The Problem
About five weeks ago, I purchased a broken TI-84 Plus C Silver Edition on eBay, hoping to score a relatively cheap third TI-84+CSE to add to my collection. I anticipated that the problem might be the user forgetting to charge the calculator, or at worst, that the battery was bad. Unfortunately, once I got the calculator, it turned out that this was not the case. Although it was in pristine physical condition, it didn't turn on at all, even though the orange charging light turned on when I connected the USB cable. I opened it up, and there was no obvious damage, so the question was, why wasn't it working? I tried to contact TI-Cares about replacing this clearly defective TI-84 Plus C Silver Edition, but unfortunately, if you don't have the original receipt, they won't repair or replace the calculator.
The Investigation
Short of reselling the calculator or writing off the purchase as a waste of $50, there wasn't much I could do besides trying to repair the calculator myself. I sat down one evening last week, pulled out my trusty multimeter, and got to work tracing through the PCB to look for problems. To help myself, I opened up my pink TI-84 Plus C Silver Edition, which I incidentally got for $30 on eBay (and merely needed to be charged for a few hours). With it side-by-side with the broken calculator, I was able to test voltages on both calculators, and see where they were different. To make matters slightly more confusing, the new TI-84 Plus C Silver Edition has a slightly different motherboard revision, which omits a number of components from the older motherboard, presumably as a cost-cutting measure.
I started by investigating the power circuitry, and right away I ran into suspicious numbers. While power seemed to be flowing into the USB port properly (5V everywhere it should be), I noticed a number of 3.3V lines on the functioning calculator that registered either 1.7V or even ~0.015V on the broken calculator. I therefore moved my investigation down to the bottom of the board, near the battery. The potential between B- and B+ on the broken calculator was more or less correct, given that the battery was not fully charged, and up to a certain point, it was providing about 4V to the calculator, however, after a component labelled U03G on the silkscreen, I found that the working calculator had 3.3V, whereas the broken calculator had 0.015V at the output (?) of that component.
The next step was figuring out what exactly U03G was. It was only labeled on the tiny SMT component itself as "PCNI", which I worried would not be very specific. Luckily, a Google search yielded (far down the first page) a datasheet for ultra-low-power 100-mA low-dropout linear regulators: PCNI turned out to refer to a 3.3V low-dropout regulator, the TPS76933DBVT.
The Solution
Thanks to the generosity of TI's sampling program, I was able to order a pair of free sample TPS76933DBVT chips, and a week later, I sat down to try replacing what I now hoped was a faulty linear regulator. I unfortunately wasted one of my new replacements trying to solder it on, but the second time was the charm. Once I popped in the battery, connected the USB cable and tried to power up the calculator, it worked! I measured the power at the output pin of the voltage regulator, and lo and behold, a steady 3.3V was flowing to the rest of the calculator.
The question remains how this calculator sneaked out of TI's factory. Did it somehow get missed during QA? Does TI not test every calculator that rolls out? Did the linear regulator have some kind of fault that only appeared after it was in use for a short period of time? Did someone connect the battery backwards and damage the regulator in some way? At any rate, the calculator works now, and I'm happy to have three functioning TI-84 Plus C Silver Editions for project testing.
About five weeks ago, I purchased a broken TI-84 Plus C Silver Edition on eBay, hoping to score a relatively cheap third TI-84+CSE to add to my collection. I anticipated that the problem might be the user forgetting to charge the calculator, or at worst, that the battery was bad. Unfortunately, once I got the calculator, it turned out that this was not the case. Although it was in pristine physical condition, it didn't turn on at all, even though the orange charging light turned on when I connected the USB cable. I opened it up, and there was no obvious damage, so the question was, why wasn't it working? I tried to contact TI-Cares about replacing this clearly defective TI-84 Plus C Silver Edition, but unfortunately, if you don't have the original receipt, they won't repair or replace the calculator.
The Investigation
Short of reselling the calculator or writing off the purchase as a waste of $50, there wasn't much I could do besides trying to repair the calculator myself. I sat down one evening last week, pulled out my trusty multimeter, and got to work tracing through the PCB to look for problems. To help myself, I opened up my pink TI-84 Plus C Silver Edition, which I incidentally got for $30 on eBay (and merely needed to be charged for a few hours). With it side-by-side with the broken calculator, I was able to test voltages on both calculators, and see where they were different. To make matters slightly more confusing, the new TI-84 Plus C Silver Edition has a slightly different motherboard revision, which omits a number of components from the older motherboard, presumably as a cost-cutting measure.
I started by investigating the power circuitry, and right away I ran into suspicious numbers. While power seemed to be flowing into the USB port properly (5V everywhere it should be), I noticed a number of 3.3V lines on the functioning calculator that registered either 1.7V or even ~0.015V on the broken calculator. I therefore moved my investigation down to the bottom of the board, near the battery. The potential between B- and B+ on the broken calculator was more or less correct, given that the battery was not fully charged, and up to a certain point, it was providing about 4V to the calculator, however, after a component labelled U03G on the silkscreen, I found that the working calculator had 3.3V, whereas the broken calculator had 0.015V at the output (?) of that component.
The next step was figuring out what exactly U03G was. It was only labeled on the tiny SMT component itself as "PCNI", which I worried would not be very specific. Luckily, a Google search yielded (far down the first page) a datasheet for ultra-low-power 100-mA low-dropout linear regulators: PCNI turned out to refer to a 3.3V low-dropout regulator, the TPS76933DBVT.
The Solution
Thanks to the generosity of TI's sampling program, I was able to order a pair of free sample TPS76933DBVT chips, and a week later, I sat down to try replacing what I now hoped was a faulty linear regulator. I unfortunately wasted one of my new replacements trying to solder it on, but the second time was the charm. Once I popped in the battery, connected the USB cable and tried to power up the calculator, it worked! I measured the power at the output pin of the voltage regulator, and lo and behold, a steady 3.3V was flowing to the rest of the calculator.
The question remains how this calculator sneaked out of TI's factory. Did it somehow get missed during QA? Does TI not test every calculator that rolls out? Did the linear regulator have some kind of fault that only appeared after it was in use for a short period of time? Did someone connect the battery backwards and damage the regulator in some way? At any rate, the calculator works now, and I'm happy to have three functioning TI-84 Plus C Silver Editions for project testing.
