A sign of dirty connectors?

I'm not sure of what contacts and connectors you're dealing with, but consider the solder to them may be bad. The wear and tear on a flakey connector (if that's the problem) only goes up when you fuss with it more.

Bad connectors can get hot, if they're handing enough current. I've seem them go up in flames once the arcing starts. There's nothing like scrubbing black soot off circuit boards after the fireworks show is over.

Not surprisingly discolored, or melted looking connectors are always a warning sign.

Intermittent problems with heat still point to connection problems, even if on the PCB itself, and not the connector.

I fixed one board recently in a timer board that was "intermittent" that had only 7 broken and bad solder joints. How it worked at all was a mystery.

The connector in question is an edge connector on the circuit board. It plugs into another circuit board mounted connector. I will check for solder joints that are bad, that's a good idea. I hope the bad solder joints aren't on the board it plugs into. It's the board that all the other boards plug into. Eric

Does the problem board have any mechanical support other than the edge connector? Is this support aligned correctly and connected? (Example: the bracket on most PC expansion cards that screws down to the chassis.)

You might also look for metal debris, oil, old grease, dried coolant, etc, on both sides of the problem board and on the "motherboard" that it plugs into. This may mean removing the motherboard from the cabinet, which is generally a giant PITA. I know *you* would never run the mill with the cabinet wide open, but you have no idea what the low-down so-and-so who owned it before you did. :)

I like Cydrome Leader's idea of possible bad solder joints. It could also be that the edge connector on the motherboard has one or more contacts that have lost their springiness. This is partly determined by eliminating other causes, but if there are other cards with the same connector, you can get an idea by unplugging one of those cards from its slot and reseating it. Compare the force it takes to do that with the force it takes to do the same thing with the problem board. If the problem board is way easier to remove and replace, you might suspect the connector. If you conclude that the connector is bad, usually you can replace just the connector, but this may involve unsoldering a bunch of pins.

If it's a bad connection, you might be able to induce the problem by poking at the board(s) with a nice long dry plastic stick, as long as you're reasonably sure that there isn't 120 V or 240 V running around the board. Have a hand on the e-stop and/or circuit breaker when you do this, just in case you manage to send a signal for "drive the table 20 feet that way and turn on the spindle to 100,000 rpm" by poking.

If removing and reseating the board is an instant fix, I think this points heavily in the direction of a bad contact somewhere. If you had a failing component, usually you would have to wait for that component to cool down enough to start working again.

I like this idea less, but here it is: if the sensor is a relatively high-resistance device, a couple of things could be happening. 1) Some foreign glop on the board in exactly the right (wrong?) spot has changing resistance with temperature, and eventually gets low or high enough to trigger the alarm. This wouldn't tend to go away just by unplugging/replugging, though. 2) The alarm input, itself, has a high impedance, and eventually drifts to a high enough voltage to trigger the alarm. This *might* fix itself by unplugging/replugging.

Sometimes if there is a sensor that your model doesn't have, the input for that sensor will have a jumper wire or resistor across it as a dummy. Can you tell if anything like that used to be there and is now missing? You might have to compare to photos of a similar machine - this may or may not be in the manual.

You might try taking the problem board out of the machine and sticking a clean piece of stiff paper, like a folded index card, into the slot in the edge connector that is still in the machine. Cut the paper to the same length as the card edge connector - don't use a shorter piece of paper and slide it back and forth, because you may catch one of the contacts and bend it. It may also help to wrap the paper around a thin piece of material (sheet metal?) - but you want the whole stack to be no thicker than the circuit board that belongs there. Remove the paper and inspect for signs of glop or corrosion in the edge connector.

Dirty contacts will heat up if there is enough current going through them. However, I would guess that a board that monitors pressure sensors (and probably interlock switches and stuff like that too) is running on relatively low voltage and current. If there was a board driving the servo motors or coolant pump or something that takes relatively more juice, then it would have a worse time with dirty contacts.

If you can identify the low-voltage DC power supply (probably 5, 12, or 24 VDC, but it could be a lot of things), it probably wouldn't hurt to measure its output voltages with a multimeter, both when the mill is working OK and when it is throwing alarms. This probably isn't the problem, but low (or high) power supplies can cause many weird effects.

Matt Roberds

Likely there is a rack of opto-coupled sensor modules. The lube and air pressure sensor positions have likely been dummied out with jumper wires. The symptom you describe is typical of a worn-out opto-coupler where the LED light output is dying from being lit for 15 years. If you can figure out which input coupler is the one for these functions, you can probably just replace them. PLCcenter is a great resource for all sorts of obscure Japanese components like this.

Jon

The board is properly supported.

The board and the inside of the cabinet is clean. I have been running the machine with the cabinet open though, just to get a couple jobs done. I have made sure that nothing can get inside when open.

I'll try the card swap to see if I can detect any differences. If the connectors are less springy if I can't bend them back where they belong I will unsolder the connector and replace it. Unfortunately the board it plugs into is buried behind everything else and will take a lot of time to remove. And there 100 contacts in the edge connector. I carefully inspected, with a 10x magnifier, the board for any bad solder joints or cracked traces and found none.

Looking at the connector contacts on the board I can see that some have more obvious marks than others on them from the contacts in the female connector. Hmmm.

There is no foreign matter of any kind on the board and the machine has no sensors. This is why the alarm is spurious. Could failing caps be causing problems? Could unplugging the board allow some caps to discharge which then makes the alarm go away?

I'll see what I can figure out about what may be different on this board from one in a machine equipped with sensors.

I'll try your folded paper cleaning strategy and report back.

Thanks for the suggestions Matt. Eric

Greetings Jon, There are opto-coupled boards in the machine but none on this board. The Fadal repair folks told me this board is the interface board and is probably the problem and since removing and replacing it stops the alarm every time I think they are probably correct. However, I will try to identify what all the opto-couplers do and see if any of them are for the sensors that are not there. Eric

I'm surprised no one has suggested a can of freeze mist. When the alarm goes off, spray half the board to cool it down, then do what ever is needed, restart the machine, reset the alarm, and see if the alarm is still on. If it is on, spray the there half and test again. Hopefully the alarm will have stopped, the next time the alarm starts, spray 1/2 of the 1/2 that you sprayed before when the alarm stopped. Now, if this is actually stopping the alarm, just continue limiting the area until your down to a single part. Mikek

I have been using freeze spray but the results have been variable. So I'm instead going to try a heat gun to saee if I can get the board to fail faster. Eric

If you put a different card in the slot where the problem card goes, it may or may not work - it isn't a given that all the slots on the motherboard are the same. But you can at least judge the mechanical condition of the connector that way. If you do this swap, do it with

That's usually how it is.

They all should have some kind of mark, and it's normal for the marks to vary a little. If most of the marks are obvious - dug into the copper/ gold somewhat, but a few of them you have to get out the 250 W lamp to see the mark, then that's a clue.

Possibly.

Kind of strange, but within the realm of possibility.

OK. I will post to your new thread shortly as well.

Matt Roberds

Check the +5 volts to one of the LSTTL chips. If the 5 volt supply is failing, it could be a power supply issue. Look up one of the 74LS chips for what pins are Vcc (+5) and ground. For a 14 pin chip, ground is pin 7 and Vcc (+5) is pin 14 usually. Track the voltage until it fails and note any trend.

tm