Agilent/Keysight U8031A quickie repair / mini teardown - Page 1
Forum section: electronics / testgear

author: ve7xen
author_rank: Super Contributor
timestamp: August 30, 2016, 08:22:38 am
content: I received a 'for parts or repair' Keysight U8031A today. This is a lab power supply with two proper outputs, 30V @ 6A each with CV and CC, plus a fixed 5V @ 3A channel with no current programming. It's on the low end of Keysight's power supply line, but the specs looked decent and IMO the UI seemed somewhat better than the pricier E3648 series. Information on it online was sparse and mostly negative, though. It looked like this when I got it:I probably paid a bit too much for it, but given that it was branded Keysight on the front panel and appeared in good cosmetic condition, I took a gamble. Should be close to paying for it just offloading the two power supplies it will replace. Auction photos showed it powering up with no errors, but it was shown only with the outputs off.On functional testing, it was quickly noticed that the Channel 1 output was completely dead. No output at all in any setting, and when it was turned on, the supply flashed both CC and CV indicators, I guess telling me 'unregulated'. Channel 2 appeared to operate normally.So it was time to open 'er up. Four torx on the rear panel, and two Philips on the strap handle and the heavy steel outer case slid off:After some visual inspection, I was both dismayed and relieved. The construction of this supply is not very serviceable. Both vertical power boards are soldered front and back with heavy joints to the main board. Even visual inspection is difficult due to the casework blocking almost all access, visual and otherwise, to the power boards. Not only that, but the power transformer mounted dead centre with the power boards straddling it makes it even more awkward to remove the assembly of the two vertical boards and the main board. Aside from that, construction quality seems to be high, with lots of fasteners and metalwork and high quality parts throughout.Dreading having to disassemble and then try to test this monstrosity, I began trying to make a visual assessment through what limited ports were available - and found something:I couldn't find any other damage in this area, so on a hope and a prayer, I made a quick repair through the little window. Some used up desoldering braid, plenty of flux, and a hot iron bridged the gap. After a quick test, I found Channel 1 also fully working after this repair! What a surprise that it would be in such a lucky spot!On Channel 2 or anywhere else on the board, and I'd have been painfully disassembling the whole mess. The trace was still loose though; it had managed to burn and flake off most of the soldermask, so I covered it with two-part epoxy. Here's what the finished repair looks like:Unfortunately for the teardown, it's quite difficult to get good photos due to the vertical boards and casework, and since the repair didn't require it, removing them is out of the question. I didn't learn much about the topology used in the supply either, other than that it seems somewhat complicated. There is clearly a large-ish switchmode supply (too large for the control board and 5V supply alone), but also a small-ish toroidal (much too small for the 360W rated output) transformer with windings going to each supply board. Each supply board does appear to have a complete DC-DC switcher based on what can be seen from above, and probably a linear post-regulator as well, so my guess is that a DC rail is produced from the AC, then isolated and regulated by each supply board, but I'm still confused as to the purpose of the toroid. That AC input switcher and the fixed 5V output regulator is about the only thing you can get a good look at though, so here's a shot:In any case, I'm fairly happy with the supply. It seems to be well built - the failure mode it experienced notwithstanding - and after playing with it this evening it seems pleasant enough to use. After a calibration, it's within about 10mV and 10mA on both outputs across the range of outputs I tested, with noise something like 0.1mV(rms), much better than spec. The fan is a bit louder than I would like, but no worse than most other equipment on the bench, most of which is not doing power. I took some scope traces of startup with various loads and settings, and it was well behaved in all cases with no overshoot or ringing that I noticed.More photos, test results and manuals can be found athttp://gotroot.ca/u8031a/if you're curious.

author: TheSteve
author_rank: Unknown
timestamp: August 30, 2016, 04:49:34 pm
content: Hah - nicely done. I was also watching that supply and was waiting for one more price drop to jump on it.

author: bitseeker
author_rank: Super Contributor
timestamp: August 30, 2016, 10:50:52 pm
content: Good job, ve7xen. Lucky break that fault was accessible. It's not very repair-friendly. What happened to that trace? Looks like it cooked right off the board.

author: ve7xen
author_rank: Super Contributor
timestamp: August 31, 2016, 02:58:58 am
content: Quote from: bitseeker on August 30, 2016, 10:50:52 pmYeah, it looked burned pretty badly. It was also pretty tough to solder to, I guess the soldermask had left some ugly deposits on the copper. Why it happened though is beyond me - it should not be possible with a decent lab supply; it's especially weird that nothing else seemed to be damaged, as would probably be the case if someone dumped power into it the wrong way or something.

author: D3f1ant
author_rank: Frequent Contributor
timestamp: August 31, 2016, 08:47:17 am
content: That really is impressive damage, esp considering this is (isn't it?) a decent quality product but guess this is some kind accident misuse not even keysight could have envisaged.