Discrete LM3909: Drain ‘Em Dry

Given that I put them into a gadget intended to use partially dead alkaline cells, the “05” date on the cells, and that it’s been blinking since last November, I cannot complain when this happens:

Discrete LM3907 - leaky Duracells - as found
Discrete LM3907 – leaky Duracells – as found

I might just fill the battery holder with vinegar and let it fizz for a while:

Discrete LM3907 - leaky Duracells - corrosion traces
Discrete LM3907 – leaky Duracells – corrosion traces

Even my simple discrete LM3909 circuit can blink a blue LED from a battery producing under 1 V, but those cells were flat dead. Gotta look over there more often, I suppose.

OMTech 60 W Laser: Replacement HV Power Supply

The original HV power supply in the OMTech 60 W laser went casters-up just barely inside OMTech’s six month tube-and-supply warranty period. For the record, the laser controller reports this status info since mid-March:

Laser Stats - replacement supply
Laser Stats – replacement supply

I think the Total job laser on time line says the power supply failed after firing the laser for a little over eight hours. The OMTech manual says the laser tube should last 1000 to 2000 hours (low vs high power), which suggests I should stock up on power supplies.

Its replacement just arrived:

OMTech replacement HV supply
OMTech replacement HV supply

It (bottom) seems to be a knockoff of the original ZYE Laser supply (top), with a similar model number and a “serial number” resembling a date from last year. All the connectors matched up, which isn’t too surprising.

The three most interesting inputs:

  • L = controller’s active-low L-ON enable output
  • IN = controller’s PWM output
  • P = jumper to G (circuit ground) — not water flow sensor

Also note the two AC power-line terminals directly adjacent to the TEST button, then consider insulation and stand-off distances before poking the button with your index finger.

The power supply has a digital current meter, so I plotted output current against PWM input:

Laser Power Supply - mA vs PWM - overview
Laser Power Supply – mA vs PWM – overview

Taking more points at the low end, with vertical bars indicating single-digit flicker on the meter:

Laser Power Supply - mA vs PWM - 0 to 20 PWM
Laser Power Supply – mA vs PWM – 0 to 20 PWM

I have little reason to believe the meter reading indicates the true current with any accuracy and I know CO₂ laser output power does not scale linearly with the current.

But it’s cutting again, which is a step in the right direction.

OMTech 60 W Laser: Failed HV Power Supply

Setting up a piece of MDF and hitting the Frame button produced a lightly scorched line around the part perimeter, plus a slightly diagonal track leading from / to the Home position in the far right corner:

Fire while framing tracks
Fire while framing tracks

Doing another pass with LightBurn’s rubber-band frame produced the faint dotted circle.

Huh. Didn’t useda do that.

The laser should not fire while framing and, having just installed LightBurn’s 1.2.01 update, suspicion instantly fell on the most recently changed thing.

Which turned out not to be the case, as LightBurn’s tech support pointed out:

This is generally an indication of a failed high-voltage power supply, not a software issue.

OMTech’s support requested a video of the equipment bay, which didn’t seem like a useful way to convey the situation. Instead, I sent pix.

This picture shows the status of the 60 W laser power supply while the laser is incorrectly firing:

OMTech 60W Laser - uncommanded framing fire
OMTech 60W Laser – uncommanded framing fire

The power supply has two LEDs on what looks like, but is not, an Ethernet jack near the bottom:

  • Orange P LED: good water flow
  • Green L LED: controller’s PWM signal

The LASER orange LED near the top turns on when the HV output is active and the laser should be firing.

In this case, L LED is off and the LCD shows “Laser signal OFF”, but the LASER LED is on and the LCD shows 2 mA beam current: the laser beam is ON, even though the controller has not activated the PWM signal.

Not only that, but I discovered the laser would fire while framing even with the lid up and the “safety interlock” sensor active.

Totally did not expect that.

For comparison, the power supply status during a manual pulse at 49% power:

OMTech 60W Laser - manual pulse 49%
OMTech 60W Laser – manual pulse 49%

In that case, the L LED shows the PWM signal is active, the LASER LED is on, and the LCD shows 14 mA of current to the tube. That’s how it should work.

Although the function of the TEST button seems very lightly documented, pressing it did not turn on the output (the LASER LED is off), despite lighting the L LED:

OMTech 60W Laser - Test button pressed
OMTech 60W Laser – Test button pressed

OMTech confirmed my suspicion:

We are afraid that the laser power supply is defective

A replacement should arrive in a few days.

Protip: always practice laser eye safety.

Kensington Expert Mouse Scroll Ring: More Data Points

A note from Alan adds more data about troubleshooting problems with the classic Kensington Expert Mouse trackball scroll ring:

I have two comments and a question: first I made the mistake of purchasing 4 used expert mice on ebay etc and each had a different problem but 3 of 4 also had faulty scroll rings. 2nd: one of them was dated 2020 (a wireless version). so they definitely haven’t fixed this issue and it’s very wide spread (or maybe why shady sellers decide to part ways with their trackballs).

question: from reading across your quotes it’s not clear but it seems like there is no real consistent fix to this issue nor a really strong conclusion as to what causes it? My futzing with a couple of these does seem to suggest that alignment of the ring makes a difference but not a lasting one.

As far as the alignment non-fix goes, tweaking the detector position just changes the amount of light passing through the wrong side of the reversed IC, without solving the problem. That’s what we’ve all done, with essentially the same results: feels good, doesn’t last.

Kensington (whoever they are these days) may have fixed the problem with a different quadrature detector oriented in the proper direction, but that’s not something we civilians can accomplish.

It should be possible to unsolder the reversed detector (if, indeed, it is), aim the lens (if that’s what it is) at the emitter, then somehow resolder the leads to the same pads. Perhaps flip it to put the leads on the top, away from the PCB, secure it with a generous blob of hot-melt glue, and connect jumpers from pads to leads?

So far, the two new-ish units on my desks continue to work well, depriving me of sufficient motivation to dig into my junkers.

If anybody is willing to hack their defunct trackball, please let us all know what happened!

Because you may be reading this in our future, comments on this particular post will probably have been disabled to reduce the attack surface for spammers. Send me an email / use the comment form (linky over on the right), or comment on the post of the day and I’ll sort it out. Thanks!

Wheelbarrow Wheel Transplant

The rubber in pneumatic tires / tubes rots when left out in the open for a year or three, so I volunteered to replace the dead-flat tire (on the wheelbarrow I rebuilt last year) with the “flat free” solid foam tire+wheel harvested from an irreparably damaged wheelbarrow. Which, as it turned out, had lost one bearing and the remaining bearing wasn’t in good shape:

Wheelbarrow Wheel - victim bearing
Wheelbarrow Wheel – victim bearing

The bearings in the pneumatic wheel were in comparatively good shape:

Wheelbarrow Wheel - donor bearing
Wheelbarrow Wheel – donor bearing

So I knocked the good bearings out, cleaned up / re-lubed them with squirts from my lifetime supply of genuine Mobil Vactra No. 2 Sticky Way Oil, and hammered tapped them into the solid-tire wheel.

Whereupon I discovered the two wheels have different hub lengths and, unfortunately, the axle clamps in the recipient wheelbarrow lacked enough adjustment range.

Well, I can fix that:

Wheelbarrow Wheel - axle clamp cutting
Wheelbarrow Wheel – axle clamp cutting

I briefly considered cleaning and repainting the wheel, but came to my senses when I considered the tire’s condition:

Wheelbarrow Wheel - transplanted
Wheelbarrow Wheel – transplanted

I suppose when the tread flakes off, the interior foam will rapidly erode, but we’ll burn that bridge when we encounter it.

The alert reader will have immediately noted the grease fitting on that rusty wheel: you’re supposed to periodically fill the entire hub with sufficient grease to push the crud out of the bearings. IMO, that’s so deep in silk purse territory as to be irrelevant.

The remaining useful parts from the defunct wheelbarrow will, most likely, come to good use next year …

Lawn Chair Re-strapping: Countdown Hold

I planned to replace the vinyl straps on our set of (salvaged) lawn / patio chairs and made a pair of rivets for one long-missing strap:

Lawn chair strap rivets
Lawn chair strap rivets

The overall project is on indefinite hold, as a Steel-blue Cricket Hunter (*) has decided at least one of the chairs is an ideal place to start a family:

Lawn chair - wasp nest under construction
Lawn chair – wasp nest under construction

The patio under the chair is littered with blades of grass and twigs that didn’t quite fit through the 5 mm vent hole in the tube, but that long stem went in just fine:

Lawn chair - wasp nest grass stem
Lawn chair – wasp nest grass stem

We have seen the wasp airlifting crickets near the chair, so provisioning has begun. The cricket seemed not only larger than the hole, but also larger than the wasp; we assume the wasp knows what she’s doing.

The new wasp will hatch this year, pupate over the winter, then hatch and emerge next summer, but I plan to replace the straps after the construction season ends.

I have no idea how to clean out whatever’s accumulating in there …

(*) I learned them as Steel-blue Cricket Killer, but the crickets are just paralyzed, not completely dead.

Bathroom Sink Pop-up Drain Rod Status

Three years later, the bathroom drain stopper once again disconnected from the pop-up rod and gave me a chance to inspect how the brass is surviving:

Bathroom drain pop-up pivot
Bathroom drain pop-up pivot

After five years, the epoxy plug is either creeping out of the end or the end is corroding back a bit, but there’s plenty more where that came from.

The original steel rod disintegrated after nine years, so it’s still a horse race.