CD player DC motor problem or it's driver IC ?

[MathWizard]

I have a CD player whose tray sometimes won't close at all, it just stays in the extended position. The end of travel SW seems good.

It has what I think is a standard 2-wire brushed 5V motor. And when I'm probing it on the scope, as it goes forwards, or backwards, sometimes it makes a nice looking square wave, +/-3.8V, other times, I see  a few spikes, where the voltage returns to ~0V, and other times, I see a lot of chatter, between 2.8-3.8V lets say (I never properly measured yet). Thats w/ the scope rolling at 200ms/div.

I was probing the control signal in form an MCU, that tells the motor controller IC what to do, and it all looks normal, like a solid square-wave.

So if a brushed DC motor, has a commutator, and every so many degrees of rotation, it actually does short PWR to GND.....is that what I'm seeing sometimes?

Or is the motor wiring, or it's cap, shorting out sometimes ? Or does this sound like motor controller IC is failing ?

The motor measures about 2ohms, I can try and find 1 to swap in I could even measure the motor's current, see if it's bouncing around.

I can swap in a load resistor and see if the IC voltage stays clean then.


I can't remember probing motors before, whats this sound like ?

[fzabkar]

Did you check whether the drive belt is slipping?

[MathWizard]

Overall I think the gear's and belt are fine, lots of grease still, and the belt even has teeth.

Since I took it all apart and the CD drive motor is not connected, it's been doing something it never did before, like  start/stop/reverse/stop then motor above, for a few mili-sec, . The door never used to start then stop , but all the wires for this motor and sensors are hooked up....so I'm guessing its the lack of the CD/laser ribbon cable that's doing that.

Looking at it's SW, there's not usually much bouncing, if u press it 1 way, but pressing it on the edge, can cause 2-3 full bonces. It has a 1nF cap on each button of the R-ladder, which has a few other buttons for, that the MCU-ADC looks at.

But sometimes, the door would be stuck open, and you could press the button repeatedly, and the SW was working....but nothing would happen. I never really measured the motor, the control signals yet, when that was happening, since it's intermittent.

And now that I've taken it apart, I never had the CD/laser part hooked up, when I was probing it.


But for probing a motor, unless my probes are failing (which some of them are mechanically sensitive/failing) I'm assuming the motor must be shorting or the driver IC is failing, for it to look like a noisy mess, and other times, look like a really nice sqr-wave

[shakalnokturn]

That looks like a dirty commutator or shorted winding.
You'll likely see it's misbehaving for about 120°.
Try confirming with an ohmmeter while slowly turning motor by hand you should see the resistance drop a lot at one point.

Driver IC should be OK if you don't play around with the defective motor too long...

If the commutator is dirty sometimes you can get away spraying contact cleaner at it and spinning the motor.
A shorted winding means a new motor.

[andy3055]

Also, look for dry solder joints/loose connections.

[james_s]

Disconnect the motor and connect a small lamp or LED with a resistor, or sub a load resistor and watch the output with your multimeter or scope.

[MathWizard]

Thanks I keep forgetting to check for bad solder joints, but when I hooked up a 10 ohm resistor to the motor controller IC, is seemed to run fine, and and regulate to +/-3.8V, that's about right for the motor rating and safe opening speed. And there's no amount of ripple or crazy distortion (I never had the wifi working last week to save the pictures, so I'll have to check again).

I don't know enough about motors yet or the EM physics, but this is a  brushed motor RF-500TB-144156 6V 0.028A(idle) , 0.13Amax, it's actually more like 8 Ohms. And I didn't know that the resistance of the motor will drop/allow more current....as the motor is loaded and spinning slower. So I think a lot of the voltage spiking is the mechanical load changing, lack of grease on the main cammed spiral path. Probaly the only thing jamming the platter open too.



With the bare motor taken out fully, it seems to be running pretty smoothly, nothing like when it was all hooked up. The driver chip is not doing PWM btw, so I'm not just mixing that up. But there are some HF spikes and LF spikes and really slow voltage spikes. But looking at a brand new motor of the same basic size/type, it does it too. Just trying to slow them down with om finger, they acted about the same at a glace anyways.

So IDK, I better put it back together and make some real measurements, maybe the motor is getting weak.

[james_s]

The motor is not a resistive load so don't expect it to behave like one. The current it draws will depend greatly on the mechanical load placed upon it. It's entirely possible that the driver IC is detecting excessive draw and cutting power to the motor, and the problem could either be the motor itself, or perhaps more likely the mechanical system it drives. Next thing I would suggest doing is put the motor back in and use a bench supply to power it and operate the mechanism while noting current draw.

[MathWizard]

Yeah I'm learning to use the models in LTspice for motors, I sort of remember the torque physics, so it's nice to see  the same type of differential equations.

I found out that the motor IC is just a high-current op-amp, and I figured out the logic levels for the op-amp inputs. For a while I thought I was looking at some timing integrator, but it's just the MCU trying to run another unhooked motor, and my probe was AC coupled and it made a sqrwave pulse look like a slope integrator LOL.

However the chip is indeed failing. Now it can barely go in reverse even when the is gear removed, or driving another motor. But there might be a really quick fix, now that I found the pinout, there's an used op-amp on the chip. If the chip still doesn't work I might just make an op-amp+pass BJT circuit, see if that works without oscillating.