulled and also put one back into the slot where one fell a few years ago. Mill seems to be working like new. I tried it without the capacitor and the head would not move forward now it moves smoothly. According to what you said, Brucer, with the two in place I should have better torque from the motor. Now this doesn't mean it won't blow up tomorrow, however, today the $350 circuit board is fixed for $1.50. That's pretty good for me, when I bought this thing 16 years ago I was afraid to touch anything on it. Now I've been into every part and tweeked it to suit me. WM was as helpful and encouraging as they could be, thanks to all .

Slabs · April 25, 2011, 08:05:18 PM

Congratulations Bubba!!

It was a cap (Tantalum) that caused the one on my bench to go messed up. Only thing is that it took out a diode in the process but it was because the tantalum shorted, not opened.

Great to hear you got it working.

pineywoods · April 25, 2011, 08:35:24 PM

Congrats, glad you got it going cause I can't be any help. I pulled the cover off mine, and apparently it's different from yours. Only 1 cap mounded on the board and it's quite a bit smaller than yours.

Brucer · April 25, 2011, 08:51:59 PM

sgschwend · April 25, 2011, 08:55:10 PM

Glad to hear that you are back going again. But I am concerned that the solder became so hot that the parts fell out.

Basic solder can hold those capacitor up 230F. If the temperature goes any higher the parts will not stay on the board. The heavy ones would fall out first, likely why those larger caps came out.

I suppose you could apply a higher melting temperature solder, but that would stress the parts. Many hobby printed circuit board designs did not take into consideration the resistance of the traces, making them too small and therefore the traces become hot. I have built a few of these type of boards and often there is a recommendation in the assembly instruction to add wires on top of certain traces to reduce this problem.

So if you run into problems again you may want to add a few wires too.

Bodger · April 27, 2011, 04:30:45 PM

Today was the first chance I had to saw all day since the repair and the mill ran great. No sputtering or cutting out, so, I guess the problem was the other capacitor. Now it's time to ramp up for replacing the motor...my Onan has about 6000 hrs. I've found a Kohler 30hp with about 40hrs... diagnosis is 'washed rings' but it's not blowing any oil or smoke so it may be something as simple as the governor or the carb.