Advertisements

Archive for August, 2010

Monthly Aphorism: On Production Quotes

  • We can ship that many, but you must give us time to build the factory.

Once Upon A Time, back in the IBM Video Disk project, a friend (herein known as LLT) built a demodulator for the video data streaming off the disk. This being the development phase of the project, cost was not much of an object, so he used a quartet of high-speed TRW TDC1003J digital multiplier-accumulators running in pipelined parallel to handle the data rate.

While a 175 ns MAC isn’t a big deal these days, it was a state of the art TTL chip back then: a finned-heatsink 64-pin DIP package that cost approximately a bazillion dollars. The board was maybe two feet on a side in classic Wire-Wrap style.

We called them Multifryers: each chip drew 750 mA at 5 V. That board had many cooling fans.

Then, One Fateful Day, came a request to quantify just exactly how much it would cost to build a production version of the player. LLT pointed out that the demodulator board itself would cost more than a really spiffy car, but to no avail: he had to come up with a cost estimate for a fairly large production volume of the as-built hardware.

So LLT calls up TRW and asks for a quote on thus-and-so-many parts, with delivery to-be-determined. There’s a long silence, after which they tell him they’ll have to get back to him on that.

Time passes.

Eventually he gets the quote. He had to tell them to not start building the factory right away, because the project was most likely doomed. Much relief was expressed…

Advertisements

2 Comments

Banishing a Mysterious Rear-Wheel Squeak

So the bike started making a weird whistling squeak. Noises on a bike are never a good sign, but it took me nearly two weeks to banish this one…

Differential diagnosis:

  • Toward the rear: not pedals, not chainring
  • Only while pedaling: not sprocket cluster bearings
  • Depends on chain speed: not sprocket

Conclusion: it’s the chain.

My shop assistant had done a massive chain-cleaning and lubricating exercise when we got back from vacation, so I guessed that a few links (of 250-ish) had escaped proper lube. I gave ’em a dose that didn’t help, so I went Old Skool on the thing.

Coiled it flat in a saucer, immersed it in denatured alcohol to displace air and water-based cleaner inside the links, then drained the alcohol. Poured a generous layer of light machine oil over the whole affair, let it sit for a day. Drained for a pair of rainy days by hanging from a floor joist in the basement. Used up a bunch of rags while wiping the thing down (I have an oily-waste can, they’re not sitting in a wastebasket).

Misrouted chain in rear derailleur

Misrouted chain in rear derailleur

Put it back on the bike, only to discover the chain was now vibrating something awful. Checked the rear end and found that I’d managed to route the chain through the rear derailleur along almost the right path…

Fixed that and the squeak was still there. OK, it’s not the chain.

The only remaining possibility: derailleur jockey pulleys.

Took ’em off without dismounting the derailleur and, lo and behold, the steel-on-plastic bearing surfaces were bone dry and a bit dusty. They’re supposed to be self-lubricating, which is probably true for the first few thousand miles, but I cleaned ’em out and added a dab of grease.

Problem solved… for a while, at least.

The only downside is that the chain will be flinging oil for the next month, no matter how often I wipe it down. There’s a good reason I stopped using light machine oil on chains!

2 Comments

Mobile Amateur Radio Power: Check the Fuseholders

The Yaesu FT-857 I have in the car has been not turning on lately, which I feared had something to do with being cooked inside a closed van for a week on the top level of a Camden parking garage during the hottest part of the summer.

But, no, as it turned out, that had nothing to do with it: when I got the radio on the workbench, it powered up just fine. Back in the car, it’s dead.

Which implies a power problem. The radio power comes from 10-AWG zip cord, through a pair of 40 A fuses, directly from the battery. The zip cord terminates in Anderson Powerpoles (of course) under the driver seat, mated to the end of the cable that came with the radio. That cable uses craptastic Molex connectors (equally of course) that are instantly suspect when any problems arise, plus a pair of smaller in-line 3AG glass fuses.

Voltage at the Molex connectors: anything from 4.8 V to 11.9 V, depending on imponderable factors. Voltage at the Powerpoles: ditto. So maybe it’s not the Molex connectors, after all.

The 40 A fuses are the kind the high-power automotive sound system folks use, complete with gratuitous goldish-plated everything. These I got surplus at a minute fraction of sticker price and mounted on the air filter housing, thusly:

Engine compartment fuses for radio power

Engine compartment fuses for radio power

I plugged a 12 V bulb in place of the radio, then went a-measuring. Voltage downstream of the hot fuse: 0 V. Tah-dah, it’s a bad fuse!

Nope, the fuse element is intact.

The zip cord terminates in ferrules penetrated by 1/8-inch setscrews. Applying a wrench, I find that the setscrews are somewhat loose, although nothing catastrophic. Tighten all four screws and the radio turns on just fine.

Case closed!

Until the next day, when the radio doesn’t turn on. Reinstall the lamp, re-measure, once again find 0 V downstream of the hot fuse.

Pull the fuse out again and it comes apart in my hand.

Defective 40A fuse

Defective 40A fuse

Huh. That would explain everything.

I suspect the fuse was marginally defective from the factory and finally failed after that prolonged heat wave. Living in the engine compartment isn’t easy under the best of circumstances, so I’ll give this one a pass.

Being that sort of bear, I plucked a spare fuse from the ziplock baggie of fuses & bulbs that’s tucked into the van’s jack compartment, popped it in place, and the radio works fine again.

Problem solved, for sure!

Side note: those fuseholder screws go through the air filter housing, into nuts with Loctite, and I ruined the threads to absolutely prevent the nuts from coming off. You really don’t want a nut loose inside the engine air intake, downstream of the air filter and upstream of the throttle…

6 Comments

Silver-Soldering a Stainless Steel Measuring Cup

Quite some years ago, the spot weld holding half of the handle to the side of my all-time-favorite 1/3-cup measuring cup broke loose. The minuscule weld nugget suggested that the spot welder got distracted; the weld on other side of the handle is perfectly bonded.

I tried repairing it with silver solder and a torch, which simply proved that’s not within my skill set. I buffed off most of the residue and applied JB Weld epoxy, which lasted just fine until a few days ago. It’s a low-stress situation, indeed, but I’m not surprised that the epoxy didn’t really bond to a slightly scuffed stainless steel surface.

So, this time around, I did it right: sandpapered off the epoxy, scuffed up the cup and handle by shoe-shining a sandpaper strip face up and face down in the gap, then silver-soldered the handle in place using my resistance soldering gadget (which I promise to describe some day).

The setup was straightforward. Clamp the cup in the bench vise with soft copper jaws (hammered from ordinary pipe) that also grip one electrode from the soldering unit.

Silver-soldered handle - left side

Silver-soldered handle - left side

Silver-soldered handle - right side

Silver-soldered handle - right side

I used a strip of fancy Brownell’s Silvalloy 355 silver solder ribbon (which is 56% silver instead of the chintzy 4% junk I normally use) with some truly toxic silver solder flux. About ten seconds of heat melted the solder and produced a pair of nice fillets along the sides of the handle.

The flux washed off in hot water and a few licks with fine sandpaper cleaned things up just fine. The carbon electrode left a bit of schmutz on the handle, but nothing a Dremel brass brush wheel couldn’t solve.

The inside has a bit of heat discoloration, but the sandpaper knocked that back reasonably well, too.

Heat discoloration inside cup

Heat discoloration inside cup

The final product looked a lot better than these in-work pictures, but it’s tough to photograph subtle differences in a shiny silver object.

Anyhow, as you might expect, we value function over form in this household.

3 Comments

Dry Ice Sublimation Rate

For reasons I’m not at liberty to discuss, we had a cooler of dry ice pellets in the freezer for a few days. I used about a pound of it a time to mumble.

I started with “10 pounds” of dry ice in a half-pound Styrofoam container with 1.5-inch thick walls; the total weights include the container. For what it’s worth, dry ice costs $3.50/pound under 10 pounds, then $2.75/pound over that. It used to be plenty cheaper in the old days, evidently, but everything else was, too.

In between withdrawals, the cooler sat in the freezer and and the dry ice quietly sublimated; here’s how the weight varied between uses.

Starting weight: 9.2 lb gross, so I lost quite a bit in transit. Which, as it happened, was about half an hour in a bike trailer during a rather hot afternoon.

A) 7.2 to 6.7 lb -> 0.5 lb / 15 hr = 0.033 lb/hr

B) 3.8 to 3.0 lb -> 0.8 lb / 11 hr = 0.072 lb/hr

C) 2.7 lb to 2.0 lb -> 0.7 lb / 11 hr = 0.064 lb/hr

I’m suspicious of that low number for the first stay, too. Maybe a side effect of having the cooler’s cavity nearly full of dry ice? Or the freezer ran defrost cycles for the other two?

Anyhow, to a back-of-the-envelope resolution the cooler loses a bit over 0.05 lb/hour of dry ice. Call it 15 hr/lb.

The temperature of sublimation is, according to Wikipedia, -109°F. The freezer is around 0°F: a differential of 109°F across 1.5 inches of Styrofoam. Assuming the cooler foam has R=4 with units of (ft^2·hr·°F) / (BTU·in ) and an internal surface area of 304 in^2, the cooler leaks heat at 38 BTU/hr. Call it 11 W.

Cross check: Wikipedia says the enthalpy of sublimation at STP is 571 kJ/kg. Sublimating 0.07 lb = 0.031 kg requires 18 kJ (18 kW·s) and doing that over the course of an hour requires 5 W.

Well, considering the rough-and-ready measurements and the fact that the freezer isn’t at STP and that I’m ignoring gas leakage and a bunch of other stuff, a factor of two error is spot on.

If I were you, though, I’d double-check those calculations before leaping to any particular conclusions. Fair enough?

When all was said and done, I found this thing in the bottom of the cooler. It wasn’t there when we started, soooo

Dry Ice Thing

Dry Ice Thing

13 Comments

Stopping Bike Helmet Strap Creepage

My bike helmet sports a mirror, microphone boom, and earbud, so I generally hang it from the top of the seat on my Tour Easy. There’s a convenient peg seemingly made for capturing the triangle of strap that normally goes over my ear and, up to the point where I set up this helmet, everything was good.

Helmet hanging on Tour Easy seat

Helmet hanging on Tour Easy seat

After about a week, I noticed that the buckle was grossly off-center under my chin: the straps had shifted to one side.

Come to find out that the front strap on this helmet passes through an opening across the central member, below the plastic covering. Judging from the teardown of an older helmet, Bell used double-stick tape to hold the strap in place. Applying a constant force in one direction (I’m a creature of habit, the helmet always hangs from its right-side triangle) gently pulls the strap through the passage.

Front strap passing through helmet

Front strap passing through helmet

So I cut two slabs of closed-cell foam and jammed them into the opening atop the strap, one from each side, with a screwdriver. That forced the strap against the adhesive and mechanically wedged it in place.

Problem solved!

10 Comments

Chipmunks Discover Agriculture

Chipmunk atop sunflower

Chipmunk atop sunflower

One of the sunflowers in the garden started swaying wildly, despite having no breeze at all. I though it was a goldfinch plucking seeds, but a quick look through the binoculars showed a brown furry tail hanging below the topmost seed head, about five feet off the ground.

Mary reports that this is one of five sunflower plants growing in a tight group near the garden fence; she thinks a chipmunk’s seed stash sprouted. This could be the start of something big: next thing you know, they’ll be planting seeds and harvesting crops!

As we watched, the critter’s cheeks became more and more distended.

Chipmunk stuffing cheek pouches

Chipmunk stuffing cheek pouches

Eventually, however, gluttony overcame common sense.

Getting the last seed

Getting the last seed

Seconds after the shutter clicked, the sunflower head disintegrated, depositing the chipmunk on the ground with a rustle and a soft plop.

No damage done, we’re sure, and that critter’s pantry should be stuffed full in short order. Next year Mary will probably do some extensive sunflower culling to get room for the rest of her crops!

And, yes, the title is a riff on Bears Discover Fire

Leave a comment