The Smell of Molten Projects in the Morning

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

Category: Electronics Workbench

Electrical & Electronic gadgets

Thing-O-Matic: Improved EC Thermistor Connector Orientation
Given that the SMD pads fell off the HBP circuit board and I must replace the connector, I figured I may as well also replace the remarkably stiff MBI thermistor cable with a much more flexible CD-ROM audio cable. Although the EC end of the MBI cable looks like a standard CD-ROM audio connector, it’s been rewired. No problem: this is not an audio application and I’m going to do exactly the same thing.

The Extruder Controller, however, doesn’t have a matching connector and the recommended attachment involves simply jamming the connector onto the pin header, per this detail cropped from that photo in the MBI assembly instructions:

MBI EC HBP Thermistor Connector Alignment – Detail

Here’s a better closeup of my EC, taken from the other side:

MBI Extruder Controller – HBP thermistor connector

The header block breaks out the Arduino’s Analog Input pins, with A6 in the front of that photo. From left to right, the pins under the HBP connector are A6 / +5 V / Gnd. Unfortunately, the connector wiring and alignment puts the thermistor signal on the cable shield, with the Gnd and +5 V wires safely tucked inside. This is, shall we say, suboptimal.

The Gnd connection provides a low-impedance connection to the least-noisy part of the circuit, so putting it on the shield tends to prevent the relatively high-impedance signals within from picking up noise. This isn’t always successful, for a number of reasons, but it’s a Good Idea.

Although probably doesn’t make much difference (it’d just add a bit of noise to the HBP temperature signal), but if I’m going to be rewiring it anyway, the cable shield will be at ground potential with the signal wire inside. Here’s my cable & connector, rearranged to make that so:

EC HBP thermistor connector – revised

The analog audio connector on the back of old-school CD-ROM drives, back before digital audio output from the drives actually worked, had four pins:
- Left (white) and Right (red) audio channels on the outer pair
- Ground (black) on at least one of the central pair
So the red wire will be in the far right-hand socket of the connector shell; depress its locking tab, slide it out of the shell, poke it into the socket between the other two wires, push to click, and you’re set. Conveniently, this puts the +5 V supply on the red wire, which is sorta-kinda standard. Your cable colors may vary; pay attention to the actual wiring and ignore the color code!

Tape the connector in place (with the empty socket now toward the board edge) to prevent the tangle of wires in the Thing-O-Matic’s electronics bay from dislodging it at an inopportune moment:

EC HBP thermistor connector – secured

Admittedly, that arrangement still tucks the +5V wire right next to the signal wire inside the shield, but it’s a step in the right direction.

You could flip the MBI cable around, too, as long as you also rearranged the pins at the HBP end to match.
2012-01-07
Thing-O-Matic: HBP Connector Failure

This has been a long time coming, as the connector shell over that pin connecting the MOSFET to the heater has been getting crispier despite my attention, cleaning, and occasional DeoxIT application.

Burned-out HBP connector

Notice that the burned pin now stands at a slight angle to the others. The PCB pad has no additional copper traces on that side to conduct the heat away from the failing connection, so the joint got hot enough to put the solder into its semi-liquid state, whereupon the springy connector rammed it upwards through the softened plastic shell. If the PCB fab shop used 60-40 lead solder, that’s around 188 °C. Silver solder would reach 220-ish °C. If the solder was eutectic, it would turn liquid and just drip off.

What doesn’t show: the SMD pads that pulled free from the PCB surface, fortunately only under the rightmost three pins leading to the thermistor. Repairing the pads and connector makes no sense, so I think I’ll go with pigtail leads anchored to the plywood, with offboard connectors to reduce the strain on those pads. Powerpoles will be bulky, but maybe pigtails long enough to get them onto the case might work.

As a general rule, soldering wires or connectors to SMD pads with no mechanical support is a Bad Idea and applying repeated mechanical stress to those connectors is a Very Bad Idea. Doing all that on a PCB running well over 100 °C with current right up near the connector’s absolute maximum, well…

2012-01-06
Sony NP-FS11 Batteries: After the Aftermarket
The batteries I rebuilt for our much-beloved Sony DSC-F505V camera back in early 2010 have faded away with constant use. Having already sawed the cases open, rebuilding three of them didn’t pose much of a challenge; this time I added a short tab of Kapton tape to help extract them from the camera socket.

Rebuilt NP-FS11 batteries

Three batteries seems to be about the minimax for ordinary use:
- One in the camera
- One in the carrying case
- One in the charger
You (well, we) can’t keep track of more than three: it always seems one battery gets overused and another gets lost in the dark. We’ll see how three works in practice; there’s a set of six more raw cells lying in wait.

The new batteries produced these results on their first two charge-discharge cycles:

Sony NP-FS11 2011 Packs – First Charges

One battery didn’t come up to speed on the first charge, but after that they’re all pretty close.
2011-12-24
Ativa Cordless Phone Batteries

These were cheap-after-rebate phones with 2/3 AA NiCd cells that lasted nigh onto five years. We rarely talk on the phone and even more rarely use these, so they’re on the dreaded continuous trickle charge and low usage cycle that kills rechargeable batteries. Of course, they’ve been sitting there for five years…

The rebuild was no big deal, although I had to replace the original 360 mAh NiCd cells with 650 mAh NiMH cells (with tabs) because that’s what’s available nowadays. The trickle rate will be even lower relative to the capacity, of course, which may or may not be a Bad Thing.

The packs contained a simple fuse consisting of a thinned section of the usual nickel strap connecting two cells, covered with a fiberglass sleeve under the shrink overwrap. For lack of anything smarter, I harvested the fuse and soldered it in the new pack. although the risk of a catastrophic short seems fairly low:

NiCd pack with thin-wire fuse

The final result looks about as you’d expect, complete with obligatory Kapton tape wrap:

Ativa phone – rebuilt battery

The old pack is kaput and new pack delivers pretty nearly its rated capacity at an arbitrary 550 mA discharge (which is, admittedly, a bit stiff for the old pack):

Ativa phone battery tests

That takes care of one phone… the other one’s probably in the same condition, so I have enough cells to rebuild it, too.

2011-12-23
HP8591 Spectrum Analyzer Screen Dump Sizes
The script I use to fetch screen dumps from my HP8591 spectrum analyzer works fine, but it turns out that the screen images have (at least) two sizes.

The hp2xx program converts the screen dumps from HP-GL text files to PNG bitmaps:
```
for f in *hgl ; do hp2xx -m png -c 1436 "$f" ; done
```
The usual size is 593x414 pixels:

SMD 470 pF – Comm Spec

The other size is 593x395 pixels:

SMD 470 pF – Surplus

As nearly as I can tell, the spectrum analyzer mashes the Y coordinate when any of the soft keys along the right edge have reverse-video highlights, which print as outlined boxes. There may be other sizes; those are the two I’ve stumbled over so far. This doesn’t much matter unless I’m using the images in a column, in which case it’s awkward to have two sizes: a one-size-fits-all script to trim off the soft keys doesn’t produce the proper results.

Musing on how to figure this programmatically…

The file command gives the pixel dimensions, with the file name (which may contain blanks: so sue me) set off with a colon:
```
file "SMD 470 pF - Surplus.png"
SMD 470 pF - Surplus.png: PNG image, 593 x 395, 8-bit colormap, non-interlaced
```
Judicious application of cut extracts the relevant numbers, albeit with a trailing comma that requires another pass through the grinder:
```
file "SMD 470 pF - Surplus.png" | cut -d\: -f2 | cut -d\  -f4,6
593 395,
```
Although I think a sed script might be better, that requires more skull sweat than I have available right now.

Given that, then an appropriate mogrify would crop off the softkey labels; the first one is what’s in the script right now:
```
mogrify -crop "540x414+0+0" SMD\ 470\ pF\ -\ Comm\ Spec.png
mogrify -crop "515x395+0+0" SMD\ 470\ pF\ -\ Surplus.png
```
Which looks like this:

SMD 470 pF – Comm Spec cropped

The two sizes come out pretty close to the same 1.3 aspect ratio, but resizing the smaller one to match the larger doesn’t work well:
```
convert -resize '540x414+0+0!' SMD\ 470\ pF\ -\ Surplus.png SMD\ 470\ pF\ -\ Surplus\ resized.png
```
You need single quotes around the geometry parameter to prevent Bash (or Dash or whatever) from gnawing on the bang character (yes, that’s how you pronounce “!”).

The images are lossless PNGs because they consist entirely of single-pixel lines and characters; alas, resizing by non-integer factors close to 1.0 introduces nasty picket-fence aliasing artifacts:

Resize x 1.049

I resize the pix by a nice, even factor of two (which also adds aliasing artifacts, but in small and very regular doses) and set the dots/inch value so the images print at about the right size without further hassle along the production pipeline:
```
mogrify -density 300 -resize 200% whatever.png
```
Which looks like this:

Resize 2.00

Resizing from the smaller images to (roughly) the final size in one step doesn’t look quite so awful:
```
convert -density 300 -resize 209% "SMD 470 pF - Surplus.png" "SMD 470 pF - Surplus large.png"
```
Like this, still with a distinctly garbled dBm:

Resize 2.09

But it’s decidedly better than this result from a two-step enlargement, although not as wonderful as one might like:

Resize x 1.049 x 2.00

So the script needs a tweak for the file sizes, but …

Memo to Self: It’d be simpler to not have highlighted softkeys when doing screen dumps!
2011-12-21
Capacitor Self-resonance: The Madness

I’d wondered whether suppressing RFI by picking capacitors by their self-resonant frequency, so that each cap would suppress a known input signal. Turns out that’s entirely possible, even for the amateur VHF and UHF bands:

Wouxun PCB – 100 nF 680 80 pF AVX – PTT

The three caps producing that trace look like this on the brassboard PCB for the Wouxun GPS+voice interface, with spectrum analyzer input & output through RG-174 coax with 22 Ω and 470 Ω SMD resistors tombstoned on the pads at the end of the string:

GPS voice PCB – SMD caps on PTT input

The scattered solder blobs cover Z-wires connecting the top ground plane to the continuous ground pour on the bottom surface. The solder strip along the edge joins the copper tape bonding the surfaces together around the perimeter. Basically, this is as well-controlled a layout as one can rationally get, without full RF matched-impedance zaniness.

However, the whack-a-mole RFI suppression concept makes absolutely no sense whatsoever for anything other than a mass-production board with rigidly controlled component parameters, which isn’t what you see here. Basically, ceramic caps have poor tolerances, bad thermal stability, and standard values too far apart to make fine tuning practical: lining up the self-resonance with a desired frequency requires trial-and-error selection for every capacitor.

Those peaks between the self-resonances can be much higher than you’d expect, too, because they represent parallel resonances where the total impedance can approach an open circuit. Remember that caps above resonance look like inductors and caps below resonance look like caps, so two parallel caps form a nice RL tank circuit for signals between their self-resonant frequencies. The caps have very low ESR, making the Q unreasonably high.

If you were hoping for / requiring broad-spectrum RFI suppression, paralleling caps will definitely make things worse, which is probably not what you expected, either.

The whole scheme also suffers from measurement error due to parasitic inductance from the position of the SA and TG “probes”. Compare this trace:

Wouxun PCB – 330 pF – HTPTT near

Made with the SA and TG connected to the same pad:

SA and TG – same pad

With this trace:

Wouxun PCB – 330 pF – HTPTT far

Which involves moving the SA input to a pad on the other end of the trace, the better part of 8 mm away:

SA and TG – different pads

Yes, those layouts are identical when you’re talking about signals near DC.

The pigtail leads certainly contribute some inductance, as does the the PCB trace itself. I suspect you could model that effect, but I’m not sure you could generate a predictive model without a 3D field solver and a whole bunch of calibration measurements. If you really care about the location of that self-resonant peak, I’m not sure which trace / layout you’d trust.

Of course, if you use a cap with a very broad self-resonant peak, then it’s all good. Except, equally of course, that I have no idea how you’d specify one of these to your purchasing agent:

Wouxun PCB – 992 pF – HTMIC

That’s a 1 nF cap from the same assortment (made by AVX, a nominally reputable manufacturer, if the eBay vendor is to be believed) that produced the other peaks. Obviously there’s something different about those caps (and the 1.5 nF caps in the next compartment of the assortment, too): it’s not a measurement error! Notice that it has the expected high impedance at low frequencies, so you’d probably want a larger cap in parallel, which would give you at least a moderate parallel-resonant peak in between.

So if there’s a single frequency that needs squelching you can probably find a suitable cap by rummaging around in your assortment. More than that, though, just isn’t practical.

Just about the only other discussion I’ve seen about this comes from the folks at Ultracad Designs, who have run the numbers much further than may seem be reasonable, even by my standards.

2011-12-19
Weller EC1201A Soldering Handle Failure
For the last few days, my trust Weller EC1000 soldering iron (well, station) has been misbehaving: shortly after cleaning the tip, it would become covered in charred residue and slag. Today, the LED I’d hacked across the heater terminals inside the base stayed dark, even though the tip was hot, and then became sensitive to the handle position. Obviously there’s a loose wire inside, right?

So I took the handle apart by removing the two screws on the front plate:

Weller EC1201A soldering handle innards

The trick to getting the guts out is to push down on the tab inside the handle that locks the cord strain relief block into the handle. After that, everything comes apart with very little force at all.

Contrary to what I thought, the heater is in the tube surrounding the temperature sensor probe. Looking at the connector on the front of the base unit, the key is on the left side and the wires going clockwise from above the key are:
- Yellow: heater
- White: heater
- Black: sensor
- Red: sensor
- Green: shield
I would have sworn the red & black were the heater, as they have special-looking brass/bronze/copper colored pins & sockets. Wrong again.

The temperature probe comes apart thusly:

Weller EC1201A temperature probe disassembly

Basically, slide the connector and ceramic-coated sensor out of the back of the black shell, then pull the spring-loaded sheath out the front.

I hoped for a laying-on-of-hands fix, but it was not to be: the tip heats while the LED (which I wired there early in the iron’s life) across the heater power remains off. But the LED blinked on intermittently with slight pressure on the iron’s tip; a bit more poking and prodding isolated an intermittent open-circuit to the ground wire just outboard of the strain relief at the handle:

Soldering iron cable failure

A bit more poking & tugging isolated an intermittent high-resistance short (a few hundred ohms, more or less) to a section of cable half a foot from the base connector at the bottom of the cable’s natural loop when the iron’s in the holder.

Unfortunately, fixing all that didn’t restore the iron to life. It seems that the temperature sensor (a thermocouple?) has failed, allowing the tip to heat well beyond any rational temperature. Now that I’m looking, a cleaned solder layer turns blue with oxidation in a matter of seconds and rosin chars instantly. The temperature control knob has no effect whatsoever.

The date codes inside the box show it’s been with me since late 1982, so on a dollars-per-year basis the thing has been a bargain. A new sensor is $60, a new handle is twice that, and I think it’s time for a new iron… at less than the price of the sensor alone, I think that’s OK.
2011-12-14