Discharging Capacitors Where Lead Is Not Visible

J M Fahey

Member
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2,826
In most cases the voltages aren't lethal. They're definitely dangerous, but I've been hit a few times with 350-450vdc and it leaves a nasty mark, but I'm not posting from beyond the grave here. The lethality is overstated in mostguitar amps, as they tend to sit below 500vdc on the power supply. Old Ampegs, Musicman RP100's, Marshall Majors, etc on the other hand will hurt you badly and likely land you in the hospital.
Stupid dangerous post, disregards *all* Electrical Safety rules valid anywhere.
Voltages greater than 50 V applied across dry unbroken human skin can cause heart fibrillation if they produce electric currents in body tissues that happen to pass through the chest area
Living human tissue can be protected from damage by the insulating characteristics of dry skin up to around 50 volts. If the same skin becomes wet, if there are wounds, or if the voltage is applied to electrodes that penetrate the skin, then even voltage sources below 40 V can be lethal. Accidental contact with high voltage supplying sufficient energy may result in severe injury or death. This can occur as a person's body provides a path for current flow, causing tissue damage and heart failure.
 

8len8

Silver Supporting Member
Messages
13,960
You routinely take 40 volt shots? Why?
Touching and debugging circuitry that's powered up. You can't debug a circuit board with gloves on (at least that makes it difficult).

And it's not taking "shots". You just handle the circuitry and you find feel the current created by the voltage.
 
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DrainBamage

Member
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2,403
Touching and debugging circuitry that's powered up. You can't debug a circuit board with gloves on (at least that makes it difficult).

And it's not taking "shots". You just handle the circuitry and you find feel the current created by the voltage.
What's worse- A 120 ac wall shock or the 25dc volts my amp stores after power down?
 

Cirrus

Member
Messages
2,429
You've already been given it :p drain them through the V1 plate resistor pin on the socket. With no valves in the sockets, if there's voltage on the HT line that voltage will be present on the plate pins of each valve socket via the plate resistors. Draining from V1 will in most cases drain all the Filter caps, in some amps the first cap might be pre-standby switch so you'll need to make sure the standby is in the "make noise" position so the first cap can drain through it.

There are plenty of instructions on the net for making a safe charge-draining probe.

Ultimately, you're going to want to test with a voltmeter. If you can't get to the cap bases, you can almost always find some exposed metal - a valve socket, a resistor leg etc - to test the voltage at. It's a simple matter of tracing the circuit from the cap with your eyes.
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,379
To the OP.....one can trace out the B+ connections and check voltage there...even in a PCB amp. IF you can't trace the B+, then perhaps more study is necessary.
I am not averse to discharging caps with an instrument...hit a chord and disconnect from the AC source.....whether turning the power switch to off or unplugging the amp...while the chord rings. You will hear the caps drain....as the sounds die out. Then, assure that the caps are discharged and that they remain discharged.....yes, these things can mysteriously regain a charge if that drain is not maintained. A connection from V1's plate to chassis ground will maintain the drained condition....with the stand by in play if there is a stand by switch. IF the amp is not in playing condition, then one needs to drain the caps with a ground to chassis.
Keep one hand behind your back...old safety precaution. IF you take a 'hit' on one side of the body with the other hand away from a contact with the current, you may feel it but you won't die. When the current crosses your heart while traveling from one side to the other, you stand a chance of attending your own funeral as the guest of honor. I have read that it takes only .1 amp of current to kill you if it hits the heart at just the right part of a second.
 

Eric2R

Member
Messages
101
Really now? Do provide us with a list of all the people who have died this year (or the past 15) from accidental high DC voltage shock in tube amps. It's a fairly small community, those of us who tinker with them, so surely you've heard of someone If they died, right?

I don't recall saying it was harmless. What I said was the lethality is overstated. And in order to disregard electrical safety rules, I would have had to provide some form of instruction or procedure in my post, which I did not.
Stupid dangerous post, disregards *all* Electrical Safety rules valid anywhere.
 

cugel

Silver Supporting Member
Messages
4,553
I don't see how the number of fatalities, or lack thereof is meaningful to this thread at all. Why would you want to hurt yourself? Why not limit pain and suffering if at all possible?
 

Eric2R

Member
Messages
101
I don't see how the number of fatalities, or lack thereof is meaningful to this thread at all. Why would you want to hurt yourself? Why not limit pain and suffering if at all possible?
Just putting the fear in check. The so-called amp gurus and experts have some guys scared to even open their amps. It's not 100 year old sweating dynamite you're playing with.
Just get in there, pay attention and be careful.
 

DrainBamage

Member
Messages
2,403
Just putting the fear in check. The so-called amp gurus and experts have some guys scared to even open their amps. It's not 100 year old sweating dynamite you're playing with.
Just get in there, pay attention and be careful.
Why don't you go stick your hands in there? lol If you don't report back we will know why.. right? Folks drain the filter caps for a reason. My brother knows of someone who was hurt as an electrician at his work place and is collecting disability.
 

Eric2R

Member
Messages
101
If you recall from my first post, I have indeed been hit more than once over the years. Yet here I am.

Also, I never advised anyone not to drain the caps.

You lot are experts at straw-manning, apparently.
 

Steppin' Wolfe

Silver Supporting Member
Messages
5,379
A realignment of the ions in the body once a day is a healthy thing. How one goes about that realignment of ions is important. In all of the arguing here about voltage and danger, there has been little talk about how to avoid...as much as possible....getting a shock when working on these things. I reread the thread, and mine was the only post that gives one really good safety measure.....keep one hand away from the chassis. IF anyone here who works much on these circuits has NEVER been shocked, ime that person is a rare being. Those of us who have been shocked and not died are in the majority, I would think....and we survived those situations more than likely because we keep one hand away from the chassis. IT is one thing to have a sore shoulder from taking all of the caps' energy due to forgetful neglect when tightening a Marshall's pin sockets. It is another thing to have had both hands involved and putting the heart in the middle of the current. (;^)
So...to the OP....drain the caps in any and all manners that you know of. Then, check with a volt meter. Then....when you are assured that he caps are drained and they are connected to chassis ground....STILL KEEP ONE HAND AWAY FROM THE CHASSIS. The only time I am free to put both hands in there is when I have cut the caps out of the circuit and there is no possibility that there is any energy being stored in an electrolytic capacitor. AFter I have fired an amp up after a cap job, all safeguards are once more put into effect....becasue energy has been introduced to those new caps...adn danger exists.

RS635, back toyour quesiton....yes, PCb's 'hide' things a bit. But....one can always find the B+ connection. IT leaves the rectifier....tube or solid state....and connects to the first stage of filtering and thereby to the center tap of the output tranny. Study the schematics and you will see this. Then, with your chassis in front of you, find the OT's primaries and the center tap. In U.S. amps, these are usually blue, brown and red.....brown and blue for the primaries to the power tubes and red for the center tap. That center tap will give you your B+...and you can check there for voltage in the filters...even if a connection to the preamp has been interrupted. Get to know your circuit. Checking that V1 plate for voltage before draining will tell you if the circuit is 'whole'....or if the amp is processing signal from the input through the speaker one can surmise that the preamp's connections to the power supply is good....and that connection from V1's plate will drain the caps and maintain that drain.
 

amstrtatnut

Member
Messages
12,836
Turn amp on then turn off the main power while leaving the standby in the on position. Wait 60 seconds it should discharge the caps. This has been written here many times, I'm just restating it for you.
Only 60 seconds? My amp manufacturer recommended over night.
 

Guinness Lad

Senior Member
Messages
15,860
Only 60 seconds? My amp manufacturer recommended over night.
From how I understand it is you have the amp on, make sure you have a guitar plugged in the turn off main power, immediately play the remaining sound you hear as it fades is the capacitors discharging, the tubes are off and they do not store electricity. If you have a voltage potential and a path to ground, voltage will seek ground immediately, it won't sit around for days, this would happen in a closed circuit.

People get electrocuted because they provide a path to ground if you were ungrounded the electricity would not escape you, you would have potential for sure, but until you grounded yourself you would be safe. The problem would become if the potential was so high that it would jump, then at this point all bets are off.
 




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