Discharging a Capacitor

[collin]

I just recieved a Ceriatone 18W Marshall clone chassis via Fedex, and the neanderthals succeeded in smashing up part of the amp.

Luckily it's all in good shape, but the 32uF/32uF Capacitor is totally smashed, and needs replacing.

I understand that Capacitors need to be discharged of electricity before swapping out------this amp was shipped from Boston over a week ago.....is it even possible that it would still have any electricity inside at all? (or enough to hurt me?)

I have no clue how to discharge the electricity on these. I'm good with tools, and can plug-n-match wires to replace the new one, but the concept of draining excess electricity is foreign to me!


Thanks Tube techhies!!

[paologregorio]

I'm not exactly sure on this, but I think one way is to make sure the amp's unplugged, and turn the amp switch to on. I could be mistaken on this. If you're interested, you can come over and look at my copy of Pittman's Tube Amp Book. I could probably scan you a copy of the appropriate schematic if you need it.

[beatlefreak]

Since the cap's smashed, just take a pice of insulated wire and touch the bare ends to the cap's terminals. Depending on whether there's a charge on the cap, you may get a spark. Once the terminals are shorted together, the cap is discharged.

[cjj]

Yes, caps can hold voltage for a LONG time, days, even weeks. High voltage caps are what you need to be worried about, and tube amps typically have several hundred volts running around inside.

So, make sure the amp is unplugged before poking around. Then, short out the cap in question as beatlefreak said, or with a screwdriver (plastic handle please), etc. It's also not a bad idea to short a few others if they're near where you need to work just in case. In general, it's pretty hard to hurt a cap by shorting it, and if there's no power you won't hurt anything else either. If it's still got voltage, you may get a spark, the bigger the cap, the bigger the spark, so be ready for it so it doesn't make you jump and drop/break something...

[collin]

Thanks guys!

Turns out it was deader 'n' dead, so nothing to worry about.


I've heard all kinds of stories, just wanted to be on the safe side.



ps...thanks for the offer Paul---I need to make it over there soon anyways, been swamped lately.

[fatcat]

I once knew a practical joker, in an electronics class, who thought it funny to charge up a cap, then put it down on a lab bench, and yuk it up when someone picked it up and got bit.
I wonder how long it took him to re-inflate 4 flat tires

[johnallg]

The crushed cap was probably internally shorted so that was why they were deader than dead.

CJ, would a 2200uF 500V cap be considered big? Can I weld with it? I have 2.....

[jingle_jangle]

I've heard more than one story about car stereo newbies being killed dead by one of those thumper 2 FARAD caps they use to kick the subs into submission. My last install (into a Miata!) used two 2 farad caps and they were connected directly to their own Optimus battery with battery cable. The manuals had all sorts of warnings about how to safely discharge them. something about trained flying monkeys, as I recall.

[cjj]

The crushed cap was probably internally shorted so that was why they were deader than dead.

CJ, would a 2200uF 500V cap be considered big? Can I weld with it? I have 2.....

Well, those are reasonably big. Two of them charged to 500V would have about 550 Joules, 1 joule is equal to 1 watt second. This is enough energy to run a 100 watt light bulb for 5.5 seconds. Now, electric arc welding typically has an arc voltage of 15 to 30 volts at around 250 amps, so you're talking about 3750 to 7500 watts. So, you could weld for about 0.15 seconds.

Enough to weld a small screwdriver to the capacitor terminals, but you won't be repairing any automobile frames with it.

[sharkboy]

FYI Collin: I have a Ceriatone version of a DC30 and am extremely happy with it. Replace that cap, inspect the rest carefully and have fun!

-Markg

[collin]

Cool, thanks Mark! I saw the DC30's on the site, they look excellent.

I've had many Marshalls----the Bluesbreakers have always been my favorite, but they are extremely heavy, and don't sound that great unless they are completely dimed, and then your neighbors hate ya.

I wanted a Marshall 1974x reissue, but they are horribly expensive---$2200 new and around $1400 used, for a small 18 watt handbuilt amp.

Ceriatone seems to get flawless reviews everywhere. The chassiae (haha, plural for chassis?) are all hand-made and hand-wired by Nik in Malaysia, and apparently sound brilliant, especially for the price. Top quality components without the "brand name fluff" and markup.

I'm having a cmbo cab made, with a 2x10" configuration so it will fit in my car's trunk. I'll check back when it's together and rockin.

[ken_j]

I use a resistor (1k) when discharging caps as not to get an arc. When I was in the army we had a cap blow up by being dicharged to quickly. It was about the size of a coffee can and really stunk up the place when it went.

[soundmasterg]

The proper way to discharge a cap would be through a resistor to ground. The value that you choose for the resistor determines how fast the voltage drops in the cap. A 10k 2w resistor is a good choice, which will usually result in the whole amp being down to a safe voltage in 30 seconds or less, though if you are just discharging one cap, it might drop to safe levels a little quicker. It can actually damage the plates inside the cap to discharge it with a screwdriver, wire, or similar shorted directly from the positive terminal of the cap to ground, thus it is not recommended unless you are planning to replace the cap anyway. Shorting with a screwdriver is a useful way to scare people because it does create a spark and a loud bang.

You can take a 10k 2w resistor, and two lengths of wire along with some heat shrink tubing, and solder the resistor in between the wire with the heat shrink tubing covering the resistor and the exposed wire. Then get two insulated clip leads and solder those to each end of the wire. You can then use this to discharge amps and caps, and still be safe. I've made up several of these and use them whenever I work on an amp. As soon as I put the open chassis on my bench, I hook one of these up to the main filter cap, and it stays there as long as I am working on it, or at least until I need to disconnect it to do some live chassis testing. With that hooked up, there is no way I can get shocked and safety is number 1. The only downfall to doing this is sometime syou may forget that you have it hooked up when you fire the amp up, and so the resistor starts to smoke. I've done that a couple times as I'm sure any amp tech does from time to time. I'd rather burn a resistor though than my hands!

Greg

[whojamfan]

Thanks for clarifying that Greg, caps can be scary business. So, to be safe, always unplug the amp, hook up the resistor to ground cable you describe here,(main cap will drain all the others), wait a couple of minutes, and then leave it connected so the caps don't recharge themselves automatically?
Cool

[soundmasterg]

Thanks for clarifying that Greg, caps can be scary business. So, to be safe, always unplug the amp, hook up the resistor to ground cable you describe here,(main cap will drain all the others), wait a couple of minutes, and then leave it connected so the caps don't recharge themselves automatically?
Cool

Yes caps are usually one of the most dangerous things inside amplifiers as they can hold a charge for years after they have been unplugged from a voltage source. You have it basically correct with what you described, but remember after you are done working on the amp to unplug your cap to ground resistor/cable setup and plug the amp back in to test it. Also, make sure to use insulated clip leads for your resistor/cable setup or you could shock yourself. Another avenue you could go instead of building one yourself is to purchase one already made. You can get a good one for $15 at https://amptechtools.powweb.com/stick.htm . For my own use I just make what I need because it is easier and I can tailor the voltage discharge time period by using different size resistors. For me I like a little slower discharge so I use a 100k or a 10k. Usually it will discharge to a safe level within 30 seconds to a minute.

Greg