Engineer and classic computing fanatic Steve Chamberlin has written up a profitable enterprise in diagnosing a bodily fault with a circuit actually 1000’s of miles and an ocean away — sight unseen.
“A couple of days in the past I obtained an pressing word from the power in China the place the ADB-USB Wombat Enter Converter is manufactured,” Chamberlin explains. “They’d simply completed assembling 250 new Wombats, and all of them had been failing the automated purposeful checks. Was there a scientific meeting flaw on this total batch of boards? Or was it an issue with the take a look at equipment?”
With a manufacturing unit 1000’s of miles away experiencing a test-jig failure, this “horrible” schematic proved invaluable for distant prognosis. (📷: Steve Chamberlin)
The manufacturing unit was utilizing the Wombat Tester, a pogo-pin-loaded take a look at jig designed to sit down beneath the Wombat — in flip a customized adapter board designed to permit trendy USB keyboard and mice to speak to classic Apple ADB-based {hardware}, or vice-versa. The take a look at board runs by means of the assorted options of the Wombat and ensures all the things is correctly — however, in response to the manufacturing unit, had immediately began rejecting each board it sees.
“The Wombat tester that is utilized by the Chinese language manufacturing unit is the unique one which I hand-built six years in the past,” Chamberlin noets. “I had solely two items of data: the purposeful take a look at was failing ADB communication. USB communication was apparently OK; an engineer discovered that bridging pins 6 and seven on IC2 allowed the take a look at to complete efficiently. The engineer suspected IC2 had failed and was looking for a substitute.”
Chamberlin wasn’t so certain, although. “IC2 pins 6 and seven are two separate energy provides: one which’s usually provided by the ADB host, and one other that is provided by the USB host. Just one ought to be lively at a time, so bridging them collectively will not be a sound take a look at,” he writes. “As a result of bridging helped the failing ADB take a look at to succeed, it steered possibly the ADB5V provide was not turning on when it ought to. Sure, that could possibly be on account of IC2 failure, however may additionally produce other causes.”
The tons of of boards failed by the manufacturing unit turned out to be nice, with a single wobbly pogo pin guilty for the false negatives. (📷: Steve Chamberlin)
Chamberlin started strolling by means of the self-confessedly “horrible” schematics for the board — “I’ve nobody guilty however myself,” he admits — and traced the availability to a quad-OR gate, and from there to indicators generated by IC1 — and handed down by means of one of many take a look at board’s pogo pins, spring-loaded pins designed to push towards take a look at factors for speedy however non permanent connectivity.
“What would occur if that pogo pin had been misaligned or damaged? UNK3 can be disconnected and floating, however the R2 pull-down resistor would carry it low. With GNDOUT additionally low, /VBUSBON would all the time be asserted and /ADB5VON would by no means be asserted. The ADB energy provide would by no means activate, the USB provide would all the time be on, and the habits can be in keeping with the noticed clues,” Chamberlin explains.
“Mulling this evaluation over a cup of espresso, I replied to the manufacturing unit: ‘Verify if pogo pin 3 at J7 is bent.’ Their response got here the subsequent day: ‘Steve, you’re actually knowledgeable engineer! The guess you made is appropriate. After attempting once more, the tester board began working usually.’ Success! That was a really satisfying repair, primarily based on minimal data and with out bodily entry to the defective circuit.”
Chamberlin’s full write-up is out there on his weblog, Huge Mess O’ Wires.
