Cap Explosion

[Gary - WA4IAM]

I hadn't turned on my Millen transmitter since the end of the AWA AM QSO Party last week, so I went down into the basement and switched it on this evening in anticipation of working a little AM on 75 meters. I switched everything on, then went upstairs to make something to eat before coming back downstairs. I hadn't been upstairs more than five minutes before my wife and I heard a loud BOOM! We both rushed into the living room thinking the cats had knocked over something there but we could find nothing amiss. I then ran to the basement door and started going down the stairs. When I got to the bottom I turned and saw a huge cloud of smoke coming from my AM station, so I ran over to it and powered off everything and unpluged it all. It was then that I saw electronic guts splattered all over the basement wall behind the transmitter. It's mounted in an open rack, and I looked behind to see if I could spot the culprit. The homebrew power supply that was built by the original owner uses a three section Mallory electrolytic capacitor (each section is 10 mfd. at 450v) that plugs into a tube socket. This capacitor had let loose and exploded violently, throwing foil, tar and paper all over the place. What a mess! Amazingly it appears that none of the tubes next to this capacitor in the power supply are shattered, just coated with paper shreds and tar. Still, it's going to be a bit of a job to clean it all up tomorrow.

I seem to remember trying to track down a replacement for this capacitor about a year ago to see if there was something like this plugin capacitor still made. I thought I might need a replacement one day (boy, was I right!). On what's left of the outer can it says the positive connections for these three sections are on pins 3, 5 & 7. I can't read which pin the common ground is since there is some of the tar covering it. I know I could wire replacement caps undeneath the power supply chassis, but I would rather find a plugin replacement if they are still made. Anyone know of a source?

I'm going to use the orange cleaner I bought to cleanup the transmitter "snark" that had leaked out of the transformers that used to reside in my Raytheon RA-250 transmitter to cleanup the tar gunk sprayed out by the capacitor. I just hope it was the capacitor zorching on its own and not some other component that made it self destruct. This is my first experience with a "cap bomb", so I guess I'm now initiated into another facet of the AM hobby, namely hazmat cleanup, hi hi! 

[N9NEO]

Hi Gary,

I seem to recall recently buying some cans with three 500v caps inside.  I think I paid 18bucks from maybe antique electric supply.  Maybe it was for a HX-50 that I just redid.  I don't recall seeing any that plug into a tube socket tho.

A guy in our repair department stuck a big beer can electrolytic  in a drive backwards last week.  Two in series, so the one in backwards survived, but the one that was installed correctly blew the case off and shot across the room.  Nobody hurt.

Good luck with search.

73
Bob

[wavebourn]

That's why I prefer modern capacitors despite they are not original; they have more capacitance per volume and have special grooves on top to let a steam relatively jently go out.

[Jim, W5JO]

I don't think you will find the plug in type, but you might check with Frontier Capacitor in Lehr, ND.  Everett Hoard.  The last number I have for him is 701 378 2341

[k3zrf]

Years ago when Phase Linear 700 amps were the hot item I remember my then partner replacing a 10,000 ufd @ 100wvdc with a 10,000 ufd @75 volts. The rail voltage was close to + and - 100vdc. He turned it on and after a moment and the smoke the upper lid broke loose unraveling everywhere. It was frightening but no one was hurt.

[WB3JOK]

Years ago when Phase Linear 700 amps were the hot item I remember my then partner replacing a 10,000 ufd @ 100wvdc with a 10,000 ufd @75 volts. The rail voltage was close to + and - 100vdc. He turned it on and after a moment and the smoke the upper lid broke loose unraveling everywhere. It was frightening but no one was hurt.

I didn't know they made a 700. My home stereo has been powered since 1984 by a more-than-sufficient Phase Linear 400 (200 W RMS per channel) I bought really cheap because it had blown a $1 preamp transistor and the previous owner didn't want to pay $100 minimum repair charge... The main filter caps are 5900 uF @ 85 V, date coded in 1976, and there is about 80V across them under no-load. I think I'll replace them before there is a smoky bang!
-Charles

[k3zrf]

Seems the 700's (700 & 700A's) were used mostly in the touring sound circuit....they were the biggest bang for the buck at that time. They were produced as early as 1972-3. The little brother, the Phase Linear 400, was always available in that time period. The 400 is a sweet piece of gear and has plenty of headroom for a home stereo. I used to swamp the output with a .1 ufd cap and 10 ohm resistor (2 watt at least) to prevent them from taking off. Those babies were flat from DC to a long way up!

[W1RKW]

When I was in the audio business we had a tech who was working on a McIntosh 2300.  He was replacing a filter cap and put it in backwards.  He went to energize it and BOOM!. I never seen anyone turn white as a ghost and shake for hours.

And then there was SAE audio equipment was nicknamed Smokes And Explodes.

[The Slab Bacon]

If you are any kind of a conniseur of vintage gear, you should KNOW BETTER than to leave old capacitors in high voltage circuits, especially old electrolytics. It is like walking around with "KICK ME" written on  the back of your shirt. They WILL always fail. It is not a question of "if" but simply "when" It is always a sure bet that old waxed paper and electrolytic caps fail. In my early years of playing with antique radios I have even seen waxed paper caps explode violently. I used to only replace the ones that were bad, but after having more failures later on, I now arbitrarily replace all of the paper and electrolytic caps, even if the unit works fine. Ceramic disk caps and tubular mylar caps, and a few others are immune to this, as they are somewhat unaffected by moisture. And besides, when a capacitor decides to give up the ghost, you never know what else it may decide to take with it. (like a hard to find and expensive transformer) Total recapping of old gear is always a good insurance policy that it wont go up in smoke, and no matter how long you let it set idle, you always know that it will come up and play when you throw the switch!!
                                                                           The Slab Bacon

 

[K1JJ]

Back in 1973 I had a buddy who was an elec engineering student at Univ of Hartford. He lived on the top floor of the dorm. He said one gag he liked to do was solder an electrolytic cap to some zip cord and then lower it down to the dorm windows below at night.

He would plug the other end of the zip cord into the 120AC socket.  After the cap blew up he would quickly retrieve the cord and the residents below would think they got bombed from the street below.... 

T

[WA1GFZ]

I worked for a power supply co. years ago. We put out a couple hundred supplies a week so every week or so a cap was installed backwards. It was like the lottery and everyone got a turn. We got smart after a while and put amp meters in the input power so you could see it coming and kill the juice if you were lucky. Then just feel around for the hot cap.

It still didn't compare to the tire I exploded while working in a garage. The bead snapped at 40 lbs. white boys can jump....and fly

[W9LBB]

>> Snip! <<

I hadn't been upstairs more than five minutes before my wife and I heard a loud BOOM! We both rushed into the living room thinking the cats had knocked over something there but we could find nothing amiss. I then ran to the basement door and started going down the stairs. When I got to the bottom I turned and saw a huge cloud of smoke coming from my AM station, so I ran over to it and powered off everything and unpluged it all. It was then that I saw electronic guts splattered all over the basement wall behind the transmitter. It's mounted in an open rack, and I looked behind to see if I could spot the culprit. The homebrew power supply that was built by the original owner uses a three section Mallory electrolytic capacitor (each section is 10 mfd. at 450v) that plugs into a tube socket. This capacitor had let loose and exploded violently, throwing foil, tar and paper all over the place. What a mess! Amazingly it appears that none of the tubes next to this capacitor in the power supply are shattered, just coated with paper shreds and tar. Still, it's going to be a bit of a job to clean it all up tomorrow.

>> Snip! <<


I worked in a capacitor factory for a couple of years, and I can tell ya from personal
experience that electrolytics aren't the ONLY caps that can explode! Under the right
circumstances oil filled jobs can blow too!

I first became acquainted with the phenomenon back in high school, chasing DX on
40 meter CW. At 3 AM I heard a ZL calling CQ, and fired up the rig; it was a single
813 with about 1400 VDC on the plate stuck in a 6 foot enclosed rack.

When he finished the CQ, I tripped the PLATE ON switch, and I swear to Gawd the
whole rack lifted off the floor...  or at least SEEMED to...  as an 8 MFD 2000 VDC cap
let go! Scared the living fecal refuse outa me, and woke up everyone in the house!

Oddly...   it didn't pop the fuses, and the 866A mercury rectifiers were glowing like
normal; I STILL had B+ to the final after the blast! Didn't sound real good on the air
tho...   and the cleanup the next morning was gawdawful.


73's,

Mr. T.

[WBear2GCR]

I was able to make a replacement for the plug in (octal base) electrolytic for my R-388 by taking some aluminum tubing, an octal socket and a few choice axial lead electrolytics plus the judicious use of silicone rubber and epoxy to hold the thing together... looked good too - nice-n-shiny! still in the hole...

Depending on your realestate available, you can probably make a three section @ 450v... or you can still buy some "twist lock" 3 sections and splice to the appropriate socket...

          _-_-WBear2GCR