Blown mains filtering cap - Page 1
Forum section: electronics / projects

author: pyth
author_rank: Newbie
timestamp: March 07, 2013, 06:09:57 am
content: Hi all,Long time lurker, first time poster with a question. I recently picked up an Philips PM 3050 scoop in good condition, it all worked and I started going through the checks in the service manual. About twenty minutes in however a clear sound came from scope together with a very recognisable smell, but the scope kept working (I was out of reach of the power switch at that instant). Clearly a cap had blown. So I opened it up, check all de electrolytics on the power supply but all looked to be in excellent shape. A bit bumped that I could not find something that smelled so how like a cap I feared something further down was blown. However then my eye caught this:The input filtering cap was the problem, probably dried out over the year. Of course a quick replace would fix this, but my question is there another reason the cap would blow? Do I have to look for another defect that cause it?Thanks for any reply,Greetings from the low-lying country,Dirkjan

author: Rerouter
author_rank: Super Contributor
timestamp: March 07, 2013, 06:35:28 am
content: probably the best question is, what voltage is your mains? and what voltage is the scope set to? if there was a decent mismatch, say running off the 210V tap when your mains is 240V that could explain it quite well, as it may have just been one of the components on the input side with a lower toleranceequally i am now seeing those values of components in that exact layout a bit too often from test gear of that period, if it has 2200pF caps on the bridge rectifier then somehow it became the standard line filter for the time,

author: pyth
author_rank: Newbie
timestamp: March 07, 2013, 06:40:08 am
content: The voltage on the mains here 230V and this cap is straight on the input, it is literally the first component after the mechanical power switch. So it is before the rectifier bridge.Also it is a switch mode power supply, which, according to the spec should work with anything from 100 V to 240 V A.C..Thanks,Dirkjan

author: Psi
author_rank: Super Contributor
timestamp: March 07, 2013, 07:06:37 am
content: Most mains caps say something like... 250VAC 600VDC they're really underrated in order to handle spikes

author: pyth
author_rank: Newbie
timestamp: March 07, 2013, 07:12:21 am
content: Then I'll at least try to get something higher spec'd. This one was indeed rated 250 VAC.Dirkjan

author: Niklas
author_rank: Frequent Contributor
timestamp: March 07, 2013, 07:26:54 am
content: Seems to be related to the construction according to some postings on other forums. The package of the time was not completely sealed and the paper inside absorbs moisture, especially after some time of not being used. The moisture then corrodes the metallization and creates small cracks.http://www.vintage-radio.net/forum/showthread.php?p=445730http://www.retro-kit.co.uk/page.cfm/content/BBC-Micro-PSU-X2-capacitors/

author: Rerouter
author_rank: Super Contributor
timestamp: March 07, 2013, 07:35:31 am
content: if its the first thing on your mains following the switch then it may just have been aging, X2 is good to 2.5KV so find a similar value one and swap it out, or if you want to be picky, you could possibly replace it with an X1 cap of the same value, just it will be a bit larger and more expensive, (X1's can handle larger spikes)

author: pyth
author_rank: Newbie
timestamp: March 07, 2013, 07:40:30 am
content: @Niklas: Thanks for those links, exactly about the Rifa X2 cap I found. Bit peace of mind that it is a faulty constructed cap.@Rerouter: My local supplier probably has the X2 rated on stock. I'll throw a X1 my next internet purchase.Dirkjan

author: David_AVD
author_rank: Super Contributor
timestamp: March 07, 2013, 08:33:00 am
content: Those older style caps going bang (or smoking) is not all that uncommon. Replace it with the newer style and move on.