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chrisN

Idiots guide to safety with electrolytic capacitors in guitar amps needed

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Checking, checking again, discharging, rechecking, discharging, checking again and making sure as many times necessary - the right way. Not the dirty way. 

Please refer to the numbered questions below, and please answer them if you can. If you can answer the questions, feel free to then share your method of it differs. I've found that some people sharing their methods can't answer the questions, and don't offer any ways to make sure. Hell, one guy, even with his method, said that he's been shocked several times, and the guy claims 20 years of experience. Then, he suggested that if I got shocked, that I should "make a fist" with one of my hands to stop from stopping my heart... But doesn't electricity travel at the speed of light? Lol 🤣 Again, this is a tech who claims 20 years experience! 🤯 Who knows, maybe this guide could save his life too! 💩

I see a lot of incomplete or vague information about this all over the place online and have a few questions dealing with high voltage capacitors and making sure one doesn't get shocked. 

For example, someone might find that their bright cap has been clipped in their amp, but they want it back; however, access to the other side of the board is difficult. Let's say you have an amp with PC mounted pots.. You have to undo them all to get the stupid board out, but don't want to get shocked. Meanwhile, you also don't want to make at least two back-and-forth 45 minute drives to a tech to then wait around for 2 or 3 weeks until they get to it, plus the bench fee for something so simple, and then repeat the whole process again if you decide you want to try another value.. Meanwhile, the tech won't even set up a couple caps to audition in person on a rotary switch or something. All stuff you could do yourself, but the only thing standing in the way is the fear of getting shocked. 

Here are the questions:

1) Can you detect if an electrolytic capacitor in an amp is holding a charge by using a non- contact DC voltage detector such as this? 

https://www.amazon.com/Non-Contact-Voltage-Flashlight-Klein-Tools/dp/B00XJQ9ZE4/ref=mp_s_a_1_5?keywords=Non-contact+DC&qid=1581188684&sr=8-5

 

2) How do you double check an electrolytic capacitor to make sure it's holding a charge) live or not, to make sure you don't get shocked, using with a multimeter exactly? Step-by step instructions?  Here is a picture of an electrolytic filter cap in a Marshall for reference. Do you touch the black lead on the multimeter to the chassis or ground and the red to one of these that are jumpered? Exact directions, no vagueness. Please make it as idiot proof as possible. This could be the most important step to ensure safety! 

20200208_144932.jpg.4ea4161d3a9ff09c9eaf

 

3) Using a capacitor discharge stick tool, would you just clip one end to the chassis and touch the jumpered terminals in the above picture for however long (probably in the manual of the discharge tool if it has a manual) and then recheck with the multimeter?

 

4) Power and Standby On while unplugged from AC Mains and connected to a Cab trick:

Turning you amp off, unplugging it from the wall, still having your speaker cab connected, and then turning power and standby on... Does this help drain the capacitors? Reading around, some amps are designed to self-drain. Also, this sounds kind of hokey, because there's no way of knowing if the capacitors are drained enough, without rechecking... But does it work? Of course one would then recheck using the answer to question 2.

 

I've spoken to some "techs," even ones that do repair for music shops, and a couple of them don't even know the answers. One just said, "I just touch a screwdriver to the chassis and then touch the possitve on the capacitor. If it pops, then I know it was charged." - and that's it. Seems to me that popping caps all the time would shorten their life no?  (this came from the same tech who said to make a fist btw hehe) 

Thanks in advance for any accurate and complete responses that are non-vague, from any of you who know what you are doing. Who knows.. Maybe fist pumping could save you too from a lightning strike! 

 

20200208_144932.jpg

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First, I am not an engineer, nor a 'real' electronics tech, nor an electrician...but I have been servicing my own gear for decades, and I am still alive. I worked in hi-tech electronics, aerospace and the musical equipment industry on and off in varying capacities during my career, and I know just enough to be dangerous😉

1: I have never seen anyone use a detector like that on a DC circuit. As I understand it, those are used for detecting AC, not DC.

2: To my knowledge there is no accurate method for the average person to safely check if a capacitor is charged. A VOM is not designed for or capable of doing this. Many years ago I learned that when dealing with DC circuits, always assume all the caps are fully charged. There are devices one can build that will detect charge in a capacitor, but this is typically not something the average person would build.

3: Yes the 'stick' tool is what I've used for years, and it works. Discharging a high voltage capacitor with a screwdriver to chassis/ground is probably the least safe method, as I have seen screwdrivers literally weld themselves to a chassis [not on an amp, though]. An engineer I used to work with advised me that the first discharge only reduces about 65% of the charge in the capacitor, and the safest process is to use the tool five times on each capacitor. You will never get the charge down to 0V, but low enough to pose no lethal threat. [ I'm told that ~40VDC is considered to be reasonably safe {non-lethal} by most techs, but that will still make you numb for a while]....and as we used to say, it isn't the voltage that kills you, it is the amperage...

Have a look here:

http://www.repairfaq.org/sam/captest.htm#ctcds

4: No, this will not make your situation any better, as there is no current flow, therefore no reason for the capacitors to discharge. and waiting a couple of days while the amp is unplugged isn't the answer either.

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21 hours ago, daddymack said:

1: I have never seen anyone use a detector like that on a DC circuit. As I understand it, those are used for detecting AC, not DC.

 

Thanks for your reply.

Most of these devices are made to check AC voltage. This one (and a few others) are supposed to check both AC and DC. 

I also have an ESR meter, but haven't used it or understand how to. All I know is that it is used to test capacitors while in circuit.

 

I'm surprised most techs don't have a clue what it is. Air Conditioning technicians use them to check the large caps in air Conditioning units without the need to remove them from the circuit. I got one to work on my AC, but discovered the problem was corroded relay switches before the ESR meter arrived, so I never got around to using it. 

BTW, it's frustrating working with A/C Techs. 5 different ones couldn't fix the problem (or lied about of it) and they all tried to up sell me units costing $8k or more. I had a similar experience with amp techs. One guy got me to get rid of my perfectly fine, legendary Sylvania STR tubes, when the problem was simply a matter of cleaning the fx loop jacks on my 5150. The amp has never sounded the same since those tubes were "taken" from me. I just didn't know any better.. But that's another story. 

Edited by chrisN

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21 hours ago, daddymack said:

😉 

2: To my knowledge there is no accurate method for the average person to safely check if a capacitor is charged. A VOM is not designed for or capable of doing this. Many years ago I learned that when dealing with DC circuits, always assume all the caps are fully charged. There are devices one can build that will detect charge in a capacitor, but this is typically not something the average person would build.

3: Yes the 'stick' tool is what I've used for years, and it works. 

There are some discharge tools that have a light that dims or some kind of buzzing sound, to let you know the capacitor is at a low enough voltage to handle, plus capacitors have the ability to shock you, so I don't understand why a volt meter isn't capable of testing if the capacitor is charged. I've been told you can test a capacitor while  it's in a circuit, with the exception of using an ESR meter, but this isn't about testing capacitance, but actual voltage/current, so it doesn't make any sense to me why one shouldn't be able to use a volt meter to do this. 

The only reason I didn't get a discharge stick with a light or buzzer is because some reviews claimed they use the wrong resistors etc, making the process take way too long. The stick I ordered says you only need 5 seconds to discharge a cap, but the device doesn't provide any feedback whether the discharge was indeed completed, so it seems like russian roulette using one. 

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the thing with caps is you don't know it has a charge until it discharges...which is why a VOM isn't going to read it unless it pops.

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Most caps will discharge when the amp is powered off depending on the circuit design.  I worked on amps for a living for half my life and the only time I'd accidentally get zapped was when working on the amp with it powered up.  Most of the troubleshooting you do to find problems is done with the amp powered up so being careful around hot spots is important if you don't want to glow in the dark.  

If I were doing a cap job, then I'd drain off any residual voltage jumping the Hot and cold leads just to be sure but rarely if every seen actual sparks like you might with the cap removed from the circuit then charged up. Power caps are typically across transformers and resistors so they discharge very quickly in most amps. 

How dangerous is the voltage stored in a cap?   They can give you a nasty blast but how dangerous depends on the path the charge takes and how large and long it is.  AC "current" is the stuff that will kill you.  Caps can only store DC voltage, not AC.  The voltage can be high, but their current is typically very low.  It also dissipates instantly so its lot like AC that will freeze all the muscles and prevent you from letting go.  Caps typically give you nasty reminder like a sharp blow that stings like a  mother.  Can it kill?  Possibly.  The paths needs to be across the chest like having on hand on the chassis and the other touches the hot lead.  This might stop a weak heart when it discharges.  to be honest I been zapped hundreds of times and always recovered. 

There is an important rule you're taught when working on gear.  Its called the one hand rule.  When you're probing around in a circuit you "only" use one hand. You keep the other hand in back of you or in a pocket. Never Hold the chassis or any ground when probing. This prevents a path from hot to ground across your chest.  You can still get bitten badly but its typically localized  to the hand only from the palm to finger etc. Still a nasty bite I wouldn't want anyone to deal with but you take yo a profession like electronics, shocks and fire come with the trade.  Trick is to simply take common sense precautions and avoid doing stupid stuff by being careless. 

 

 

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22 hours ago, daddymack said:

the thing with caps is you don't know it has a charge until it discharges...which is why a VOM isn't going to read it unless it pops.

Found this... 

Http://www.learningaboutelectronics.com/Articles/How-to-test-a-capacitor

 

"Test a Capacitor with a Voltmeter

Another test you can do to check if a capacitor is good or not is a voltage test.

Afterall, capacitors are storage devices. They store a potential difference of charges across their plate, which are voltages. The anode has a positive voltage and the cathode has a negative voltage.

A test that you can do is to see if a capacitor is working as normal is to charge it up with a voltage and then read the voltage across the terminals. If it reads the voltage that you charged it to, then the capacitor is doing its job and can retain voltage across its terminals. If it is not charging up and reading voltage, this is a sign the capacitor is defective. 

To charge the capacitor with voltage, apply DC voltage to the capacitor leads. Now polarity is very important for polarized capacitors (electrolytic capacitors). If you are dealing with a polarized capacitor, then you must observe polarity and the correct lead assignments. Positive voltage goes to the anode (the longer lead) of the capacitor and negative or ground goes to the cathode (the shorter lead) of the capacitor. Apply a voltage which is less than the voltage rating of the capacitor for a few seconds. For example, feed a 25V capacitor 9 volts and let the 9 volts charge it up for a few seconds. As long as you're not using a huge, huge capacitor, then it will charge in a very short period of time, just a few seconds. After the charge is finished, disconnect the capacitor from the voltage source and read its voltage with the multimeter. The voltage at first should read near the 9 volts (or whatever voltage) you fed it. Note that the voltage will discharge rapidly and head down to 0V because the capacitor is discharging its voltage through the multimeter. However, you should read the charged voltage value at first before it rapidly declines. This is the behavior of a healthy and a good capacitor. If it will not retain voltage, it is defective and should be replaced. "

 

 

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zackly...it only reads the discharge, not the potential....and it will not read '0' volts...there is always  a little something left behind

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