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Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.

Tag: Repairs

If it used to work, it can work again

XFCE Window Manager Recovery
The XFCE window manager, at least in its Xubuntu incarnation, seems surprisingly fragile. Every now and again, it won’t start up: all the auto-starting application windows pile atop each other on a single workspace, with no title bar or window decorations, with no way to move them around or change focus. In some cases, the mouse will be active and the keyboard will be dead. This is Not Good.

Rebooting that sucker isn’t productive, as the failure seems to occur most often after a normal system update that, inexplicably, clobbers the window manager’s state information. After that, the window manager will wake up dead every time.

The usual recovery technique involves activating a terminal window and entering xfwm4 --replace to forcibly restart the XFCE window manager, clear the state, and ensure it’s the default. That is remarkably difficult with a nonfunctional keyboard and can’t be accomplished remotely without access to the jammed user’s X session.

What has worked is to SSH in from another PC and delete the XFCE caches for the affected user:
```
cd ~/.cache
rm -rf xfce4
rm -rf sessions
```
You can blow away the entire .cache subdirectory if you prefer.

That this should not be necessary goes without saying. Remember that XFCE is currently the least-awful Linux Desktop Environment; all the rest have even greater complexity and much larger problems.
2013-02-01
Windows 7 Home Premium Remote Desktop: The Missing Link

The tiny Lenovo Q150 has become the dedicated Windows box for running TurboTax this season. In earlier years, I used the Token Windows Laptop through a remote desktop session that appears on a Xubuntu desktop, but the Q150 runs rings around the old laptop.

This time, no matter what I tried, I couldn’t connect to the Windows 7 desktop on the Q150 from my Xubuntu desktop. The usual search results suggested Windows configuration settings that didn’t quite match what the Q150 provided; a bit more searching revealed that Windows 7 Home flavors of the OS (this one is Home Premium) lack the Remote Desktop Protocol server required to export the desktop. The Q150 could act as a client that controlled another machine’s desktop server, but not the other way around.

The suggested solutions required applying patches, in the form of EXE files downloaded from sketchy websites, or dropping in replacement DLLs obtained from similar sites. All that seems like Bad Practice, particularly for a Windows box used to prepare our taxes, and I was unwilling to proceed along those lines.

Instead, I fetched UltraVNC, installed it on the Q150, and it works perfectly. Remmina occasionally requires a resize-window-to-match-server at startup and then it’s all good.

From what I hear, Windows 7 doesn’t display the classic Blue Screen of Death nearly so often as before, although I did manage to lock it up during the course of this adventure. That’s OK, I can still use my favorite Windows wallpaper image:

Windows 7 Home – UltraVNC via Remmina

2013-01-18
Cheap LED Flashlight Switch Disassembly

The pushbutton switch on the end cap of a cheap LED flashlight became intermittent, for reasons that should be obvious:

LED Flashlight switch – intact

Pulling the spring contact out revealed the usual situation inside:

LED Flashlight switch – spring removed

I thought that the discolorations around the central plug indicated a solder joint between the two, but the scratches showed that the plug was actually a press-fit plastic cylinder. Having nothing to lose, I pried the rubber dome off the outside of the switch, balanced the cap’s outer rim on the bench vise, centered an aluminum cylinder over the switch post, and gave it a hammer shot:

LED Flashlight switch – guts

It appears the Basement Warehouse Wing inventory lacks a push-on switch that fits the cap, so this one goes on the pile of potentially useful parts. If a suitable switch appears, I know what to do with it, but if I should need a nice aluminum cylinder that fits a trio of AA cells before then, well …

2013-01-12
Tektronix 2215A Oscilloscope Power Switch Rebuild

My trusty Tek 2215A oscilloscope might be useful for a Larval Engineer engaged in late-night debugging away from the lab, but the power switch has become flaky: sometimes the ‘scope didn’t turn on at all, sometimes the switch required multiple pokes, sometimes everything worked fine. Removing the cover revealed there’s a long plastic bar connecting the power button on the front panel (to the right in the picture) to the power switch near the rear panel AC line socket, tucked under the EMI filter with the red sticker:

Tek2215A – internal top view

Removing the high voltage shield below the PCB reveals the switch has DPDT terminals, but it’s wired as DPST:

Tek2215A power switch – PCB terminals

This knowledge will come in handy later…

Unsoldering the switch and wriggling the bar out of the front panel puts the switch on the bench, solder terminals upward. A plastic shell snapped around the actual switch insulates the top of the six terminals from prying fingers:

Tek2215A power switch – bottom

Remove the shell, remove the toggle-action U-shaped steel pin, release the spring, and pull off the top plate:

Tek2215A power switch – internal

Remove the plunger hardware, remove the rocker arms and their springs:

Tek2215A power switch – disassembled

One contact on each rocker shows signs of distress, but the other button remains pristine (having never seen any voltage differential):

Tek2215A power switch – rockers

Pull out the fixed contact tabs and note that they’ve been scorched a bit. The one on the right corresponds to the bottom rocker above:

Tek2215A power switch – contact tabs

I cleaned everything with a fiber wipe wetted in DeoxIT, then decided that I’d take the easy way out. The tabs have heavy silver plate on both sides, so I flipped them over and reinstalled them with the unused side facing the rockers. The rockers went back in with their unused contact buttons facing the flipped tabs, so we now have fresh, shiny new contact surfaces. Reassemble the switch, soldered it in place, button up the case, and a firm push on the button lights the ‘scope exactly the way it should.

While I had the cover off, I measured the ESR of all those electrolytic capacitors: they’re in fine shape!

The next time the switch needs repair, in another couple of decades, someone can swap in the completely unused tabs from the other end of the switch, then pick whichever contact buttons look best… [grin]

2013-01-11
Wall Switch Failure

Here’s what happens to a really old wall switch:

Wall Switch – innards

A closeup of that broken contact:

Wall Switch – detail

This switch controlled an outlet, so I’m sure it’s hot-switched far too many vacuum cleaners, clothes irons, and suchlike over the last half century or so.

Our house is a bit fancier and originally had top-of-the-line mercury-wetted switches: the contacts sealed in the capsule don’t burn, but the springy supporting structures outside the capsule eventually wear out.

They’re still more reliable than X10 switches, though.

2013-01-07
Furnace Heat Exchanger: Temporary Repair

Which small spot on this hot-air furnace heat exchanger isn’t like all the other small spots?

Pinhole in furnace heat exchanger

Correct! The orange one at the corner of the rectangular exhaust gas flue that’s lit up like the sun, because you’re looking directly into the oil burner flame.

With the fire off and everything cooled down, it looked like this:

Pinhole in furnace heat exchanger – detail

Now, this calls for a new furnace (because replacing the heat exchanger costs as much as a new furnace), but as it turns out this was in an unoccupied house during the week before Christmas. So I scrubbed off the debris with a steel brush, bent up a snippet of 2 mil brass shimstock to fit the corner, applied a layer of JB Industro-Weld epoxy to the problem, and positioned 200 W of incandescent bulbs to help it cure slightly sooner than forever:

Furnace heat exchanger – temporary fix

That is most certainly not a final repair, not just because the heat exchanger’s normal operating temperature exceeds the epoxy’s 500 °F rating, but because where there’s one pinhole there’s bound to be more. The goal was to let us keep the furnace running until we could schedule the replacement after the holidays. Remember, the building isn’t occupied and neither of the smoke / CO detectors went off at any point in the proceedings.

Houses are trouble!

2013-01-06
HP3970 Scanjet Lid Hinge Repair

When the second hinge on my father-in-law’s scanner broke, he asked if I could fix it:

HP3970 Scanjet Lid – broken hinge

It’s a flatbed scanner, so the lid is nearly 18 inches long and weighs 2.2 pounds with the slide / negative backlight illuminator. The stress raiser notches, located exactly where the cracks started, look like a perfect example of how not to do these things.

I solvent-glued the hinges back together, with a square brass tube applying clamping force to the joint overnight, but this certainly won’t last for long:

HP3970 Scanjet Lid – crude repair

HP used to have some really smart engineers, but this looks like it was done by a Newkid (I was one, once, so I know the type) after a solid modeling and simulation session convinced him that those two thin plastic webs had enough strength for the job.

No. They. Do. Not.

Of course, HP provides no Official Way to repair that failure, as the hinges emerge seamlessly from the injection-molded plastic lid frame: you must scrap the scanner and buy a new one, because the lid would cost more than a new scanner. Equally of course, the fact that they don’t have a Windows driver beyond XP makes replacement a foregone conclusion.

It runs under Xubuntu 12.04, mostly, which is what I set him up with after the XP PC got compromised.

2013-01-04