Ed Nisley's Blog: Shop notes, electronics, firmware, machinery, 3D printing, laser cuttery, and curiosities. Contents: 100% human thinking, 0% AI slop.
Prompted by the condolences on the death of my Beckman DM73 Circuitmate, I brought the carcass back to Squidwrench, took it apart, annoyed the switches, and re-soldered the battery connections:
Beckman DM73 Circuitmate – restored
I worked minute dots of Caig DeoxIT into the switches, without disassembling them, with magical thinking guiding my technique. One of the battery connections seemed suspect, but we’ll never know.
In any event, it beeps happily when turned on (*), the LCD behaves properly, it’s no longer pressure-sensitive, and life is good! It surely needs calibration, but that’s definitely in the nature of fine tuning.
Thanks for nudging me into Doing The Right Thing™.
(*) Including the double beep with the AC/DC button held down.
I’d added Mad Phil’s trusty Circuitmate to the tool kit I carry along to Squidwrench:
Beckman DM73 – new ground clip
Over the last few months it became increasingly erratic, eventually got to the point where slight pressure on the case would blank the display, and finally didn’t turn on at all. Yes, I replaced the batteries.
So I took it apart:
Beckman DM73 Circuitmate – case open
Nothing seemed particularly broken and, even after resoldering all the joints, it continued to not work at all:
Beckman DM73 Circuitmate – PCB
If you want to try your hand at instrument rehabilitation, let me know.
… with any of the doc for the generic AD8950/51 DDS modules you’ll find out on the Interwebs. This snippet from the seller’s schematic will suffice:
AD9850 module schematic – cropped
Here’s a closer look at the 2×7 header in the upper left corner:
AD9850 module schematic – J5 detail
Don’t blame me for the blur, the schematic is a JPG.
Compared it with the board in hand:
AD9850 DDS Module – swapped GND D7 pins – detail
Yup, the D7 and GND pins are reversed.
Some careful probing showed the silkscreen is correct: the pins are, in fact, correctly labeled.
Should you be laying out a PCB in the expectation of using any DDS module from the lowest-price supplier, remember this high truth: Hell hath no fury like that of an unjustified assumption.
Fortunately, I’m hand-wiring the circuit and caught it prior to the smoke test.
Given the intended LF crystal-measurement application, a hulking 51 Ω metal film resistor sprawled across the ground plane will work just fine. All three ceramic caps measure a bit under 1 µF; I intended to solder the input caps atop the existing 10 nF caps, but that didn’t work out well at all.
I should harvest the InLo SMA connector to prevent anyone from mistaking it for an actual input.
With that in place, the log amp output makes more sense:
AD8310 – modified – 100 kHz 150 MHz – 0 dB atten
That trace tops out at 150 MHz, not the previous 500 MHz, but now the response is flat all the way out. The log amp generates plenty of hash when the tracking generator isn’t producing a valid signal.
The 60 kHz response looks different:
AD8310 – modified – 60 kHz 1Vpp
So it’s really the log amp response to the absolute value of the sine wave (or, more accurately, to the sine wave re-zeroed around Vcc/2), with minimum output at the input’s zero crossings. At 500 mV/div, the log amp says the input varies by 42 dB = 1000 mV/(24 mV/dB), which might actually be about right for a zero-crossing (or zero-approaching absolute value of a) signal; logarithms don’t deal well with zeros.
The AD8310 datasheet and AN-691 suggest the 2.5 V output corresponds to +10 dBm = 12.5 Vrms input, which flat-out isn’t the case. However, the actual 500 mVpeak = 350 mVrms input is 2.5 mW = +4 dBm, so maybe it’s within spitting distance of being right.
AN-691 recommends 10 µF input caps for “low frequency” use, showing results down to 20 Hz; 1 µF seems to get the circuit close enough to the goal for use near 60 kHz.
It also recommends a cap on the BFIN pin (pin 6) to reduce the output stage bandwidth = “video bandwidth” and improve the overall accuracy, which remains to be done. The datasheet suggests rolling VBW off at 1/10 the minimum input frequency, which would be around 3 kHz for use with 32.768 kHz crystals. The equation, with reference to the internal 3 kΩ bias resistor:
CFILT = 1/(2π 3 kΩ VBW) – 2.1 pF = 18 nF
For a bit more margin, 1 kHz would require 56-ish nF.
The PCB has a convenient pair of pads labeled C6 for that capacitor. This may require protracted rummaging in the SMD capacitor stash.
Rolling off the VBW should reduce the hash on the 100 kHz end of the frequency sweep and filter the 60 kHz response down to pretty much a DC level.
Applying the 10 dB and 20 dB SMA attenuators to the input from the tracking generator and recording the log amp output voltage produces this useful table:
AD8310 Log Amp – mods and log response
With the terminating resistor on the correct side of the input caps, the log amp seems to be working the way it should, with an output varying a bit under the nominal 24-ish mV/dB over a 30 dB range.
We need caps! Lots of caps!
A quick search with the obvious keywords suggests nobody else has noticed how these modules work over a reasonable bandwidth. Maybe I’m the first person to use them in the LF band?
Once again, the single moving part on my first-generation Kindle Fire stopped working. As before, the switch contacts accumulated enough fuzz & contamination to prevent any current flow, but this time the (soft) solder joints attaching the switch body to the PCB failed:
Kindle Fire power switch – failed anchor
My joint cleaning & fluxing wasn’t up to contemporary standards, as shown by the obviously un-fused footprints left in the upper pads:
Kindle Fire power switch – failed anchor joints
The switch frame seems to be unplated steel, which shouldn’t be an excuse.
So I dismantled the switch, cleaned the contacts and tactile bump plate, put it all back together, and did a much better job of surface preparation:
Kindle Fire power switch – rebuilt – right anchor
The other joint:
Kindle Fire power switch – rebuilt – left anchor
And, for completeness, the switch leads:
Kindle Fire power switch – rebuilt – switch pads
I don’t like the way the joint on the right looks, either, but we’ll see how long the whole affair holds together.
This may be the last time I can repair the Kindle, as a bypass cap came loose while I was working on the PCB, the screen has been accumulating dust at an increasing pace, and several latches securing the back of the case have cracked.
Methinks it’s getting on time for a new pocketable memory device; if only Pixel XL phablets had a bigger screen and didn’t cost night onto a kilobuck.
So: jouncing over the larg(er) potholes / pavement discontinuities / debris on the roads around here wobbulates the front fender enough to pull the stays out of those tidy 18 mm = 6 diameter deep sockets on the fender clip.
Waving a heat gun around a 3D printed part seems fraught with peril, even with PETG’s glass transition temperature around 80 °C = 175 °F, as ordinary polyolefin tubing shrinks at 140-ish °C. Aiming the hot air stream more-or-less away from the clip (and the tire!) carried the day. PLA would surely have gotten bendy.
The proper solution surely involves screw clamps and suchlike. I really dislike fiddly hardware: I hope this hack survives.
The small garage door opener I tote around in the Tour Easy’s underseat bag failed after many years of exposure to the elements, so I paid a few bucks more for a cheap replacement in order to get fast delivery from a (US!) eBay supplier:
Garage door opener remote controls
For whatever it’s worth, before buying the replacement I tried:
Cleaning the battery contacts
Installing a new CR2032 battery
Programming the hitherto-unused buttons to open the door
The remote control would occasionally work, but none of the “repairs” made much difference; I suspect corrosion hidden under the components or cracked solder joints.
The eBay item description clearly, if inarticulately, specifies the compatibility requirement:
key chain remote control
compatible for purple learn button
So I trotted out to the garage and inspected the button:
Sears Garage Door Opener – purple button
Looks purple to me, but, being that type of guy, I also read the adjacent instruction sticker:
Sears Garage Door Opener – instructions
Nobody, nobody, maintains the documentation. [sigh]
I figured if they went to all the trouble of ordering a bazillion switches with purple caps, then the PCB surely holds the corresponding RF filters & firmware & whatever else that button signifies.
Seeing as how we have exactly one garage door opener and no lights or other doodads, I told the opener to obey both the 1 and 2 buttons, thereby dramatically reducing the dexterity required to open the door while pedaling up the driveway. The opener can remember an unspecified number of transmitters, so I didn’t go for all four buttons.
The Smell of Molten Projects in the Morning 