Blown PS Capacitor on a LeCroy 9304A Oscilloscope - Page 1
Forum section: electronics / repair

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: July 31, 2020, 07:30:37 am
content: I was taking some measurements today on my LeCroy 9304A Oscilloscope when I heard a crackling and sizzling sound followed by the smell of electronic smoke. I couldn't reach the power switch on the back quickly enough before the power went out on the scope. That slightly sour and fishy odor smells of a blown capacitor.The LeCroy power supply is surprisingly easy to get to and remove. A bit too easy, I thought, since the PS is not held down by any screws and just slides into a groove on the side of the chassis. Nothing looked unusual from above the PCB, but the smell definitely came from one corner of the PS.When I removed the PCB from the PS chassis, this is what I saw (see photos 1 and 2).PS Filter capacitor C203 blew out the bottom. Molten electrolyte sprayed and burned all the way through the PCB and melted the plastic insulating sheet lining the PS chassis. The damage took out some the trace on the bottom of the board as well.Removing C203 shows the damage from the top (see photo 3). Removing the cluster of four capacitors shows that only C203 blew, but caused some melt damage to the surrounding capacitors (see photo 4).I cleaned up the area (see photos 5 and 6) and the damage looks confined to the bottom of C203. But there is a gaping hole where the positive lead of C203 used to be and the feed-through on the PCB is gone as well. See the photo of the damaged cap and you can see some of the feed-through via still welded on to the positive lead.Does this PCB look repairable? Is there anything that I can do to fill the melted area?I plan on replacing the capacitor and soldering the leads to the remaining traces on the bottom of the board. And I plan to replace all the other capacitors of the same kind as well.Any advice on what else can be done?

author: goaty
author_rank: Regular Contributor
timestamp: July 31, 2020, 09:08:26 am
content: - Probably remove more of the carbonized FR4 area because it might be conductive (Measure with good Ohm-meter)- It is very likely only double sided, so no problem there- Just filter caps that you can directly solder to the next traces nearby

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: July 31, 2020, 09:43:41 am
content: Quote from: goaty on July 31, 2020, 09:08:26 amThank you. I was hoping that it would be as simple as that.

author: bd139
author_rank: Super Contributor
timestamp: July 31, 2020, 01:13:46 pm
content: Similar repair I did here on an older HP scope:https://www.eevblog.com/forum/testgear/hp-1740a-power-supply-burn-up/Should be fine. I’d replace all the capacitors however if you can.Edit: actually looking at that are you sure it was the cap that blew? Possibly arcing on the bottom of the board could do that too?

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: August 04, 2020, 05:05:30 am
content: Quote from: bd139 on July 31, 2020, 01:13:46 pmThanks for the suggestion. I was giving your idea some more thought, that this might have been caused by arcing, and I think that you are right.The PCB damage looks like it was burned by a very high temperature plasma torch. I now doubt that this kind of damage was due to boiling electrolyte alone. Also, I measured the capacitance of the blown 220 uF electrolytic and the other identical capacitors nearby, and all the caps measured good! Furthermore, their ESRs were all roughly the same.I am guessing that the blown 220 uF was slowly leaking electrolyte since the scope did have a very faint and vaguely fishy smell for quite some time. I just ignored it as being “old equipment smell”, but I should not have let it go. So perhaps enough electrolyte leaked out causing the capacitor to short and created the plasma torch arcing damage.Anyhow, I am replacing all the electrolytics on the board along with RIFA safety caps that look to have cracked casings. The power supply board is about 30 years old so all the electolytic caps must be dry and physicaly leaky by now.

author: bd139
author_rank: Super Contributor
timestamp: August 04, 2020, 08:17:39 am
content: Sounds like a good plan. If you clean the board up it should survive this

author: shakalnokturn
author_rank: Super Contributor
timestamp: August 07, 2020, 11:04:04 pm
content: It is due to leaking electrolyte mainly, I doubt the capacitors short, but enough electrolyte getting into the PCB and possibly electrolyte corroding the capacitor legs or solder can give spectacular results.After a good wash and recap it should be good to go.

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: August 14, 2020, 07:01:49 am
content: Quote from: shakalnokturn on August 07, 2020, 11:04:04 pmYou are right about the leaking electrolyte. When I removed the other set of large electrolytic caps in my blown power supply, I saw what looked like dried puddles of liquid on the PC board at the base of the caps. The dried puddles were underneath the cap and hidden from view until the cap was removed.I was reading a Nichicon application note about their electrolytic capacitor failure modes. The Nichicon app note says that their caps rarely short, but it is a common failure mode for their electrolytics to fail open and to leak electrolyte due to excessive temperature, high voltage abuse, physical damage, or just from prolonged use. I interpret "prolonged use" to mean age. For me, that statement alone justifies replacing all the electrolytic capacitors in old equipment.

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: August 14, 2020, 07:22:10 am
content: Repair update:My new caps finally came in. I replaced the offending blown capacitor, reinstalled the PS, applied power but the scope would not restart. So there were other parts that blew out also. But at least there weren't any pops or sizzling sounds this time around.I went ahead and replaced all the other, dozen or so, electrolytics in the PS board. This time the scope started up!So, it appears that there was more than one blown electrolytic cap in this failure. One was the blown 220 uF electrolytic and the other bad cap was a 2200 uF electrolytic. Thankfully, the protection circuits in the switched mode power supply shut down quickly enough to prevent damage to other components; especially the parts on the main scope board.Thank you all for the good advice and for the help to bring another broken piece of TE back to life.

author: bd139
author_rank: Super Contributor
timestamp: August 14, 2020, 07:53:59 am
content: Great to hear it’s working again and nice job

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: August 14, 2020, 08:39:50 am
content: Here is the URL for the Nichicon Application Note that I mentioned:Application Guidelines for Aluminum Electrolytic Capacitorshttps://www.nichicon.co.jp/english/products/pdf/aluminum.pdfSection 2-2 Failure Modes of Aluminum Electrolytic Capacitors2-2-2 Failure Mode in the FieldQuoteMy scope is over 25 years old and the bad caps in my PS are both located very close to and above (PS is vertically positioned in the scope) a couple of large heat sinks. Fortunately, the new replacement caps are less than half the height of the old caps and they are now physically farther away from those hot heat sinks.Even Nichicon says that heat and time kills the electrolytic caps. I now have my answer about whether or not to recap old equipment.

author: bd139
author_rank: Super Contributor
timestamp: August 14, 2020, 09:06:04 am
content: I wouldn't worry as long as you put half decent 105oC capacitors in it.I think the best example I've seen are a couple that failed in 2015 in an old Compaq Deskpro computer at a client of mine's. They were rated at 105oC and the PC was running inside a metal filing cabinet for 15 years, the inside temperature reaching 65oC in summer. The power supply was probably hotter than that.

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: August 14, 2020, 09:26:31 am
content: Quote from: bd139 on August 14, 2020, 09:06:04 amThanks, yes, my replacements are 105C rated, except for one cap that is only 85C, but that one is farther away from the heat sinks.Wow, it's amazing that those PCs continued to run for that long. Those temperatures would definitely kill the other components besides the caps.Did they use the PC with the monitor in a metal filing cabinet also?

author: bd139
author_rank: Super Contributor
timestamp: August 14, 2020, 09:38:56 am
content: The monitor was actually in the filing cabinet as well. Fortunately turned off as it was running as a workgroup server. The only things coming out the back were a bit of ethernet and a mains lead. I'm very surprised that nothing else went bang either. Oh and there were no backups of this either.

author: Kinkless Tetrode
author_rank: Regular Contributor
timestamp: August 14, 2020, 10:36:48 am
content: Quote from: bd139 on August 14, 2020, 09:38:56 amThat is hilarious.It would be surprising if backups were considered. But I guess being a workgroup server, nobody would need to see it or think of it day to day, just use it.