By turns: tinker, engineer, husband, author, amateur raconteur, recumbent cyclist, father, ham radio geek. So many projects, so little time!
While staying at the Witherup House in Franklin PA, I found :
It was published in 1946, when memories were fresh and ISBNs hadn’t yet been invented:
Paging through it, I found a photo similar to one I’d grown up with (clicky for more many dots):
None of those guys look like Dad.
Many of the events in World War II made little sense until the declassification of the Enigma decryptions and the ensuing Ultra / Magic programs showed the value of weaponized math …
A loud crack during a windy thunderstorm announced this mess:
Some deft bow saw work cut it down to size:
Whereupon our neighbor arrived home and we dragged the carcass off the driveway.
Fortunately, it missed everything important, as have several recent branch falls in our yard. The same cannot be said for the many downed trees around the immediate area from recent storms; some folks are hurtin’ bad.
Verily, houses (and trees) are trouble!
We’re waiting at the end of Burnett Blvd, with the signal red and the clock at T = -0.17 seconds (photo numbers in 1/60 second frames):
You can’t hear the car (barely visible) approaching on the far left, but we can.
T = 0.00 – We get a green light and the (more visible) car is accelerating hard:
T = 1.00 – The car reaches the crosswalk:
Note that the driver of the car to our right isn’t moving, either.
T = 2.03 – Car passes through intersection:
The view from above, showing the distance between those two positions is 100 feet:
Do the math: 100 ft / 1.03 s = 97 ft/s = 66 mph.
There’s a reason we don’t start moving instantly when a traffic signal turns green.
T = 3.17 – We start moving, as does the car to our right, with our signal still green:
T = 4.88 – Whoops, our signal turns yellow:
T = 9.28 – Our signal turns red:
The signal timing hasn’t changed over the years:
- Green = 4.88 s
- Yellow = 4.40 s
Elapsed time from green to red: 9.28 seconds. No problem if you’re a car, death if you’re a bike.
T = 10.42 – We’re pedaling hard in the intersection:
The white car to our far right started moving into the intersection about the time we did. If you’re going to say we shouldn’t run the light, you gotta deal with cars first, OK?
Note the car approaching from the right on the far side of Rt 55. That’s a 40 mph zone, the driver sees a green light, and we’re still in the intersection.
T = 12.50 – We’ve been moving for 9.33 s, which puts Mary directly in the path of the oncoming car:
T = 14.83 – The oncoming driver having spotted us and slowed down, we’re asymptotically approaching the right-hand lane of Rt 55, passing beyond the steel manhole cover:
If you plunk “burnett signal” into the search box at the upper right, you’ll find plenty of previous incidents along these lines.
Despite bringing this hazard to their attention many times (“We appreciate and share your interest in making our highway systems safe and functional for all users.“), NYS DOT obviously doesn’t care.
If any of their employees commuted to their office building (which overlooks this very intersection), perhaps they would care, but they don’t: we have yet to see a bicycle in the DOT’s token bike rack.
DOT says they’re in favor of Complete Streets, but, seven years on, it’s just another day on the only route between Arlington and the Overocker Trailhead of the Dutchess County Rail Trail.
One of my Tektronix AM503 Hall Effect Current Probe Amplifiers (B075593, for future reference) lost its DC Level zero-ing capability:
The front-panel knob produced only positive output voltages from maybe 50 mV to the amp’s upper limit around 200 mV (into a 50 Ω termination, Tek not being one to fool around with signal quality & bandwidth). Other than that, the amp seemed to work fine, but you definitely want a 0 V baseline corresponding to no current through the Hall probe.
The manual includes troubleshooting recommendations:
Because I didn’t understand the circuitry, I check the supply voltages, then started at U350, the differential amp rubbing the DC level knob against the input signal, and worked outward in both directions (clicky for more dots):
The PCB looks like this:
U350 is the round epoxy package in the the square spider-leg array over on the far left. Contrary to what you (well, I) might think, the index mark denotes pin 16, not pin 1:
Which puts pin 1 at the upper right corner of the package on the PCB. The part listing in the manual says
MICROCKT,LINEAR:VERTICAL AMPLIFIER /
SELECTED, which makes perfect sense given Tek’s oscilloscope business; if you needed a high-speed differential amplifier, that’s what Tek’s internal catalog would surely suggest. Newer AM503 revisions use somewhat less unobtainable op amps, although they replace the DC Level knob with one of those newfangled microcontroller thingies for some sweet auto-leveling action.
Nothing seemed out of order. The unable-to-zero condition pushed the bias voltages off the expected values, but nothing seemed completely out of whack / stuck at the rails / broken.
The problem turned out to be in Q230, the first item on Tek’s checklist after the power supplies, even though its bias voltages looked OK. It produces the “Attenuated AC Signal” seen above and lives on another page of the schematics:
Q230 is clad in the natty red heatsink in the PCB picture above. CR226 is the metal TO-18-ish can partially hidden by the orange-red-brown ribbon cable from the DC Level pot.
For future reference, C234 and C244 aren’t installed in this PCB; they’d fit in the conspicuously vacant spots to the right and in front of Q230.
What may not be obvious at a first glance: Q230’s pins sit in teeny individual sockets installed in the PCB. One might remove and reinstall Q230, should one be so inclined and, given that it’s the first active device after the input attenuator, one might imagine such an action being necessary after a catastrophic oopsie.
At this late date, finding a suitable dual JFET would be … difficult, even were one were willing to compromise on the hermetic metal TO-78A package.
Seeing as how Q230 has been sitting quietly in its socket for the last three decades, I proceeded cautiously:
- Turned the power off
- Waited for the supply voltages to drop
- Pulled Q230 slightly upward
- Wiggled-and-jiggled it around
- Shoved it back down
- Turned the power on
I heroically refrained from pulling it completely out of its socket to dab DeoxIT on the pins; JFETs being notorious for susceptibility to static damage and, likely, lube would make no difference anyway.
Fired that devil up and the DC Level knob resumed doing exactly what it should:
The output now has the usual ±200 mV range centered at 0 V. The waveform shows a 100 mA signal at 50 mA/div, produced by a bench supply into a 100 Ω power resistor switched by a DC-DC SSR.
Whew & similar remarks.
Moral of the story: it’s always the connector!
The gap in the rivets along the main truss show where someone pried off the bronze plaque surely commemorating the bridge. The scarred surface suggests a bronze-steel battery was in effect for quite some time.
I’m a sucker for big ironwork:
It’s a look at engineering done in the days of slide rules and limited data, when overengineering wasn’t nearly as bad as ensuring the thing never, ever fell down.
The bolts holding the beams and struts together show considerable confidence:
Each bolt counts as single point of failure, but this one can rust for a long, long time before the risk becomes important.
Each of those gazillion rivets required a crew to heat white hot, shove into the hole, and hammer tight.
They don’t make ’em like that any more and I suppose it’s a good thing …
Posted in Electronics Workbench on 2018-08-14
Ex post facto notes from the fourth Squidwrench Electronics Workshop.
We finally talk about (bipolar, NPN) transistors as current-controlled current sources / sinks, ruthlessly restricted to DC operating conditions.
Scribbled notes of things to cover, contrast-stretched to be slightly more readable:
A bag o’ samples:
Nomenclature, regret expressed as to conventional vs electron current flow, schematic pictures vs. reality, why different packages. All six possible pinouts loose in the wild: always check datasheet and confirm device pin polarity.
Not all TO-92 packages contain transistors: voltage regulators, references, AM receivers, dual diodes, you name it, you’ll find it. When you order a million of something, you can get whatever you want.
junk box parts drawers contain some genuine Mil-Spec 2N2222 transistors in genuine TO-18 metal cans, packed in individual containers labeled with their warranty expiration date. They still make ’em like that, just not for the likes of mere mortals such as I.
Reading data sheets and tamping down optimism: (large print) max voltage and max current ratings always limited by (small print) max power dissipation. Safe Operating Area bounded by datasheet limits, power becomes graceful curve on linear scales = straight line on log-log scales. Handwaving description of secondary breakdown issues, story about killing those ET227 bricks.
DC current gain β = hFE, font flourish catastrophes, uppercase subscripts = DC vs. lowercase = AC, temperature dependence, process dependence, expected spread = don’t count on any particular values.
Just to show what the results should look like, I measured an MPS3704 by hand before class:
Which required two power supplies and three meters:
Which, in turn, prompted me to festoon the class meters with conspicuous masking tape labels!
Seen a bit closer to the origin, with a fixed 100 μA base current and the scope’s arbitrary function generator producing a voltage ramp:
Obviously, you’ll want automation when you do this more than once.
The whiteboard of introductory scribbles, with a plot of expected results:
Small values of collector voltage to remain within allowable power dissipation! Discussion of switching behavior: high current at low voltage, low current at high voltage, avoid crossing the non-SOA (pulse vs DC) expanse, another mention of secondary breakdown.
After painstakingly measuring another MPS3704, compute actual current gain(s) and power dissipation:
With data in hand, we carefully increased the collector voltage with constant base current, ventured slowly into the non-SOA, and eventually measured the same base current producing no collector current at all. No smoke, much to the disappointment of all parties.
The benefit of actually measuring a (sacrificial) transistor cannot be overstated. Lots of baling-wire setup, plenty of mistakes and fumbles, hard lessons in how difficult it is to get useful numbers.
A good time was had by all, despite the absence of non-SOA smoke …
Baofeng UV-5 radios can (mostly) eliminate the loud hiss heard at the end of a transmission before the squelch kicks in after the received carrier drops:
Menu → 34 STE → ON. A detailed description of the option suggests it’s a 55 Hz subaudible tone sent for 250 milliseconds after the sender releases the PTT and before the transmitter stops sending, with the receiver muting its audio during the tone. Obviously, this requires a Baofend radio at each end of the conversation, which applies to our bikes.
Saying “laaaa” while kerchunking (into a smaller dummy load than the hulk) with STE OFF:
Compared to the received audio, the squelch tail hiss is really really loud.
Then with STE ON:
You can see the STE tone reception start about 250 ms before the audio cuts off, although it’s not at all clear the audio is muted on either end. In any event, there’s no squelch tail worth mentioning, even if there’s an audible tick when the STE tone starts.
Saying nothing with STE ON:
It’s unlikely the audio output would include the subaudible tone, but you might convince yourself something happens in the 250 ms between the STE blip near midscreen and the final pop (now clipped) as the audio drops.
All in all, a definite improvement!