PC power supply rectifier output skip/dip symptom - Page 1
Forum section: electronics / beginners

author: tamermetwalli
author_rank: Newbie
timestamp: June 14, 2017, 08:10:48 am
content: Hey everyone, I have a PC power supply (OCZ500MXSP) and have searched this forum but found solutions to other issues (PG signal mostly).My problem is different.I decided to enter a new stage in my electronics learning and thought why not go all the way and fix this? I bought a used OScope earlier (Leader LBO-514) and decided to probe around the circuit with it. Our mains output has no ground so I should be safe going to town.In testing the power supply circuit, I can hear a continuous ticking noise if the unit's fan was connected and it would switch off if anything else was connected or nothing at all , meaning that I can only connect the power supply's cooling fan to see outputs. Testing the fan alone with another power supply showed no fault with it. Anyway, the power supply rectifier output shows a dip every other cycle with the fan connected. But in standby mode, it is smooth and the 5VSB is smooth and steady (TNY247). I suspected the rectifier to be at fault and replaced it but that didn't change anything. All outputs show a "fuzz" in sync with the ticking noise which is around 5Hz in frequency (just a ticking noise). There might by an IC at fault but before going all out throwing parts at it, I'd like to know what to search for next. It feels like something is shorting briefly but in step with something.For reference:TEARDOWN & REPAIR (we hope) of an OCZ MODXSTREAM-PRO 600W ATX PShttps://www.eevblog.com/forum/repair/teardown-repair-(we-hope)-of-an-ocz-modxstream-pro-600w-atx-ps/I did everything in that thread with no effect.At the moment I have de-soldered the primary side MOSFETS trying to test parts out of circuit but nothing seems out of the ordinary (no obvious shorts found using a DMM).I'm confident this can be solved and rather easily but running around such a high density circuit makes me dizzy not to mention dangerous when powered on. So I thought I'd ask. As a learning experience, time is no object really.Thanks for reading this far!

author: Kleinstein
author_rank: Super Contributor
timestamp: June 14, 2017, 03:38:24 pm
content: Even (or maybe even more) without a ground connection of the mains supply, probing at mains level is dangerous. So one should use an isolation transformer. At the very least having a GFI circuit is a good idea. Also it would be more like setting the scope to the suitable settings and connections and only than turn on power to the circuit and no more touching the scope.The ticking sound is usually a sign of the SMPS to start up and find a problem and than shuts down again for protection. It could be something like a broken rectifier at the output side.

author: tamermetwalli
author_rank: Newbie
timestamp: June 15, 2017, 09:10:56 am
content: Thank you for your reply.I'll de-solder the secondary side and test the rectifiers there.Update:All rectifiers measure .102 - 0.097 in diode mode on the DMM. And open in reverse. I have a suspicion this is how they should be. Thoughts?Could the PFC controller also give such a strange dip symptom? The reason I suspect the primary side more than the secondary is because the ticking is at a very low frequency. I would suspect the secondary side to be running in the kHz range.

author: ocset
author_rank: Super Contributor
timestamp: June 17, 2017, 08:00:23 pm
content: What about having a dummy load on a switch...start the PSU up with just the fan on..(becuase as you say thats the only way it gives an output of sorts)......then suddenly switch in the dummy load and see if the PSU powers it.

author: ocset
author_rank: Super Contributor
timestamp: June 17, 2017, 08:03:14 pm
content: It sounds like it is maybe one of these super efficient things which just kind of shuts down in light load.....and maybe it wont start up into full load with or without fan...there are often reasons why PSUs liek to have theri load switched in after they have powered up to the regulated output voltage.The fact your psu doesnt work without the fan makes sense......PSU is probably not meant to be operated without the fan....so i think you need to get some load on it as its a 600W beast and will just loose interest and start 'tree-hugging' antics when on lighter load....ie energy saving behaviour

author: tamermetwalli
author_rank: Newbie
timestamp: June 19, 2017, 05:32:59 am
content: Thanks for your suggestion.I'll reconsider my assumption of the primary vs. secondary fault as 5Hz is about a 200ms period which is indeed inline with the PS224's protection system. However the unit needs a full power cycle (main switch toggle) in order to be able to get it up and running when I load it. I have an old WD 8GB HDD I don't use (noisy motor bearing) and upon connecting it, the unit shuts off immediately.But I'm still curious as to how could there be a dip on the rectifier output? What could potentially cause a rectifier to suddenly drop/dip in step every 200ms or so? Perhaps a capacitor is at fault? I checked most of them (admittedly not all) and they turned out fine and none have any sign of bulging or leaking. In fact, all the caps were upgraded to reputable brands earlier (Nichicon, UCC, Panasonic) and none of the original Teapos remain.