A few days ago, I tried to cut into the PCB of my TI-84 Plus CE (pre-A) while it was powered and with metal wire cutters. I figured things would be fine because there were no traces where I was cutting. However, the moment I made the first cut, the calculator died and wont turn back on. I got it on video here: https://cdn.discordapp.com/attachments/466808269789200387/862157326630780998/MVI_3217_Trim.mp4

Here's where I cut. There's no visible traces that I can see. Only the ground plate built into the pcb.

(Click for full size)

I'm not convinced the cut is the culprit. The battery had some bare leads soldered to it that were loose. My guess is the lead brushed up against something it shouldn't have and shorted. Unfortunately, I've had a difficult time diagnosing what exactly is broken. I don't have another pre-A so I tried testing the resistance between ground and different test points then comparing the values to my rev-C. I did get different values for some test points but I don't know if it's due to a short, or the hardware difference.


I may have figured it out though. In the photo below I marked a few things. The exposed battery lead was the positive end which means the current found the path of least resistance to ground (Places that connect to ground I circled in orange). This could mean that irreplaceable components like the ASIC and FLASH are OK. The lead could only reach a certain distance so I marked its reach in pink. I had quite a lot of the PCB protected by Kapton tape (marked in yellow) so I know it couldn't have shorted through that.

Therefor it could only have shorted through the reset button (I don't think it did because of the way I was holding it), the big capacitor (the shell is isolated from the PCB so it couldn't be the issue), or the pads for the missing capacitor (marked in red). The top pad is connected directly to ground so that couldn't have fried anything. Therefor I'm pretty sure the battery shorted through the bottom pad. The light blue area is potential path the short could have gone though.

Thank you so much to DrDnar for the super high res image! You can click it for the full size guys.

I measured the values of each component in the light blue area and marked the values in orange. I then compared it to my rev-C and marked that in purple. The values were a bit different, but again, I'm not sure if that's due to a short or the hardware revision. I'm not sure how to test the black box I put a question mark over or even how to find a replacement.


Hopefully this area is my issue and not anything more serious. Does anyone know what this circuit even does? Do you know if my values are nominal for my hardware revision (pre-A)? Where can I get replacement components? Wish me luck fixing this.

So besides the obvious fact that you should never cut into a PCB if you don't know what is underneath *especially* while power is applied (let alone with dull wire cutters) -- here are some other things that you should follow:

1) Use a properly grounded ESD mat or table. For all you know you zapped a chip with a buildup of static electricity on your body.
2) Use proper ESD protections such as a wrist grounding strap whenever you work with bare electronics! Even just moving them around can easily damage them.
3) Use flux and isopropyl alcohol when soldering. Those solder joints need some love.

Anyway, good luck fixing

I happen to have a working Pre-A 84+CE. I measured these resistances between test points and ground:

All values are in ohms.
3.85k Ohms is across the capacitor, which is what I think you measured. If it was from the circled point to ground, it's about 5 ohms.

I also tested resistors of known values on my multimeter:
Nominal 1k: measured 0.99k
Nominal 100k: measured 102.2k
My multimeter doesn't have the capacitance option, so I'm not sure what the capacitors should measure. If they're undamaged, it's best to measure them desoldered from the board.

So it looks like for the same board, we have considerably different measurements. I'd hazard a guess that your hunch is true, but I don't have enough experience to say for certain. Either way, good luck with the repair

Thank you Pi Time! That information is invaluable! I believe the slightly lower resistance you were getting on those capacitors are just because they weren't fully discharged. This means I know both my capacitors are toast and I'll have to get new ones of those.

Thanks to Zeroko and TIny_hacker, I can now get the exact voltage regulator IC I need! The datasheet gave the the general part type: RP130x and the tiny WJ marked on the component lead me to the exact part: RP130K421D-TR. (fun fact: this is the same voltage regulator found on the Nspire CX II)

You can't measure capacitance/resistance like that at all? Unless you have a proper ESR meter or similar? And getting the values of components at this stage is basically useless.

The steps to diagnosing a PCB are:

1) Visual inspection, look for corroded solder pads, blackened parts, loose metal, slivers, etc.
2) Physical inspection, power the board and use an IR sensor to look for hot points which might indicate a short.
3) Voltage probing, probe various points on the PCB based on the layout of the board
4) Logic analysis, use an oscilloscope or logic analyzer and hook it to various buses such as a flash chip or spi interface.
5) Only then do you start worrying about what parts you need to replace.

Please don't just rip out random components and replace them with things you find; you are only going to make things worse until you determine what is actually causing the issue. You need to start creating a schematic of the various pieces of the pcb as well.

1. Nothing looks out of the ordinary
2. I don't have an IR camera
3. That I can do, see below
4. I don't have an oscilloscope or analyzer
5. Noted.

Mateo also suggested I make a voltage map of the area:

(Click for full size)
As you can see, the voltage is obviously not behaving the same. It's stopping at the voltage regulator IC on the broken CE. The voltage is also way higher on the right side of the PCB where the two transistors are. It seems as though resistors R04G and R46G are acting as jumpers?

I noticed that there's a trace on the back of the PCB that leads to the CE pin of the voltage regulator and I squared the connections in purple. It looks to be that the transistor Q04G is suppose to set that pin but something is causing the transistor to divert the power up to resistor R46G?

Hopefully I'm interpreting my own map remotely correctly. My limited knowledge of ICs is making this painful.

I don't feel like disassembling my calculator again, but the TI-84+CE might have a multilayer board, meaning there are traces inside the board. Notably, it's common for four-layer boards to distribute power and ground on the middle two layers, forming a capacitor. If that's the case, you would have shorted the internal power planes.

That was exactly right! I spent a good 45 minutes cleaning up the cuts. Once I plugged in the battery, I saw the backlit turn on and the calculator actually booted up! TI's engineers may not know how to make a sensible calculator, but they sure know how to make a robust calculator!

Thank you so much everyone for your help! I've learned a lot in the past week. Hopefully I won't have to learn this stuff the hard way again.

I'm glad that you've managed to get it working again!

Yay! That's awesome!

I'm happy to hear you got it fixed! And to learn a couple more ways to diagnose electronics.



Thanks for bringing this up here. I had thought about cutting a circuit board elsewhere and might have made the same mistake had I not seen this.