Extremely Flabbergasted - Laptop power jack converts DC to AC - Page 1
Forum section: electronics / repair

author: LEECH666
author_rank: Frequent Contributor
timestamp: September 22, 2023, 06:36:29 pm
content: Hi guys,long time no see. I am currently trying to repair an Acer E1-572 laptop and I thought I traced the issue down to a broken DC jack but now I am extremely flabbergasted and questioning all my understanding of electronics.So I first checked the external PSUs voltage on the barrel connector. It's kinda awkward to get the probes in but it measures 19V DC on my Fluke 87 IV.Next I measured the voltage on the little 4 pin connector that connects to the mainboard (don't know the type exactly, JST I think). Nothing. Nada. Zip. 0 Volts DC on all contacts.Then I measured the voltage directly on the DC jack where the wire is soldered to it. Again nothing.So I thought okay this jack has contact issues but even after trying to bend some of the metal contacts on the jack nothing was changed.I measured the continuity from the DC hack to the kittle white connector. All fine, showing around 0.32 Ohms.So since I didn't know if there was anything else defect at that point in time I cut the DC jacks wire and hooked up my lab supply to it with 19V DC and sure enough the power led became active. After putting back in the RAM sticks the laptop turned on and booted up into a boot disk error on screen but that was to be expected since there was no drive connected.So I said okay the jack is broken and doesn't make contact with the barrel connector of the external PSU which is an FSP manufactured unit, which I'd not considered a cheap knock off unit.So now after some delays my newly bought DC jack cable came (bought on eBay, probably used) and I plugged it in but disappointment strikes and again nothing happens.So I measure again on the new white connector (2 Pins are VCC the other 2 are GND) and again 0V DC. But this time I noticed the +- indicator flickering. So I switched to AC and I was greeted with 19V AC. What the fuck. So I measure a couple of times again and it's intermittent. I've seen 2V, 5V, 19V all AC ... how???I measure again with the cut off side of the ild cable. Same thing. I see 19V AC relatively constant but after moving the probes to take a photo it's down to 5V AC, 6V AC, 7V AC. But why AC??? There is a little ferrite bead on the new cable but it behaves the same on the cut off end of the old cable without that bead.When I take off the DC jack cable the PSU is back to 19V DC.... I am completely stumped as to what I am seeing here.I considered getting the scope out but it's so awkward to measure so I haven't done this yet and I probably will do it wrong anyway.My next idea would be to cut off the PSUs barrel connector and to solder it directly to the cable so that no barrel connectors are there to interfere but I thought I'd ask for help here first before cutting another wire.Has anyone any advice on what to test next or any hints what might be going on here?

author: mikerj
author_rank: Super Contributor
timestamp: September 22, 2023, 06:41:52 pm
content: Very unlikely to be AC. It sounds like your laptop charger may be in "burp" mode where the output pulses if it's in an overload state. Since you can power the laptop up with a bench supply, your charger may be faulty.

author: LEECH666
author_rank: Frequent Contributor
timestamp: September 22, 2023, 07:12:40 pm
content: Indeed, after some extreme finger yoga with two sharp probes, my bench power supply and the new DC jack it turns out that it's powering fine through the new cable. Holy crap that was awkward to hold. So the FSP PSU seems to be broken even though it presents itself as being okay while unloaded.However what I do not get is how a cut off cable and only a multimeter connected does load the PSU in any meaningful way. Maybe because the sense pin (if there is one) in the barrel jack actually gets connected?

author: LEECH666
author_rank: Frequent Contributor
timestamp: September 22, 2023, 07:29:12 pm
content: I mean I somehow fail to see the significant difference between this (pic below) ...... plus the respective multimeter leads connected and this (pic below) ...... plus the multimeter connected respectively that would warrant the difference in behavior but yeah I guess I will be ordering a new PSU now.

author: Kim Christensen
author_rank: Super Contributor
timestamp: September 22, 2023, 07:51:22 pm
content: QuoteI think you're just picking up AC via the Y capacitor in the power supply.

author: Chris56000
author_rank: Frequent Contributor
timestamp: September 22, 2023, 07:53:52 pm
content: It may be one of those like Dell and H.P. use that has a very thin pin for data/identification, in which case you won't get a good reading if you try and measure between the inner and outer of the female plug on the end of the adaptor cord – the negative contact of the plug is often an internal tube inside the plug that's inaccessible!However, if you can get the correct type of mating connector from an old laptop power/charger board of the same type you'll be able to identify the positive, negative and data pins more easily!Chris Williams

author: thm_w
author_rank: Super Contributor
timestamp: September 22, 2023, 10:41:07 pm
content: Quote from: Chris56000 on September 22, 2023, 07:53:52 pmThats what I thought but it just a normal two pin jack:https://www.ebay.ca/itm/274595473390

author: LEECH666
author_rank: Frequent Contributor
timestamp: September 22, 2023, 11:35:32 pm
content: I wish I could provide more photos of the weirdness but it's extremely fiddly with what I have available to even measure anything here. Maybe if humans were born with six hands.The weirdness doesn't stop by the way. I've just wetted an Euro Cent with my Pincel on this power supply.So I think I am back to that the barrel connector doesn't mate correctly with this DC jack socket.//EDITLooks like the Pinecel has a substantially larger center pin and that is probably the cause of all my issues.I am repairing this for the dad of a friend and this charger is probably not the original one. Genuine chargers from Acer from that time frame seem to be made by LiteOn mostly though this may probably vary by region / availability. In any case I think this is not the one the Laptop came with and I can't tell if this laptop ever ran with this charger. I have my doubts.//EDIT2So it turns out I had another compatible charger on hand that fits perfectly (coincidentally given to me a couple of years ago together with another Acer laptop by the same friend for whom's dad I am attempting the repair now). The dad of my friend must have given me the wrong charger. So the error is quite simple and I am aware that there are different pin sizes, but since this was given to me and was said to have worked before I didn't even consider this could be an error source and those weird weird AC measurements really threw me off. Well I guess I give her the charger back so she can pass it on to her dad in case he can't find the correct charger.