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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Fri 15, 2021 8:53 pm 
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azenithnut wrote:
Oh you mean the RCA D22-1 correct?
It was RCA's biggest dog for 1936 Afterward it would be the QU8 which was like a Capehart with the flip-over changer. -Steve
Yup, RCA D22-1 that's the one. That auction was 25 years ago...

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Fri 15, 2021 9:03 pm 
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Turbologic: I think you should add excessive leakage to your list,

DM


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Sat 16, 2021 4:14 am 
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devilsmist wrote:
Turbologic: I think you should add excessive leakage to your list,

DM


... then the leakage must be VERY excessive (if you consider shorting as a special form of leakage)

Leaky electrolytic caps could bulge and loose some electrolyte but never explodes so violently and quickly. All electrolytic caps (including the old ones) have some kind of protection against explosion (usually a vane to slowly release internal gasses) and only a very heavy overload or abnormal condition will lead to this kind of rapid destruction.
But of course there might be some exceptions since all caps are not built alike, especially the old ones.
My post's purpose was mainly a caution: I think it would be wise to check the circuit before putting a new cap in the set, for instance an open cathode bias resistor will put nearly full B+ (trough the tube) across the parallel decoupling capacitor and you can easily imagine what could happen when 200-300VDC hit a capacitor rated for 35VDC: a loud BANG !! And this is only one possible scenario amongst many others.
Better be safe than sorry...


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Sat 16, 2021 3:20 pm 
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Tubologic, you are sadly lacking in experience. I have encountered enough capacitors that suddenly exploded with no warning signs of leaking, hissing, bulging or whatever. I don't know where you got the idea that "all" electrolytics have some kind of explosion protection, because some of them fail to provide it.

John


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Sat 16, 2021 4:56 pm 
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I've seen more than a few electrolytics which blew up in normal service, usually because something else failed and caused them to overheat, or because they were not spec'd properly in the first place. It's very rare that an internal defect will cause a cap to burst, especially with electrolytics from the 1970s and newer.

In order for an electrolytic to explode, one of three things has to happen. Either it has to heat up so fast the vent cannot operate in time, the vent has to be plugged so it does not operate due to a slower rise of internal pressure, or there has to be a build-up of hydrogen in the cap from electrolysis. (Microscopic sparks due to scintillation on the dielectric layer set the hydrogen off if it cannot escape through the seals).

Rapid destructive heating would imply something catastrophic happened, such as reverse polarity, application of excessive voltage, or a total breakdown of the dielectric layer. Sudden, complete breakdown of a dielectric does happen but it is rare and about as likely in new caps as in old ones. Slower heating against a plugged vent with or without hydrogen being evolved would require excessive leakage current or a very high ESR. Those should have been checked before the cap was put back into service. If it was hydrogen build-up and scintillation, it would suggest that the reforming current was too high or the reforming did not really "take."

While I am a big proponent of reforming caps where possible, I'd be the first to say that it simply isn't possible for many capacitors once they get beyond a certain age or a certain number of hours of operation. Most of the older cardboard covered axial electrolytics I've come across had red or white rubber end seals which dried out and cracked long ago, so they lost too much electrolyte to reform well. If I see a cracked, perforated, or shrunken seal, or evidence of dried electrolyte residue outside the cap, I don't bother going any further with it, out it comes.

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Sat 23, 2021 4:45 pm 
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Adding to what Chris posted, electrolytic capacitors have a negative resistance property with regard to temperature. If voltage is applied to a leaky capacitor with no current limiting, the capacitor will start to heat up. As it does, the resistance goes down, increasing the current flow. The process continues until the capacitor blows, due to boiling of the electrolyte or failure of the dielectric. I have seen it happen!

The subject of reforming has been discussed here many times. The bottom line is, it is a crapshoot, depending upon the quality and condition of the capacitor in question. With new capacitors being fairly cheap, it isn't worth the trouble; especially with a customer's set.

The familiar cardboard tube and carton electrolytic capacitors used in AC-DC radios seem to be the worst; I suspect that the electrolyte evaporates due to the porous nature of the cardboard.

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Sat 23, 2021 5:06 pm 
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This is quite true; the term for it is "thermal runaway." Before 105 degree C capacitors were common, engineers sometimes tried to get around it by using higher voltage caps than necessary from the circuit considerations alone if the capacitor was going to be used at high temperatures. The higher the voltage rating of an electrolytic the lower the leakage current, so the less likely it would be to reach an internal temperature where runaway occurs.

A classic example is the main filter cap in an HP 606A signal generator. They are rated for 475 volts even though the highest voltage in the circuit is well below that. Anybody familiar with those generators knows they get nice and toasty warm inside after they've been on a short while. Some years ago somebody here on the forums tried to use 450-volt replacement caps and found out about this the hard way.

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Tue 26, 2021 9:37 pm 
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Tubologic wrote:
Electrolytic capacitors (old or new) only explodes when submitted to one of the following conditions:

- Reversed polarity (backwards connection)
- Connected to AC voltage
- Overvoltage (DC) in the order of 150 to 200%

I suspect there is something wrong in your set-up or radio. Check again and monitor the voltage (and polarity) across the substitution cap with a hand on the power switch ready to cut power off in the event anything goes wrong. Before exploding there is allways a noticeable overheating of the cap. (Don't check with your fingers and wear protection googles !)


I worked for Burroughs and was in a training class for a NEW mainframe computer around 1980. This had a lot of large capacitors, around 3" diameter and 8" long. The power cabinet was about 3 feet from the wall and a guy was standing there when the capacitor exploded, sounding like an M-80. Scary! I assume the engineers did not think ripple current was important.


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Wed 27, 2021 12:28 am 
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I had my computer located in the basement until I was able to make a room for it elsewhere in the house. One day I powered up the computer which was 3 years old. All of the sudden there was a very loud explosion sound that was aggravated by concrete walls.
A capacitor failed in the power supply. Exploded its guts all over inside the supply case. 2000mfd @ 50v. Changed the supply out after changing my underwear.

Keith


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Wed 27, 2021 1:18 am 
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I blew one up real good once when restoring a Jackson 658 tube tester some years back. I simply installed it backwards.


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Thu 28, 2021 1:21 am 
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shango066 (tv resurrection - most-lately) on YT has many goofy videos. putting line voltage to several caps and bang

https://www.youtube.com/user/shango066

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Jan Sat 30, 2021 3:15 pm 
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Many years ago, I had a radio with capacitors that would whistle after the radio warmed up. Steam came out of the top vent. This was before I knew to routinely replace old filter caps.

Dennis


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sat 10, 2021 6:37 am 
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Chris108 wrote:
This is quite true; the term for it is "thermal runaway." Before 105 degree C capacitors were common, engineers sometimes tried to get around it by using higher voltage caps than necessary from the circuit considerations alone if the capacitor was going to be used at high temperatures. The higher the voltage rating of an electrolytic the lower the leakage current, so the less likely it would be to reach an internal temperature where runaway occurs.

A classic example is the main filter cap in an HP 606A signal generator. They are rated for 475 volts even though the highest voltage in the circuit is well below that. Anybody familiar with those generators knows they get nice and toasty warm inside after they've been on a short while. Some years ago somebody here on the forums tried to use 450-volt replacement caps and found out about this the hard way.


Chris, do you have any details you can point me to about the 475V vs 450V caps? I just picked up an HP606a from a thrift store - looks great and so far the only problems I've found is a blow B+ fuse, and one 12B4A in the -200V regulation section that is bad (looks to have lost its seal as nearly all the getter is gone). I was going down the path of the original electrolytics being bad but after swapping the 12B4A with one of the others, the -200V is now working. That said, the original black paper covered cap is marked 450V, and all are listed as 450V in the parts list.

Dave


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sat 10, 2021 7:44 am 
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Hello Guys,
yes I have had those paper capacitors go bad and one put a hole in the drywall in my College electronics classroom .


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sat 10, 2021 1:29 pm 
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Quote:
Chris, do you have any details you can point me to about the 475V vs 450V caps? I just picked up an HP606a from a thrift store - looks great and so far the only problems I've found is a blow B+ fuse, and one 12B4A in the -200V regulation section that is bad (looks to have lost its seal as nearly all the getter is gone). I was going down the path of the original electrolytics being bad but after swapping the 12B4A with one of the others, the -200V is now working. That said, the original black paper covered cap is marked 450V, and all are listed as 450V in the parts list.


The 606A was in the catalogs for about 10-15 years and there were a lot of different production runs. My impression is the original 450-V electrolytic cans were only lasting a couple of years in some situations so they went to 475-V parts in some of the later runs. I've looked at a number if different 606As and owned two, that's what it looked like to me anyway.

The thing was, a lot of places turned those generators on and left them on 24/7 to try to get the temperature to equilibrium and ensure maximum stability. They wouldn't get shut down again until something broke or it was time to send them to the cal lab--whichever came first. The generators do run hot with those regulator and RF output tubes, so the caps "cooked" rapidly. Perhaps you came across a 606A that was not run so hard, or the generator was sent back to HP for repair and they put new caps in as a preventative measure (it was common that they did). I'd test them to make sure they are not on their last legs before I'd put any trust in them.

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sat 10, 2021 3:51 pm 
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Chris108 wrote:
The thing was, a lot of places turned those generators on and left them on 24/7 to try to get the temperature to equilibrium and ensure maximum stability. They wouldn't get shut down again until something broke or it was time to send them to the cal lab--whichever came first. The generators do run hot with those regulator and RF output tubes, so the caps "cooked" rapidly. Perhaps you came across a 606A that was not run so hard, or the generator was sent back to HP for repair and they put new caps in as a preventative measure (it was common that they did). I'd test them to make sure they are not on their last legs before I'd put any trust in them.


Thanks Chris. I did perform the -200V test where the 300V supply is deactivated and a 1M is tacked across one of the tubes. That test did not result in the -200V measurement on a section of C105 (the other section was over 300v). Maybe the one section is leaky enough to pull down the 1M series impedance. I'll test the electrolytics.


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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sat 10, 2021 7:22 pm 
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I can win the prize with only ONE word.....YES!! ---All you have to do is have enough overload such that the pressure in the package causes a structural failure

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sun 11, 2021 4:39 am 
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bang

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sun 11, 2021 6:10 am 
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In doing a recap, I accidentally installed a 160 WV unit in a 300+ volt plate circuit. Oh my did it give a loud report upon applying power, as one end of it came flying out of the chassis. Taught me to pay a bit more attention... :wink:

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 Post subject: Re: Do Electrolytic Capacitors Blowup?
PostPosted: Apr Sun 11, 2021 9:54 am 
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When I was powering up a CTC17 chassis, a big ole multipart electrolytic capacitor blew, leaving black gunk every where under the chassis.

A few years ago, I was using a lytic for filter, but I didn't see the voltage. An ear piercing notice occurred, and the sucker flew to the moon. It seems 16v electrolytic capacitor don't like 170v...


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