Advertisements

Posts Tagged Repairs

Tektronix AM503 Current Probe Amplifier: DC Level Fix

One of my Tektronix AM503 Hall Effect Current Probe Amplifiers (B075593, for future reference) lost its DC Level zero-ing capability:

Tek AM503 front panel

Tek AM503 front panel

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:

Tek AM503 Amplifier - troubleshooting zero set problems

Tek AM503 Amplifier – troubleshooting zero set problems

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):

Tek AM503 Current Probe Amplifier - p 61 - Output Amplifier schematic

Tek AM503 Current Probe Amplifier – p 61 – Output Amplifier schematic

The PCB looks like this:

Tek AM503 - Q230 PCB detail

Tek AM503 – Q230 PCB detail

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:

Tek AM503 Amplifier - Tek-unique IC pinout reference

Tek AM503 Amplifier – Tek-unique IC pinout reference

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:

Tek AM503 Amplifier - Q230 detail

Tek AM503 Amplifier – Q230 detail

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:

Tek AM503 - Q230 reseated

Tek AM503 – Q230 reseated

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!

Advertisements

4 Comments

Park Spoke Tension Meter vs. 20 inch Wheel Spokes

Obviously, the good folks at Park Tool never anticipated a three-cross spoke pattern on a 20 inch wheel:

Park Tool Spoke Tension Meter vs 406 wheel

Park Tool Spoke Tension Meter vs 406 wheel

It’s my trusty Park Tool TM-1 Spoke Tension Meter, unchanged since shortly after the turn of the millennium.

For future reference, the rebuilt wheel spoke tensions came out around 25, slightly lower than the 27-ish I measured on Mary’s bike; it didn’t occur to me to measure the tension until after I’d relaxed the spokes. I’ll ride it for a while before doing any tweakage.

The spoke pattern is pretty close to four-cross, due to the large-flange Phil Wood hubs:

Tour Easy Front Spoke Pattern

Tour Easy Front Spoke Pattern

Which makes for a hella-strong wheel, particularly seeing as how it’s very lightly loaded. The Tour Easy we got for our lass came with a radially spoked rim around a Phil hub.

I transferred the hub and laced spokes intact to the new rim by the simple expedient of duct-taping the spokes into platters, removing the nipples, stacking the rims, sliding the spokes across into their new homes, reinstalling the nipples, then tightening as usual.

3 Comments

Presta Valve to Schraeder Hole Adapter

The front rim on my Tour Easy developed a distinct bulge, of the sort usually caused by ramming something, but I’m not Danny McAskell and the bulge got worse over the course of a few weeks, suggesting the rim was deforming under tire pressure. Having ridden it upwards of 35 k miles with plenty of trailer towing and too much crushed-stone trail riding, the brake tracks were badly worn and it’s time for a new rim.

An Amazon seller had an identical (!) rim, except for the minor difference of having a hole sized for a Schraeder valve stem, rather than the Presta valves on the original rims. One can buy adapters / grommets, but what’s the fun in that?

The brake track walls are 1.5 mm thick on the new rim and a scant 1.0 mm on the old rim, so, yeah, it’s worn.

A few measurements to get started (and for future reference):

Presta to Schraeder Adapter - dimension doodle

Presta to Schraeder Adapter – dimension doodle

If you don’t have an A drill, a 15/64 inch drill is only half a mil larger and, sheesh, anything close will be fine.

Introduce a suitable brass rod to Mr Lathe:

Presta-Schraeder Adapter - parting off

Presta-Schraeder Adapter – parting off

Break all the edges and drop it in place:

Presta-Schraeder Adapter - installed

Presta-Schraeder Adapter – installed

One could argue for swaging the adapter to fit flush against the curved rim, but commercial adapters don’t bother with such refinements and neither shall I.

The 7.0 mm length got shortened to fit flush with the center of the rim:

Presta-Schraeder Adapter - valve stem installed

Presta-Schraeder Adapter – valve stem installed

It’s brass, because the rim is heaviest on the far side where the steel pins splicing the ends live, and, with the tube & tire installed, the rim came out almost perfectly balanced. Which makes essentially no difference whatsoever, of course.

The shiny new rim sports shiny new reflector tape (from the same stockpile, of course).

That was easy …

, ,

4 Comments

Monthly Science: Cheese Slicer Epoxy Coating vs. Water

Ten months ago, I cleaned the corrosion off our favorite cheese slicer:

Cheese slicer - aluminum corrosion

Cheese slicer – aluminum corrosion

After cleaning, I coated it with XTC-3D epoxy:

Cheese Slicer - epoxy coat - detail

Cheese Slicer – epoxy coat – detail

We’ve been using it daily ever since and it spends most of its life drip-drying in the dish drainer. I added a third opening to the cheerful orange measuring spoon holder just for the slicer.

A few weeks ago I noticed corrosion once again growing on the handle:

Cheese Slicer - epoxy coat - corrosion - detail

Cheese Slicer – epoxy coat – corrosion – detail

I think the rot comes from water diffusing through the epoxy, rather than gross leaks through damage or pinholes. The tip of the handle has the most corrosion, probably due to the water drop hanging there, even though it also has the thickest epoxy coating: it cured with the handle pointing downward.

Verily, rust never sleeps …

Leave a comment

Cheap Flashlight Boost Converter: Capacitor FAIL

A long time ago, a pair of white LED + red laser flashlights powered by an AA cell diverged: one flashlight worked fine, the other always had a dead battery. The latter ended up on my “one of these days” pile, from which it recently emerged and accompanied me to a Squidwrench Tuesday session:

Small Sun flashlight - original wiring

Small Sun flashlight – original wiring

The black wire trailing from the innards goes to the battery negative terminal, with the aluminum body providing the positive terminal connection to the wavy-washer spring contact visible atop the rear PCB inside the front shell.

The switch connects each red wire to the battery negative terminal, so there’s a color code issue in full effect. The two red wires burrow through holes in the rear PCB (shown above) and connect to the negative terminal of the laser module (the brass cylinder near the top) and the negative terminal ring on the front PCB holding the seven white LEDs:

Small Sun flashlight - original wiring - LED laser board

Small Sun flashlight – original wiring – LED laser board

Continuing the color code issue, the black wire from the laser is its positive terminal. The out-of-focus wire (an LED pin) sticking up near the top of the picture carries the positive connection to the LED ring. The red wires from the switch are the negative connections for the LEDs and laser.

Voltages applied to the LED ring and the currents flowing therein:

Small Sun flashlight - 7x white LED current vs voltage

Small Sun flashlight – 7x white LED current vs voltage

Seven LEDs at 20 mA each = 140 mA, so the voltage booster must crank out slightly more than 3.2 V. They’re not the brightest white LEDs I’ve ever seen, but suffice for a small flashlight.

A crude sketch of the PCB layout, with a completely incorrect schematic based on the mistaken assumption the SOT23-3 package was an NPN transistor:

Small Sun flashlight - schematic doodle

Small Sun flashlight – schematic doodle

Obviously, that’s just not ever going to oscillate, even if the 2603 topmark meant a 2SC2603 transistor, which it doesn’t.

A bit more searching suggests it’s a stripped-down Semtech SC2603A boost converter, normally presented in a SOT23-6 package. If you order a few million of ’em, you can strip off three unused pins, do some internal rebonding, and (presumably) come out with an SOT23-3:

Small Sun flashlight - correct schematic doodle

Small Sun flashlight – correct schematic doodle

That topology makes more sense!

Before going further, I had to rationalize the colors:

Small Sun flashlight - rewired LED laser board

Small Sun flashlight – rewired LED laser board

Soldering longer leads to the PCB allows current & voltage measurements:

Small Sun flashlight - LED current test

Small Sun flashlight – LED current test

With the LEDs and laser disconnected, the converter seems to be struggling to keep the capacitor charged:

Small Sun flashlight - V boost I 200mA-div - idle

Small Sun flashlight – V boost I 200mA-div – idle

Those purple spikes come from the current probe at 200 mA/div: maybe half an amp in 5 μs pulses at 6 kHz works out to a 15 mA average current, which is pretty close to the 11 mA I measured; it’s not obvious the Siglent SDM3045 meter was intended to handle such a tiny duty cycle.

Obviously, the output capacitor is junk and, after removing it, the AADE L/C meter says NOT A CAPACITOR. Perhaps it never was one?

Measuring the cap in the good (well, the other flashlight) suggests something around 100 nF, so I installed a random 110 nF cap from the stash. The current peaks are about the same size:

Small Sun flashlight - I 200mA-div - 110nF cap

Small Sun flashlight – I 200mA-div – 110nF cap

The cap voltage (not shown) is now nearly constant and the 50 Hz PWM rate reduces the average battery current to 100-ish μA:

Small Sun flashlight - I 200mA-div - color-grade - 110nF cap

Small Sun flashlight – I 200mA-div – color-grade – 110nF cap

Not great, but tolerable; a 1000 mA·h battery will go flat in a few months.

The LED current runs a bit hotter than I expected:

Small Sun flashlight - I 200mA-div - LED current - 110nF cap

Small Sun flashlight – I 200mA-div – LED current – 110nF cap

The bottom is about 200 mA and the average might be 350 to 400 mA.

Compared with the other flashlight:

Small Sun flashlight other - I 200mA-div - LED current

Small Sun flashlight other – I 200mA-div – LED current

So the cap is maybe a bit too small, but it likely doesn’t matter.

Done!

,

Leave a comment

Bathroom Sink Drain: Epoxy Touchup

The glaring white ring around the drain comes from Magic Porcelain Chip Fix epoxy:

Bathroom Sink Drain - Epoxy fill

Bathroom Sink Drain – Epoxy fill

What looks like a blob on the left side covers the missing chip, with the rest of the ring filled in to make it look like I knew what I was doing. The drain dried out while we were on vacation, having been scrubbed clean before we left, making for the best surface preparation I could provide.

As it turns out, our resident iron bacteria took about a week to set up shop along the bottom of the ring, producing a pair of small rust-colored dots that will inevitably spread to encompass the whole thing. They’re endemic in the plumbing, impossible to kill off, and nothing more than an unsightly nuisance.

Leave a comment

Amazon Basics AA Cells: Capacity

Being that sort of bear, I (sometimes) note the date on cells when I change them, as with this notation on the AA alkaline cells in the Logitech trackball:

Amazon Basics AA cell - mouse runtime

Amazon Basics AA cell – mouse runtime

These Amazon Basics AA cells lasted almost exactly two years, compared with 15 and 20 months from the previous two pairs of Duracell AAs. A few months one way or the other probably don’t mean much, but the Amazon cells aren’t complete duds.

The new Amazon Basics cells have a gray paint job, so they’ve either changed suppliers or branding.

,

2 Comments