Alps mini plotter stepper motor not stepping

[hjalfi]

I've just acquired a Tandy CGP-115, mint in the box, one of those ubiquitous four-color ALPS plotters which are so fiendishly unreliable. Mine doesn't work, of course, otherwise I wouldn't be here, but mine has failed in an interestingly different way.

The two miniature nylon gears which always break are, on mine, intact. There are two things faulty on mine: firstly the little plastic lever which connects the pen solenoid to the bar which pushes the pens against the paper had snapped. I managed to fix that by fusing the pieces together with a soldering iron, but while I was working on that the paper feed stepper motor slowly stopped working. It's not the gear, which is still intact, but the shaft no longer rotates.

So, this sounds like it's either the driver electronics (big chunky through-hole transistors, easy to replace) or the motor itself, which looks much less easy to replace. Or something else I haven't thought of. Does this look at all familiar to anyone?

 

[hjalfi]

Update: the stepper motors appear to have failed. One of them grates when it rotates and I measured the torque on the other one and it's 0.4 mN.m, which is way way too small. I think the bearings have collapsed.

Lucky me! I have discovered a completely new failure mode for these plotters!

 

[DeltaDon]

With the creation of low cost Chinese 3D printers there should be a large quantity of cheap steppers to purchase to replace the failed ones. Good luck!

 

[ldkraemer]

hjalfi,
I'd OHM out the windings of the motor, as it could be a cold solder joint.

Larry

 

[hjalfi]

I've figured out that applying pressure to the stepper motor shafts in a particular direction cause the motors to start working again (suggesting a defunct bearing). Unfortunately, now the pen load solenoid has stopped working. Manually actuating it makes it operate fine, so I'm now suspecting the driver electronics...

 

[hjalfi]

...the solenoid hasn't failed. It just doesn't like buck regulators.

The plotter has (had) two 7805 5V linear regulators attached to a gigantic heat sink. I thought one had failed, so I removed it, but it turned out to work fine; but as I'd made a bit of a hash of the board I replaced both regulators with modern buck regulators and got rid of the heat sink. However, with a buck regulator attached the solenoid won't fire. It works absolutely fine with a linear regulator (including one from my junk drawer). Just not with a buck regulator.

The only thing I can think of is that the buck regulator can't provide enough current (1A for very brief pulses) but checking the voltage with an oscilloscope shows nary a ripple. It looks completely rock solid steady.

So confused.