Mill power feed has died; what do I check?

[cathead]

Thanks, I'll check this out.


Thanks. That doesn't seem to be the issue; I checked the stop and it doesn't appear to be sticking.


No bulb. That's an odd feature.


Brushes look fine.

I have seen where the brushes on first inspection look fine but the springs have reached the wear stop that keeps them
from putting adequate pressure on the commutator. I'm not saying this is the problem but one needs to
look it over and verify that the springs are indeed putting pressure on the commutator. Some motors are made
this way to protect the commutator when the brushes wear to the wear stop.


Another thing that comes to mind that would cause the symptoms mentioned would be the variable speed control
resistor. Inside it is generally a carbon resistor with a wiper that can be problematic. I'm very careful about turning
my speed control any more than necessary since turning it eventually will wear out the carbon part.

 

[WobblyHand]

Wonder if the schematic is different than the one Blondihacks took apart? I see a connector where the neon bulb went, but no neon bulb. What fires the SCR? Or if the board was totally redesigned.

Maybe Vevor saved the cost of the neon bulb and the light pipe with this design, which indicates that this circuit is different. If it was me I'd trace out the schematic, so I could figure out how it works.

Speed issues go back to the potentiometer. CRC sells QD contact cleaner for sensitive electronics that you can buy, at least outside of California. Don't know if there's a Cali version. That should help maintain that unobtainium pot.

 

[RJSakowski]

The light near the rapid feed; does it connect to the circuit board where blondihacks' was? If so, it might be that as well, you could check it for resistance too, though now that it works, perhaps not!

A neon lamp will not show any resistance. It conducts when a breakdown voltage for the neon gas is exceeded, usually around 70 - 90 volts,IIRC. Even then, the current is very low.

I'm not sure about the description of operation that Blondihacks gave. The neon lamp operation should be the same whether the motor is running in forward or reverse. Neon lamps are symmetrical in design and have two identical electrodes and so are not affected by the polarity of the voltage. Motor direction would be determined by the polarity of the power as determined by the two microswitches.

Here is a description of the type of control circuit that is apparently being used. https://www.industrial-electronics.com/emct_2e_4a.html
There are a few additional components on your circuit board but the operation us basically the same. The motor is a universal motor similar to that used in small portable power tools. This is verified by the field windings and the brushes. A universal motor works equally well with either ac or dc and the armature windings and field windings are connected in series. To change the direction of rotation, the field winding connections have to be reversed.

 

[ErichKeane]

A neon lamp will not show any resistance. It conducts when a breakdown voltage for the neon gas is exceeded, usually around 70 - 90 volts,IIRC. Even then, the current is very low.

I'm not sure about the description of operation that Blondihacks gave. The neon lamp operation should be the same whether the motor is running in forward or reverse. Neon lamps are symmetrical in design and have two identical electrodes and so are not affected by the polarity of the voltage. Motor direction would be determined by the polarity of the power as determined by the two microswitches.

Here is a description of the type of control circuit that is apparently being used. https://www.industrial-electronics.com/emct_2e_4a.html
There are a few additional components on your circuit board but the operation us basically the same. The motor is a universal motor similar to that used in small portable power tools. This is verified by the field windings and the brushes. A universal motor works equally well with either ac or dc and the armature windings and field windings are connected in series. To change the direction of rotation, the field winding connections have to be reversed.

I wanted to say 'continuity', but was unsure if OP would get that. I was going for "not open" there, figuring a blown out bulb might be the problem.

I DO want to know though: is that bulb plugging into the same spot on the board as Blondihacks' bulb? I presume so, but we should make sure. If so, that would confirm we're looking at the same circuit.

 

[pontiac428]

Feed stops and protection schemes should all be simple interlocks to break continuity that could be checked with a multimeter. Motor coils, pots, wiring connections, and so forth are all components that can be quickly verified with an ohm test. The so-called "neon" light is interesting, but seems strange to me that they would use an alternator charging light type circuit for any purpose in this appliance. I had a knock off Align that had a green power ready LED. My Servo has a red feed engaged LED. The primary overload protection for power feeds is mechanical, it's in the gear train. I'd seriously focus on connections and components, find what went wrong and see if you can fix it.

 

[RJSakowski]

On my RF30 clone, there is a measurable difference in the force required to move the table to the left vs. to the right. This is because moving the table to the left forces the lead screw to the right, increasing pressure on the thrust bearing. Tight gibs exacerbate the problem. A universal motor has relatively low torque at low speed and the motor will slow down or stall with increased load. Because of the difference in stiction and moving friction, this will cause jerky motion at low speed. IMO, this was the cause of Blondihack's symptoms.

The potentiometer shown in the video above is fairly well sealed from contaminants. Potentiometers are typically rated for tens to hundreds of thousands to operations before failure. The use in the power feed is relatively low impact and I wouldn't expect its early failure. I had a similarly constructed potentiometer use for a motor speed control in a marine environment. I used the control for thirty years before I had to replace the potentiometer. I wouldn't open the potentiometer unless I had positive proof of failure as indicated by a resistance test.

In any event, the symptoms exhibited by the OP's power feed are different than Blondihack's symptoms. A bad connection would be a prime suspect. Check all the connectors and check the circuit board for a cold solder joint. In particular the second from the bottom on the potentiometer jack in the photo of the solder side of the board looks suspicious. Although unlikely, a wire can have an internal break. This is more likely when wire are continuously flexed but shouldn't be eliminated in an investigation. Finally would be a component check. Active components are not likely as usually when they fail, they stay failed which isn't consistent with the symptoms reported. The resistors used are carbon film except for the 10 watt wirewound and are usually reliable. The two disk devices should be capacitors but are labeled F1 and F2 which is confusing. An examination of the printing on the part could clear that up. What is supposed to be the SCR or triac (bidirectional SCR) doesn't come up on a Google search for a data sheet.although it did show images for similar devices so the number is probably valid. It would be a simple exercise to draw up a schematic for the board and a schematic would be useful in understanding the operation.or lack of.

 

[pontiac428]

RJ, thanks for supporting my recommendation to go over the basics and pointing out that Quinn was chasing a different fault.

Remember the source of this drive, it's a Vevor. That's Ali Express with an easier to pronounce name. High quality kit that is "rated" to last "tens or hundreds of thousands" of cycles is not to be expected- the thing wasn't exactly built by Westinghouse in 1959. With cheap kit, the basics are overlooked and the basics fail, so check the basics. I don't think the "neon" light has anything to do with your premature failure, that's way to deep in the weeds here. Troubleshooting 101: Is it plugged in? Always go easy early. This is why mechanics and engineers don't get along.

 

[RJSakowski]

An update on the components. The search for the PJA7908 showed an image of devices in a family of insulated gate bipolar transistors which are completely different in function from an SCR or triac.

 

[ARC-170]

Thanks for the help, everyone. I really appreciate all of you taking the time to go into detail about what this could be.

I'm going to clean the grease off, make sure the motor connectors are good and tight and then shrink-tube them on, check the wires on the board and elsewhere, add some sort of insulator sheet, then put this back together, being careful to stuff the wires carefully back in. I'm also going to check if anything flexes in any way.

I'll try and report back on what happens.

 

[tq60]

In looking at the photo of the back of the board, at the far right where there are 2 pins attached, it looks like a line or circle in the middle of the puddle.

Some others toward the middle look ball shaped, could be Flux or not flowed.

The line is likely a Crack in the solder, common when the pins are only supported by the solder.

Do NOT resolder.

You can gently wiggle the pins and see if they move.

Reconnect everything carefully and get the motor running.

Apply some side stress to the connector and see if it hiccups.

If it does, reflow but you could add some single strand wire around the pin to are a stronger connection.

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