Need help fixing apple time capsule power supply - Page 1
Forum section: electronics / general

author: kripton2035
author_rank: Super Contributor
timestamp: September 25, 2023, 07:06:08 pm
content: Hi Folks,I have an apple time capsule (the flat ones) that got hit by lightning. the powersupply is dead. the fuse has exploded.I tested the main diode bridge, the primary mosfet by unsoldering them and testing with a multimeter and transistor testerboth are good. there is no short on the plus and minus primary as seen by an ohmmeter.I installed another fuse, and boom exploded as the first one...what other component can be checked ?attachment: picture of the power supply, and probable schematic.thanks.

author: kripton2035
author_rank: Super Contributor
timestamp: September 25, 2023, 07:07:33 pm
content: forgot to add: I checked the two mains 47uF 400v capacitors and their esr is around 0.2Ω

author: srb1954
author_rank: Super Contributor
timestamp: September 25, 2023, 07:54:27 pm
content: Quote from: kripton2035 on September 25, 2023, 07:06:08 pmThe controller chip may have been damaged. Even if the MOSFET is OK a damaged controller chip could permanently turn it on and the current drawn will continue to rise until the fuse blows.I would replace the MOSFET and bridge rectifier as well. They may well test OK on a multimeter or transistor tester but could have been overstressed by the lightning surge and will eventually fail.

author: tom66
author_rank: Super Contributor
timestamp: September 25, 2023, 08:39:32 pm
content: I would agree with the controller chip being a candidate. I've repaired a number of power supplies in my time and it was about a 50-50 chance that when the main FET blew as to whether the controller IC blew. And then it could be either that the controller IC shorted the gate pin to ground or to Vcc, if it was Vcc then bye-bye fuse. If the power supply was subject to a surge it's possible the controller IC was damaged and the FET looks OK.Also don't rule out the possibility of the FET having suffered some less-than-obvious failure. If you test the resistance with just <1VDC from a multimeter it wouldn't reveal any leakage current that appears under high bias voltages.I had an LG 50PS3000 plasma TV that had a bizarre failure like this. A 100V rail on the Z-sustain board, used to bias the plasma panel for some part of the display cycle, was generated by linear-regulating the 200V sustain supply down. It had a moderately sized heatsink on it and average current measured around 50mA in operation. So probably dissipating around 5W. As the FET increased in temperature the Vzbias voltage would increase progressively until it hit 150V, at this point the plasma discharge was very bright and the main PSU would shut the TV down (the picture essentially just became brighter and more distorted, at one point the PSU sounded very angry and ~1000W was pulled from the wall! Normally it is about 400W.)The cause was that the FET was bad, I guess years of being at a sustained temperature had somehow caused a slow leakage to build up. But take the FET out of circuit, drain-source measured >1Meg. Heat it up with a heat gun and it dropped to ~100k, but still didn't seem obviously bad. It seemed whatever effect there was, it depended on the voltage across the device.A little trick: if you have access to a current-limited 80V+ PSU, many small flyback based universal-voltage AC PSUs will start up at quite a low voltage. You can then try to see what is going on with a scope without fearing (too much) about getting a shock or weird grounding etc.Since the PSU only produces 12V and 5V, maybe it is easier to find a replacement PSU, or add an external PSU like used by external drive bays that outputs 12V/5V.

author: kripton2035
author_rank: Super Contributor
timestamp: September 25, 2023, 09:38:19 pm
content: I checked the mosfet with a transistor tester, and it appeared ok. but I aggree it could fail short only when powered...

author: tooki
author_rank: Super Contributor
timestamp: October 06, 2023, 11:44:55 am
content: FYI, replacement power supplies don’t cost a lot on eBay.

author: kripton2035
author_rank: Super Contributor
timestamp: October 06, 2023, 12:27:30 pm
content: as the diode bridge and mosfet are quite small and I don't have replacements that fits in the box, I will surely go that way.and also I have a lot of donor time capsules ...this was just for the "pleasure" of fixing it.