HP Z800 power supply won't start - Page 1
Forum section: electronics / repair

author: perdrix
author_rank: Frequent Contributor
timestamp: July 31, 2019, 10:13:10 am
content: With the supply out of the box it is supposed to come up to standby, perform a POST, and go to standby again.It doesn't get that far - the fans start and the green LED comes on for about 1/2 a second then it shuts down again waits about two seconds then repeats.I've done a big capacitor replacement with Panasonice and Wurth parts, and it was working for a while, now it's not playing again.Has anyone worked out why these just trun up their toes?Does anyone have schematics or aother diagnostic data?This is driving me slowly nuts - I'm normally moderately good at this sort of stuff, but this one is *way* complex!David

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: August 01, 2019, 11:32:39 am
content: Not sure how much help I can be, but I have a working and a faulty 850W Z800 smps in front of me now. The faulty one showed your symptom when it failed. The working one I replaced with a 1250W supply as I have to have a working spare on standby. Long story... but I can now try debugging by comparing the two.I get very little time to work on them but do have some diagnostic data and can likely check some things for you on the good supply. I replaced secondary caps in the bad one - to no effect. Considerable checking of components using lcr meter and octopus tester has not yet revealed fail point. Need to spend more time trying to decipher circuitry.Have had no luck finding schematics and the like using HP PNs etc. If u do find schematic it would be great to get a copy. If you end up looking at a replacement pm me for some advice on pitfalls.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: August 01, 2019, 12:17:59 pm
content: One thing you might want to check is that both fans are working. The supply will definitely shut down immediately after flashing the LED if one of the fans is faulty.

author: perdrix
author_rank: Frequent Contributor
timestamp: August 01, 2019, 01:49:59 pm
content: Well, blow me down, this time round it is sitting there on the desk as happy as Larry, with the green LED on (both fans definitely spinning).Of course I can almost g'tee that if I put it back in the computer that it will do the run for 5secs, click, then stop trick (and maybe repeat until bored)!Hmmm totally wierd - it worked this time - still don't trust it as far as I can throw thoughOnly obvious difference is that it's a bit cooler today (22C or so, versus about 28C).David

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: August 01, 2019, 11:31:17 pm
content: I’ve had the same symptoms on my first 850W supply - sudden computer shutdowns and occasional failure to start on the bench. But very intermittent and has been working for weeks in the Z800 recently. That’s why I got a replacement supply so it’s just for backup now. It continues working/starting fine at present while using it for debugging.

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: August 06, 2019, 12:52:41 am
content: Don't want to hijack this thread but this seems a sensible place to post some diagnostic notes and pictures re this sort of Z800 start-up problem. Good/bad board comparison shows I have two 20N60C3 MOSFETs that are not switching (marked on attached photos). As far as I can tell these are driving the main 12V outputs. Everything else in the vicinity seems to check out fine as far as I can tell with lifting legs/components. Figure I'll replace these, crank it up carefully and see whether there's anything else amiss. I'm able to start up these supplies OK with 80VAC input and no load. My 240V 300VA isolation transformer can drive them in this startup state without blowing a fuse. Not at all clear why these MOSFETs have seemingly blown.This link to a 460W SMPS teardown/repair has a useful (for non-specialists like me) breakdown of somewhat similar circuitry. I found it helpful in working through the Z800 board.

author: perdrix
author_rank: Frequent Contributor
timestamp: August 13, 2019, 02:22:11 pm
content: Mine is back to misbehaving again - out of the case the green LED comes on for a short time then it shuts down - waits a while and then retries.D.

author: perdrix
author_rank: Frequent Contributor
timestamp: August 16, 2019, 03:00:18 pm
content: Stranger and stranger - if I leave it for a number of hours out of the computer, the BIST will complete and the green LED comes on fine.I can then put it back into the system and it will boot.Shutdown and come back later, and it is back to not starting againAnother supply stolen from another Z800 works just fine ...D.

author: perdrix
author_rank: Frequent Contributor
timestamp: August 26, 2019, 12:56:40 pm
content: Well I know now that the problem is dependent on ambient temperature. If it's over about 25C or so the supply just refuses to play!Cold days it's OK! I'm guessing there's a thermistor somewhere in there that's faulty or has some faulty parts around it.HmmmDavid

author: perdrix
author_rank: Frequent Contributor
timestamp: August 31, 2019, 08:38:16 am
content: Just an off the wall thought: Does anyone have any service information for any Delta PSUs of this vintage and power output?Clearly this supply is specific to the Z800, but it's probably closely related to their other supplies.ThanksDavid

author: Lorenzo_1
author_rank: Regular Contributor
timestamp: September 02, 2019, 11:40:36 am
content: I’ve spent quite some time searching for this or similar Delta power supplies but without any success yet. Sorry for that bit of bad news.