A3977/Mach3 Direction changes randomly

[kiltjim]

I am running 3 A3977 driver boards, 1 of which is acting funny. The direction seems to change randomly.

I have switched the cards around to isolate the problem, and it seems to be the card itself. I had the machine running somewhat smoothly, and it seems to have come about recently.

If I am jogging the z-axis up/down, it will run for about 1-2", and then change direction. If I continue to run the axis, it may run for a little while in that direction, and then change.

I am running Mach3, and have changed out the resistor values specified on PMinMO's website. I have the pulse set to 5uS, as required as well. All the other axis seem to be working fine (electrically). I am not sure what it could be, since it doesn't seem to stall, just change direction.

Any suggestions?

[pminmo]

Which version? 1st try resoldering the pins associated with the direction signal. 2nd replace the 74HC14 since it should be easy to change. The last would be the 3977 itself.

[kiltjim]

Well, I followed the traces back to the A3977 with my oscilloscope. Bad news for me, it's the chip. The signal looks nice and strong, all the way up to the direction pin.

So, because I am not going to try to remove the chip and replace it, I am just going to replace the whole card. Maybe sometime after the holiday, I will have time to trouble shoot it, or maybe even replace it.

[pminmo]

Are you measuring on the actual pin of the 3977, not the pad? i.e. above the solder joint on that pin of the 3977?

Removing the chip really isn't that difficult. Take a sharp break off blade utility knife. (like a big exacto knife/wallpaper break off blade knife) Cut the pins where they fold into the plastic case by applying downward pressure. They cut pretty easy as long as the knife is like new sharp.

[kiltjim]

Yes, I traced it back to the A3977, and then measured it right on the pin, not the pad. So I thought it must be the chip. Taking your advice, I replaced the chip. It is still doing the same thing!

So, just for giggles, I replaced the 74HC14 too. Now it doesn't do it, unless I rapidly change direction while jogging. If I hold the button down, and let it go for a little while, it will continue in the right direction. If I let it stop, pause, and change direction the motor responds correctly. However, if I tap the jog button, and then hold it down, you can see the motor stutter, and change direction. Could it be a cap discharging maybe causing the 74HC14 to freak out?

The schematic is pretty straight forward, I can't figure out where the problem could be coming from.

[pminmo]

Bottom line does it run a g-code program correctly? The jog - quick key test could be keyboard, or windows issues.

[kiltjim]

No, it seems to do the same thing, but at a greater speed. I just tried to run a simple circle (I've swapped everything around, it's now on the Y-axis motor), and it just twitches. Occasionally, it looked like it was working, and then it would switch directions.

[pminmo]

I would have a tendancy to believe that you have a solder joint or trace problem based on what you have said. Which version did you build and I can give you some things to look at. Also make sure you have the timing set in the motor tune screens setup. You could have one board that has marginal filter component values.

[kiltjim]

The version I built is the one on your website, the single sided version. I checked over my traces, and everything looks okay, not to say I didn't miss something.

I did change out the resistors (R4, R17, and R18) as stated on your wiki, as well.

[pminmo]

Swap IDC cables and see if it might follow the cable. I have seen cables made that had ittermittents. If your using a socket for IC1 make sure the IC is well seated and if so consider replacing the socket. You might replace C17 and and R18 since it might be a delay issue. Also make sure it's a 74HC14, not any other of the 74 series. Make sure where the trace from R18 thru JP1 and IC1 pads is good and not a hairline short between pins. Take a magnifying glass and look at it really close....I've seen some pretty hairline strands of copper that remain after etch. it wouldn't hurt to take a soldering iron and reflow the solder on IC1 pins 1,2,3,4, JP4 pin 3, and JP1 pins. Another idea, you said you had a scope, disconnect the IDC cable and jumper pin 3 of the 10 pin connector to the adjacent ground. Connect the scope to JP1 the pad you are using, and mildly flex the board and see if the signal is solid or changes as you flex it.