Repairing a LeCroy WaveAce 222 Oscilloscope That Doesn't Power On - Page 1
Forum section: electronics / repair

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 05, 2019, 01:27:47 pm
content: I recently got my hands on a used LeCroy WaveAce 222 scope that won't power on. I popped the cover and verified that the fuse is intact. With the case open I plugged in the power cable and hit the power switch and noticed that the fan spins for about half a second, shuts off, and half a second later starts spinning again. This on-off-on-off fan spin cycle continues until I turn off the scope again. Does anyone have any thoughts on what might cause this issue? Some sort of surge perhaps? The fan seems to spin freely, so if there's any sort of fan speed sensing going on I don't think that's the issue.If a video would be helpful, feel free to let me know and I'll see about taking one.Thanks a lot!Matt

author: Psi
author_rank: Super Contributor
timestamp: April 05, 2019, 01:31:15 pm
content: Very likely to be power supply related problemPSU is starting up sensing a fault/short and then shutting down + repeatMeasure voltages and look for buldging caps or shorted diodes.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 05, 2019, 01:48:12 pm
content: Oh of course! The first commandment of troubleshooting: Thou shalt check voltages!I'll post back here after I have a chance to probe around the thing.Cheers,Matt

author: fable
author_rank: Regular Contributor
timestamp: April 05, 2019, 01:49:52 pm
content: If possible disconnect psu from main board and check all voltages

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 05, 2019, 07:27:19 pm
content: During my lunch break this afternoon I had a chance to to crack open the scope and remove the power supply board. I inspected it for bulging/leaking caps and I checked all of the diodes using my meter, and all looked good. This thing is stuffed with Rubycon caps, so I doubt any of them are faulty.I then powered it up on the bench without the rest of the scope attached (I removed the main ribbon cable as well as the cables for the fan and the backlight). After pressing the power switch I could hear the board "chirping" in the same pattern as the fan was turning on and off when it was still connected to the board. By "chirping" I mean I hear the switching supply starting up and shutting back off again.I am getting energized capacitors on the secondary side of the switching transformer, but without any reference for what the voltages should be I'm just stumbling around in the dark. I did measure low resistance across the secondaries of the power supply (around half an ohm, give or take), but this did not concern me much knowing that the only resistance would be that of the winding itself. Regardless, I removed the transformer from the board and measured the resistance directly across the windings. I also measured the resistance at the pads. I measured the same low resistance across the secondaries of the transformer, and open circuits/high resistance on the PCB, so the secondaries do not appear to be shorted on the PCB.To confirm that the transformer was working, I then connected the transformer to my function generator set to output a 150kHz 0-10V square wave (similar to what I expect the circuit does on the board, based on the application circuit for the TOP246YN off-line switch) and probed the secondaries with the scope. The primary-to-secondary ratios appear to be around 5:1 and 7:1, though I don't know how accurate these measurements were. I was guessing on the frequency. Anyway, assuming rectified US mains being switched across the transformer I expect this to result in (roughly) 34V and 24V on the secondaries. Unfortunately I am not able to get an accurate measurement of these values because the supply switches off before the output caps can become fully energized, so any measurements would not be accurate.That's about where I left off. I plan to work on this a little more during the weekend, but I'm not quite sure where to go next. Any suggestions?I will see about posting photos a bit later.Thanks,Matt

author: tautech
author_rank: Super Contributor
timestamp: April 05, 2019, 07:40:10 pm
content: I'd be pulling secondary side caps and testing them based on the info given thus far.uF value first and of they're 10+% off replace them. ESR tests should return well less than 1 ohm, preferably under 100 mOhm.If they run a cap after the diode and another after an inductor the ones closest to the diodes will likely be more stuffed.Determine which rail gives the feedback to the SMPS controller and concentrate on it initially.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 06, 2019, 12:35:44 am
content: Quote from: tautech on April 05, 2019, 07:40:10 pmI suppose you're right, that probably ought to be my next step. I assumed the caps were fine because they looked ok and were a reliable brand (Rubycon). Regardless, they would be next in line. Hopefully I'll be able to get back to work on the scope tomorrow morning.I'll keep you guys posted. Thanks a lot for the suggestions.Matt

author: Psi
author_rank: Super Contributor
timestamp: April 06, 2019, 10:29:57 am
content: Also have a look for any dry joints.They can be hard to spot though, a microscope is a good tool to look for them.Another diagnostic test you can try is to solder a new low esr cap across an existing one. This is quick and easy since you dont need to remove anything.If the capacity of the current caps is low because they have all dried up then adding capacitance may get it to turn on properly.

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 06, 2019, 10:57:05 am
content: Quote from: Psi on April 06, 2019, 10:29:57 amI checked the board thoroughly for dry joints and was unable to spot any. Great trick with the low-ESR caps though!

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 06, 2019, 01:21:39 pm
content: As promised, here are some initial (general) photos of the layout of the PCB. I would be happy to take more detailed photos upon request.UPDATE: I desoldered all six of the main capacitors on the secondary side of the transformer and all of them test ok. No issues (as expected, considering their brand). The bad news, of course, is that since the caps appear to be fine, I am not much closer to solving the issue.UPDATED UPDATE: Well Gents, I take it back. I now have a working scope. Looks like it may have been a cracked joint after all. I resoldered all of the caps back in place with fresh (leaded!) solder, et - voila! The scope works!Thanks very much for all the help guys!Matt

author: tautech
author_rank: Super Contributor
timestamp: April 06, 2019, 07:56:25 pm
content: Good job but don't you hate it when that happens ?You ask yourself what was learnt......to inspect more carefully or was it a dodgy header connection ?Anyways, it's running and that's all that really matters.BTW, who manufactured those Waveaces for LeCroy ?Are there any clues on the mainboard ?

author: The_PCB_Guy
author_rank: Regular Contributor
timestamp: April 06, 2019, 08:02:10 pm
content: Quote from: tautech on April 06, 2019, 07:56:25 pmYep. You hope it's an interesting problem to fix and that you learn something really neat in the process of troubleshooting/repairing it, but it ends up being something as dumb as a bad solder joint? Oh well, I am glad it's working anyway. Most likely there was a microscopic crack in the solder joint(s) that wasn't visible through a magnifier and reflowing it/them fixed it right up.QuoteI never pulled out the main board and didn't look too closely (other than seeing a vertical double-stacked ceramic cap). I'm afraid I couldn't tell you who actually builds them.

author: johnny.tifosi
author_rank: Newbie
timestamp: November 20, 2024, 09:47:07 am
content: Quote from: The_PCB_Guy on April 06, 2019, 01:21:39 pmHello,I am trying to repair my WaveAce 112 power supply which happens to be exactly the same and just registered in order to revive this topic (sorry for that). Could you please tell me which are the nominal voltages in the secondary side? Some info I figured out myself is the board has five DC voltage rails. Four are generated by the larger transformer and routed to a ribbon cable, of which one is negative, one is the master positive rail, and two are 3.3V and 15V rails generated by LDOs from the master rail. There is a fifth one generated by the smaller transformer and routed to a pin socket.Initially, the power supply was not functioning at all and making a periodic chirping sound. Then I replaced a shorted TVS diode in the primary side snubber circuit with a beefier one and I get some voltage readings in the secondary, that is -28V, 3.3V in an LDO regulated rail, 9V to 15V fluctuating in the master rail, 15.4V in another LDO regulated rail and 0V in the pin socket (multimeter measurements). But I still get burned 5ohm resistors (R49) in series to the optocoupler output. I have replaced the TL431 and 4N35 ICs in the feedback circuit to no avail. I attach some schematics I made of the secondary side and feedback circuits by observing the board (still WIP and may have some inaccuracies), I am preparing the primary side schematic too. Any help is much appreciated! Cheers

author: johnny.tifosi
author_rank: Newbie
timestamp: November 20, 2024, 08:41:57 pm
content: Here are the updated schematics, for which I dedicated far too many hours to figure out. Transformer pinout is not certain but you get the gist. Initially the TVS D2 in the primeary snubber was burning, I replaced it with two in parallel and I got two rails to work as described in my previous reply. The master rail is fluctuating a lot though, and the 5 Ohm R49 and Shottky D4 in the feedback circuit are burning after a while. I wonder how the opto output is has only two 5 ohm resistors in series, I haven't figured out the operation of this circuit. It is connected to a pin on the primary side of the transformer, but internally it may be a secondary winding because there is no way it could handle 320V.

author: johnny.tifosi
author_rank: Newbie
timestamp: December 15, 2024, 08:25:39 pm
content: I got access to a differential probe and measured the DC rectified voltage which was perfectly stable at 315V, the V_DS voltage of the MOSFET and the output voltage V_OUT2 (waveforms below). The V_DS was ok-ish with some significant ringing, V_OUT+2 though has some weird voltage drops similar to RC discharges about 0.66s apart. Any clues?