Once the magic smoke escaped... - Page 1
Forum section: electronics / projects

author: uwezi
author_rank: Unknown
timestamp: October 07, 2015, 08:13:03 pm
content: Look what happened to the control unit of ourelectron beam evaporatoryesterday: the magic smoke escaped from a capacitor, taking with it the insulation of several cables and it even desoldered a thyristor from the bolt which kept it on its heatsink since 1976!Luckily - as Dave recently also mentioned in his repair video on the HP 'scope - it was usual at that time to provide the full schematics. And even more amazingly, you can still buy the exact same type of thyristor today! Try that with electronics from the 1990s...The rewiring and restauration of this unit, however, will probably keep me and a colleague busy for quite a few days to come

author: JoeB83
author_rank: Regular Contributor
timestamp: October 07, 2015, 08:20:31 pm
content: Yikes. What happened to make the cap explode? Or did it just happen?

author: uwezi
author_rank: Unknown
timestamp: October 07, 2015, 08:26:40 pm
content: Quote from: JoeB83 on October 07, 2015, 08:20:31 pmWell, it's 1976 vintage and might have seen some abuse during its days. Of course it was an X2-type cap like it's neighbor and should have failed open... ok, it did at lastThis part of the circuit regulates the heating current through a filament. It is driving the primary of a transformer from mains voltage and the transformer gets the voltage down to a couple of volts and the filament current up to 100 A.

author: SaabFAN
author_rank: Frequent Contributor
timestamp: October 07, 2015, 09:07:18 pm
content: So about 100A blew up that cap?This looks like the broken floppy-drive I destroyed with firecrackers when I was a kid

author: uwezi
author_rank: Unknown
timestamp: October 07, 2015, 10:21:03 pm
content: Quote from: SaabFAN on October 07, 2015, 09:07:18 pmNo, the 100A are on the secondary side of the transformer, this drives the primary by phase-angle firing. I don't know how much current went through it, but it only blew a 6A fuse.The diagram states that the wires around are 0.75mm2

author: AF6LJ
author_rank: Unknown
timestamp: October 08, 2015, 12:41:15 am
content: You have your work cut out for you.Hopefully that cap was all that went, (being optimistic )

author: uwezi
author_rank: Unknown
timestamp: October 08, 2015, 07:04:58 am
content: Quote from: AF6LJ on October 08, 2015, 12:41:15 amYes, I know that's optimistic - well, I already know that the thyristor also has to be replaced and I bought 2 of them to replace the other one as well. The worst thing is the wiring whichkeepskeptall the parts connected...

author: Gyro
author_rank: Super Contributor
timestamp: October 08, 2015, 09:26:28 am
content: Damn! Those Rifa caps get you every time... and make such a mess!The Rifa X1 cap across the IEC input in my HP3310A went the same way. There was clearly flame because it charred the wiring sleeving above it but it didn't blow the fuse. The same thing happened to the one in my electronic ignition years ago, it burned a big hole in the PCB (leaving the traces hanging in mid air) while it was still operating (very briefly!).I think that once they crack and absorb moisture they initially try to self heal but that triggers a sort of 'avalanche' self heal until it just catches fire.P.S. You can see it's a Rifa - metal foil label that wraps over the top inside the resin is clearly visible.

author: Pjotr
author_rank: Frequent Contributor
timestamp: October 08, 2015, 11:33:52 am
content: Yes that's a Rifa metal-paper cap. Usually indestructible when not severely overloaded/abused. You cannot say what causes what here and the cap is "guilty". Those wire wound resistors look pretty cooked and can have overheated the thyristor as well.

author: Wolfram
author_rank: Frequent Contributor
timestamp: October 08, 2015, 03:01:11 pm
content: Quote from: Pjotr on October 08, 2015, 11:33:52 amThe cap looks like a Rifa PME271. These are notorious for catching fire:http://www.vintage-radio.net/forum/showthread.php?t=69128.

author: AF6LJ
author_rank: Unknown
timestamp: October 08, 2015, 03:15:07 pm
content: Quote from: Wolfram on October 08, 2015, 03:01:11 pmInteresting thread, a bit scary to find a 250V cap across 220V mains.

author: Gyro
author_rank: Super Contributor
timestamp: October 08, 2015, 04:33:42 pm
content: QuoteIf it's a PME271 then that's an AC rating.... 250V AC @ Class X (for across the line use).Edit: That means it's not its excuse for failing - Age and resin cracking = moisture ingress. If you see one with any sign of cracking, crazing or bulging it's a replace on sight (...yesterday!) )

author: uwezi
author_rank: Unknown
timestamp: October 08, 2015, 04:38:16 pm
content: That was some interesting input, yes obviously these particular metal-paper capacitors from Rifa are responsible for quite some damage in electronics - Swedish quality fromRadioindustrins Fabrikaktiebolag...First of all, to me the sequence of the events is quite clear and it started from the capacitor, because what is not clear from the pictures I posted is that the board was originally mounted vertically with the thyristor directlyabovethe caps.Given that probably nobody in 1976 expected this electronics to be still in daily use in 2015 the caps have survived quite well. They are encased in epoxy, and even without cracking some humidity might have diffused into the cap over the years, compromising the self-healing functionality.When it comes to the voltage rating - these caps are actually not installedoverthe mains voltage, i.e. not between phase and neutral. But they will be replaced by modern 630V rated caps

author: Gyro
author_rank: Super Contributor
timestamp: October 08, 2015, 05:28:33 pm
content: Interestingly PME271 X and Y caps are still available to this day, made by Kemet / EVOX/Rifa. The datasheet indicates " Self-extinguishing UL 94V-0 encapsulation material", probably not something that could be claimed for the old ones (even though they carried the relevant approvals makings).

author: AF6LJ
author_rank: Unknown
timestamp: October 08, 2015, 05:50:54 pm
content: Quote from: Gyro on October 08, 2015, 04:33:42 pmIndeed I get the AC rating, I wonder if the equipment the OP is working on has MOVs. If so they should also be replaced.

author: uwezi
author_rank: Unknown
timestamp: October 08, 2015, 06:00:38 pm
content: Quote from: AF6LJ on October 08, 2015, 05:50:54 pmNo MOVs - don't worry... Only a couple of fuses.

author: AF6LJ
author_rank: Unknown
timestamp: October 08, 2015, 06:57:51 pm
content: Quote from: uwezi on October 08, 2015, 06:00:38 pmGood.