[MENDED] Only just out of Warranty - Agilent DSO1014A - Page 1
Forum section: electronics / repair

author: nfmax
author_rank: Super Contributor
timestamp: November 05, 2015, 10:10:28 pm
content: Less than 10 months out of warranty, my lightly-used Agilent scope blew both the mains and internal PSU fuses. And I mean blew. No sign of combustion anywhere else, but IMHO the PSU board has been rather messily reworked at some point in its life. Keysight have quoted me a get lost 'repair' charge, amounting to an exchange unit at a price only just less than I could buy a new one for! There is, naturally, no service manual available.I am assuming the fault lies in the non-isolated side of the SMPSU. So long as it isn't in the transformer I think I stand a fair chance of fixing it. The first step is to identify all the parts & trace out the circuit: however, I thought I'd ask here if anyone has already done this.This is one of the rebadged/subcontracted Rigol designs Keysight sell. It's not my main 'scope so I'm not desperate. It's just I hate the idea of paying what amounts to £300/year for a 'disposable' oscilloscope!

author: tautech
author_rank: Super Contributor
timestamp: November 06, 2015, 04:51:18 am
content: Oh dear, that's not what one would expect from a reasonably new unit.If you are lucky the SMPS design is one listed in datasheets as a "typical application" for the controller IC used.Always worth checking this out, seen it many times.Often then the component values used are exactly as those listed in datasheets and then repair is normally straight forward.Let us know how things go.Pics?

author: nfmax
author_rank: Super Contributor
timestamp: November 06, 2015, 06:54:31 am
content: Quote from: tautech on November 06, 2015, 04:51:18 amThat's a very good thought. I've had to put this (unwanted) job to one side for a while, but I'll follow that up when I get back onto it.

author: nfmax
author_rank: Super Contributor
timestamp: November 09, 2015, 09:28:02 pm
content: Well, the SMPS controller is an NXP TEA1610T half-bridge zero-voltage switching resonant converter controllerhttp://media.digikey.com/pdf/data%20sheets/nxp%20pdfs/tea1610t_p.pdf. This is now obsolescent, but it looks like a good choice for a quiet SMPS suitable for a scope. I found an application notehttp://www.nxp.com/documents/application_note/AN99011.pdfand started tracing the Agilent circuit from the AC input and comparing it with the application note. The Agilent (?) design has an additional inrush limiting thermistor, followed by a TVS across the first filter capacitor: it also has a second stage of LC filtering. It looks like somebody was concerned about conducted EMI.Following the filters, the bridge rectifier is - or more accurately was - a 2A 1000V TBP208http://www.genesicsemi.com/images/stories/genesic_images/products/Bridge/kbp206_thru_kbp210.pdf. One of the diodes has failed dead short, making it look like a single diode across the AC supply. Yep, I think that would blow the fuses! After removing the rectifier, continued testing still showed a ~0.3 ohm short across the rectified mains. I removed the main capacitor and tested it: OK. However, both the series power FETs (IRF840) seem to have failed dead short drain to source. I wonder what failed first, and blew what else in what order? And why? It might have been a transient, but nothing else on the same mains supply suffered any problems. Removing one of the FETs leaves a high resistance across the rectified mains output (about 1 Meg at 5V) so that may be everything. I don't have spares to hand, so I'll have to order some and see if that fixes it.Fingers crossed!

author: nfmax
author_rank: Super Contributor
timestamp: November 29, 2015, 05:40:14 pm
content: Fixed it! I replaced the bridge rectifier (with a KBP10M for a bit of extra margin), both IRF840A's, and the main input capacitor for good measure, since it might have been briefly reverse-volted. I powered up the converter from a DC bench supply, with the start-up dropper resistors shorted so the control chip saw the full input voltage, and 'scoped the waveforms. Everything looked healthy, so I removed the short, put the supply back together & fetched my ancient VARIAC from storage in the garage. Problems. I had to completely dismantle it & clean the contact areas of the winding with an abrasive contact cleaning block, finishing off with Electrolube contact cleaner lubricant. Also replaced the ancient rubber-insulated mains lead (you may have gathered it hasn't seen much use - it was a dumpster dive score when the branch of Racal I was working for closed down in 1999. I knew it would come in handy one day)Anyway, a quick bench test with a restive load looked OK, assuming the output voltages were correct - I've no idea what they are supposed to be. Reassemble the scope, and success!

author: Fraser
author_rank: Super Contributor
timestamp: November 29, 2015, 05:59:24 pm
content: Great result. Well done.I was interested to read that many SMPSU circuits are very close to the controller IC's OEM application schematic. After all these years I had not considered such and have always reverse engineered SMPSU schematics. It's obvious when I now think about it ! Why 'reinvent the wheel' if the controller OEM has already done the work for you.Fraser

author: nfmax
author_rank: Super Contributor
timestamp: November 29, 2015, 06:00:56 pm
content: Some pictures.To get inside you need a Torx T9 driver. There are two screws next to the front feet, and two lurking beneath the handle hinges. The back cover comes off leaving everything attached to the front. The PSU is beneath the secondary metal shield - the fixing screws throughout are Torx T9. It's all quite nicely made. I didn't attempt to dismantle any further, and I only had a quick look at the main board. The processor section is very Rigol - an Analog Devices based processor, Hynix RAM and a single NAND flash device, with what look like a couple of FPGAs. However the acquisition section looks more like Agilent - two pairs of chips, presumably a 2GB/s ADC and an acquisition memory controller/trigger device, each with heatsinks. The ADC (?) is a PQFP, the memory controller is a BGA. There is also another Hynix memory chip over here, but I don't think it is fast enough to be the sample memory.Finally a shot of it all back together and working, also with the dead bits.

author: nfmax
author_rank: Super Contributor
timestamp: November 29, 2015, 06:03:12 pm
content: More pictures

author: nfmax
author_rank: Super Contributor
timestamp: November 29, 2015, 06:08:32 pm
content: More pictures, and my 'estimate' of the circuit diagram. There is also another onto-coupler used in linear mode to carry the AC input waveform across the isolation barrier to provide the scope with line trigger, but I didn't have room for it on the paper.