Help with Problem X axis Stepper Please

[howser37]

Ok im gonna have a go at this repair tomorrow but just want to check a few things first with you guys as my electronics knowledge is minimal lol,so a few questions.

1. For the track jumper repair i can only find/source solid core wire with 0.6mm core unless i use something ridiculously thick like a core from lighting twin and earth, however im slightly worried by the max amp rating of 1.8 amp for the 0.6 solid core wire as i am not sure how much current goes through this track. Would i not be better using say 3 amp stranded wire?.

2. I would like to ground the heatsinks to avoid the need for the micah washers to isolate the driver chips agsinst the heatsink. However is this simply a case of grounding the mounting bolt of each heatsink to the case via maybe one of the standoffs on the corner of the pcb?. Or ground it to where the incoming earth is grounded to the case? Or is it more complicated than that?. Grounding seems to be a bit of a dark art reading about it on some threads.

3. I want to check the wiring from the db9 to the stepper motor as suggested for a short or breaks etc, is there a method for doing this, i have a multimeter but ive not much idea how to use it other than the continuity test.

Thanks
Adam

[GND]

Hi Adam,

To answer your questions;

1. I had suggested solid core wire simply because it is easier to work with on the back of a pcb, but your concerns about ratings are legitimate, and so I'd say try the stranded wire and see how you get on. Strip the ends, twist the strands together, and then tin them before you try soldering to the board. Sometimes stripping a bit overlength is good, so that once tinned you can trim them right back to something that will neatly sit on the PCB pad. All you need is a couple of mm exposed at each end ideally for the actual repair.

2. Grounding is a whole subject in itself! In simple terms, the issue is that wires and connections aren't perfect, and don't have zero resistance. Hence although you might connect things together to form a common ground, there are always tiny differences in voltage across them. This is especially the case when large currents are involved, of if there are multiple paths between the grounded items. And since ground is your reference for everything, then this can cause issues in operation. Hence care has to be taken as to how you handle grounds, and that is why it's a big deal. In your case, I know the mica washers are a pain, but the original designers specified them for a reason. I'd be reluctant to change things and ground the heatsinks, even if it looked like it should be possible. With a system that is already a bit unreliable, I think such a change would be very unwise.

3. I guess we're looking for short and open circuits on the interconnections to the motor. So, to check for open circuits, put the meter into "Ohms" mode, and select a low resistance range (if ranges are selectable!). Disconnect the motor, and probe each end of the cable assembly on matching pins, and check that the reading is low - ie. there is a connection. Should be one or two ohms max. I'd also flex the cable when doing this - although that might need three hands! - to make sure there isn't an intermittent fault. Try and test as much of the connection as you can, so start on the PCB at one end, and finish at the motor connections themselves if at all possible. Access to connections may however be limited, so you may need to compromise this, but the closer to the PCB and motor you can test, the more of the system you are actually checking.

Open circuits - ie. broken connections - are one thing, but since you are blowing your chips, the short circuits are the more likely culprit, as these are more likely to cause large currents to flow. So you have to try and find where your motor wires are shorting to. Here you should again disconnect the motor (to avoid its low resistance coils giving you false readings) and then pick a motor drive pin and apply one of your meter probes. Then touch the other probe onto each of the other motor wires in turn. You should not see very low resistance readings, otherwise it indicates a short circuit. Again flex the cable when doing this, especially near the connectors, as the fault is likely intermittent. If all are OK, move onto another pin, checking if it is shorting to any of the others. If the cable is still connected to the PCB, you may get erroneous readings due to the driver components, which sit across the motor connections - especially protection diodes - so you may have to just check the cable itself, totally disconnected. See how it goes....

Hope that helps!
Graeme

[Robin Hewitt]

Looks to me like that track had a problem, someone tried to fix it by gobbing solder over it, found the solder wouldn't bridge the gap until some of the tin had boiled out of it, so he delayed and destroyed the glue holding the track down to the PCB.

After 30+ years soldering for a living I could fix that in a jiffy. OTOH I can remember what it was like when I started so I feel your pain