Repairing Cheap Chinese Tablet - Identifying Blown IC - Page 1
Forum section: electronics / repair

author: shmuelzon
author_rank: Contributor
timestamp: August 23, 2018, 06:54:46 pm
content: Hello,I'm trying to repair a cheap Chinese tablet that, at this point, fixing it became a matter of principleThe original symptom was that the battery seemed to be dead and wouldn't charge when connected to power via USB cable.I opened up the tablet and the battery measured a 0V flat. I swapped it out with another cell I had on hand and the tablet booted up and charged (albeit was very warm).After I disconnected it from power, it survived for about 20 minutes and then died again. The battery was depleted ~2.6V. I buzzed out both battery terminals and they were shorted.A closer inspection revealed some SOT-32-5 IC had blew up (see attached image). As can be seen, any marking that was on the IC is now long gone so I don't know what to replace it with.At first I thought it was the Li-Po charger (e.g. MCP73831T) but it doesn't seem to fit the traces, which are also attached.The PCB is marked as: AL-MT83XX-706M-V1.0And also with: EQ-2017.06.15I've tried to search online for images of this PCB but only found other revisions (V1.1 and V1.2, thought they have earlier date stamps) and don't seem to have this IC near the battery connections.Does anyone have any idea what this IC might be and/or what I can replace it with?Thanks in advance!

author: Gyro
author_rank: Super Contributor
timestamp: August 23, 2018, 07:00:53 pm
content: From the proximity of that inductor and diode, it's say that it's more likely to be a switching regulator IC. You'll need to map out the circuit to know for sure though.

author: shmuelzon
author_rank: Contributor
timestamp: August 23, 2018, 07:41:41 pm
content: Hey Gyro,Thanks for your reply!As I have no idea what I’m doing, I would really appreciate some more guidance.What is it I should map? Is the close vicinity of the mystery IC enough or something else?Does it make sense that the regulator would be connected to two inputs (USB power and battery)?What about the charging circut? Could it be connected in parallel with the battery to the regulator?Could you send me an example of an IC that might fit the bill?Thanks again!

author: zxase258
author_rank: Contributor
timestamp: August 25, 2018, 03:25:52 am
content: If you provide a more complete partial circuit, it may be easier to determine its type. Below is a list of similar models I found. The possibilities are high to low.LPW5206ABL9198AP2138XC6204

author: shmuelzon
author_rank: Contributor
timestamp: August 26, 2018, 10:23:35 am
content: Thanks for that list!I've tried to map out the circuit with the nearby devices and came up with the attached.I'm not an EE by any means so take it with a grain of salt.I drew the schematics with the same layout as the PCB for easier viewing.The X1 and X2 devices aren't oscillators, but I'm not sure what they are. X1 is probably a cap, not sure about X2.Does that help narrow down the list?Thanks again!

author: shmuelzon
author_rank: Contributor
timestamp: August 28, 2018, 05:32:51 am
content: I was able to get photos from the seller of the PCB with the IC intact.As far as I can tell, it's marked with "CP8c".Unfortunately, I still couldn't find what device that is.I'd appreciate any help if anyone knows it is.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 28, 2018, 08:05:13 am
content: http://www.farnell.com/datasheets/330435.pdf

author: shmuelzon
author_rank: Contributor
timestamp: August 28, 2018, 08:24:30 am
content: Hey PA0PBZ,Thanks for that. Can you please explain to me why you think this is the part?As far as I can tell, it doesn't fit the markings described in the datasheet, nor do I understand how the Li-Po fits in...Thanks in advance.

author: PA0PBZ
author_rank: Super Contributor
timestamp: August 28, 2018, 08:37:34 am
content: Sorry, here is a better datasheet with the correct markings:https://www.torexsemi.com/file/xc9235/XC9235-XC9236-XC9237.pdfThe SOT-25 pinout and the markings match if you keep in mind that you made an error in your drawing (pin 4 and 5 are not connected).C =XC9235, P8 = 1.8V output.Since you don't connect 1.8V to a LiPo cell there is something else wrong, maybe that is the core reason the thing failed?