EEVblog Electronics Community Forum
Electronics => Repair => Topic started by: The_PCB_Guy on April 05, 2019, 01:27:47 pm
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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
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Very likely to be power supply related problem
PSU is starting up sensing a fault/short and then shutting down + repeat
Measure voltages and look for buldging caps or shorted diodes.
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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
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If possible disconnect psu from main board and check all voltages
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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
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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.
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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.
I 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
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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.
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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.
I checked the board thoroughly for dry joints and was unable to spot any. Great trick with the low-ESR caps though!
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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
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Good job but don't you hate it when that happens ? :rant:
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. :phew:
BTW, who manufactured those Waveaces for LeCroy ?
Are there any clues on the mainboard ?
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Good job but don't you hate it when that happens ? :rant:
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. :phew:
Yep. 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.
BTW, who manufactured those Waveaces for LeCroy ?
Are there any clues on the mainboard ?
I 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.