Toshiba satellite a300 laptop - Page 1
Forum section: electronics / repair

author: Bukurat
author_rank: Regular Contributor
timestamp: August 10, 2014, 10:29:48 am
content: I have a Toshiba a300 that runs ok on battery but refuses to start when the AC adaptor is plugged in. If it's running and the AC adaptor is plugged in it immediately reboots. I did a search on the symptoms and found the culprit is usually the NEC/TONKIN capacitor that bypasses the power rail at the CPU.I found disassembly instructions on the 'net and disassembled the laptop so I could remove the capacitor. After removing it and cleaning the area I measured the resistance between the power rails, to my surprise I measured about 0.6. Ohm.Normally I would look for a short, but after rechecking the area around where I removed the capacitor, including looking at the board under a USB microscope, and finding nothing I'm beginning to wonder if what I saw was the result of circuit elements conducting due to the 1V potential at the meter terminals.The CPU ( which was removed ) runs on 1.6V according to the schematic I found.My question, to anybody with experience in laptop repair, is whether this is plausible?

author: tautech
author_rank: Super Contributor
timestamp: August 10, 2014, 08:26:42 pm
content: Try another AC adapter to eliminate yours as the problem.

author: wraper
author_rank: Unknown
timestamp: August 10, 2014, 08:50:33 pm
content: Quote from: Bukurat on August 10, 2014, 10:29:48 amDid you remove CPU when measuring? They can measure very low on power rail. Also very strange about capacitor on CPU power causing reboot because of the AC adapter. It should not mater at all and if there is a short, it should not work from any power at all because power goes from the same step down converter anyway. There might be a fault somewhere between step down converter and AC adapter. There likely is some schematic with mosfets which switches step down converter power between battery and adapter.

author: G7PSK
author_rank: Super Contributor
timestamp: August 10, 2014, 08:59:06 pm
content: Is there a handshake requirement with the power supply, I know that many Dell laptops have this and I have had this problem with one in the past, the only easy fix was a new power supply.

author: tautech
author_rank: Super Contributor
timestamp: August 10, 2014, 09:03:43 pm
content: Quote from: G7PSK on August 10, 2014, 08:59:06 pmThe A300 has only a 2 wire DC connector.Could it handshake?

author: kolbep
author_rank: Frequent Contributor
timestamp: August 10, 2014, 09:33:08 pm
content: Had similar problem with a laptop.it was the proadliser cap under the cpu that goes faulty.The laptop will charge fine, and will run fine on battery,but if as soon as you plug in the psu, it will bsod or switch off.very frustrating having to go to standby mode, let it charge, unplug the psu, start it up, work until the battery is flat, and then repeat. Apparently the cap smooths the power rails, and wnen it fails, the psu injects tOo much noise into the rail.changing the cap will fix it.Some people replace that one cap with several smaller value smd caps in paralell.Re the low readings. Your dmm probably tests resistance at about 2v, which is enough to start some of the semiconductors on the rail conducting, which is why it will show low resistance.p

author: Rasz
author_rank: Super Contributor
timestamp: August 11, 2014, 12:44:48 am
content: Quote from: Bukurat on August 10, 2014, 10:29:48 amwhy surprised? 0.6 ohm at 1V is below 2wattbtw how did you remove this cap? did you remove the correct one? big black flat square?this cap is not easy to desolder, usually takes a LOT of heatQuote1vQuoteyou need another ~1000uF capacitor in that spot

author: Bukurat
author_rank: Regular Contributor
timestamp: August 11, 2014, 01:42:35 am
content: Quote from: tautech on August 10, 2014, 08:26:42 pmThat was the first thing I tried. I also tried the adaptor from the faulty laptop on another Toshiba Laptop to verify it works ok.Quote from: wraper on August 10, 2014, 08:50:33 pmYes, the CPU was removed. This seems to be a common fault on laptops that use this particular capacitor. It usually starts to show problems after about three years according to my research.Quote from: G7PSK on August 10, 2014, 08:59:06 pmNo, the power supply is pretty much a 19V brick. the charging smarts are in the battery. the battery communicates with the laptop via an I2C bus.Quote from: kolbep on August 10, 2014, 09:33:08 pmExactly what this one was doing, it was driving my niece crazy.I read something about the CPU speeding up when the power supply is connected and this is too much for a failing bypass capacitor, causing the BSOD and shutdown. Some of the other suggestions such as lowering the CPU speed and running the laptop in low power mode or shutting down 1 core bear this out.I'm intending to replace it with 4 330uf 2.5V tantalum caps.I measured the probe voltage of the dmm I was using at 1 Volt.Quote from: Rasz on August 11, 2014, 12:44:48 amThat was 0.6 Ohm across the Laptop power rails. I started my electronic training when we used a few hundred volts on the power rails and the most of the time the active components glowed a dull red.The solid state stuff that came in later brought that down to 12 or so volts and then 5 as TTL made its entrance.I'm not used to devices that can be powered up by the measuring equipment.How did I remove it? Hit it with hot air to soften the plastic and then pulled it apart piece by piece.QuoteI'm going the 4 x 330uf tantalum route.There should be a photo of the cap in question attached to this if I've done it correctly.

author: amyk
author_rank: Super Contributor
timestamp: August 11, 2014, 11:02:28 am
content: Perfect example of "planned obolescence" - put a critical decoupling cap rated at 105C/1000h in what could be the hottest place in a laptop, right under the CPU! There are many examples you can find online of people who have replaced this cap with an identical one and the problem comes back several months to a year later.Replace with standard SMD caps of equal or greater capacitance and equal or lower ESR. Note that the original cap (datasheet here) has only 1-2 mohm ESR, so your replacements will need to be at least that low to be reliable.