Discharging Capacitors Where Lead Is Not Visible

JS635

Member
Messages
70
Title says it all. Trying to enter the world of amp modding and don't want to shock myself to death in the early going. The amp has all the components mounted directly to a printed PCB, and the caps are all radial-style, so the leads aren't visible. What's the best way to discharge the caps in this situation? I'm a little concerned about removing the board, since I'd have to physically touch it to do so. Any tips?
 

Guinness Lad

Senior Member
Messages
15,860
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.
 

RussB

low rent hobbyist
Messages
11,157
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.

That works to get the voltage down to 10 or so volts...MOST of the time. It will still sting a little. I always clip a jumper from a socket pin that has plate voltage on it, like pin 1 or 6 of a 12AX7, pin 3 of a 6L6GC/EL34...

Always
measure the voltage in the caps after draining and before starting any work.

I worked on a JCM 800 2204 that had a broken wire from the choke to the fuse. The mains caps were chock full with 500 volts in them. Because of the broken wire, the juice had no where to go. Tread carefully!
 

JS635

Member
Messages
70
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.
No standby on the particular amp I'm starting with. Nothing but a power-on and a volume. Figured I need to start from the bottom and work my way up before I start messing with my higher-end (and dollar) amps.
 

JS635

Member
Messages
70
That works to get the voltage down to 10 or so volts...MOST of the time. It will still sting a little. I always clip a jumper from a socket pin that has plate voltage on it, like pin 1 or 6 of a 12AX7, pin 3 of a 6L6GC/EL34...

Always
measure the voltage in the caps after draining and before starting any work.

I worked on a JCM 800 2204 that had a broken wire from the choke to the fuse. The mains caps were chock full with 500 volts in them. Because of the broken wire, the juice had no where to go. Tread carefully!
Yikes, that's what I'm afraid of. Not looking to shock myself to death in the process, so I certainly want to avoid that type of situation.

This particular amp is bare, bare bones; one 12AX7, one EL84. I figured I'd start simple (and cheap) and work my way up. I suppose the issue I'm having is that I don't really have access to the leads of the capacitors to measure the voltage without actually grasping the circuit board to remove it and potentially juicing myself. Can I accurately measure the cap voltage value without seeing the leads?

By the way, thanks for the response!
 

damian1

Member
Messages
615
Always make sure with a multimeter. The standby trick usually works but like Russ said in some instances it does not. I like to have my multimeter handy when I use a large cement resistor to ground (chassis) and I can watch the voltage drain out and make damn sure.
 

DrainBamage

Member
Messages
2,403
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.
That's terrible advice and should be removed. My 18 watt will still have 25 volts that way and have most of those volts for some time
 

8len8

Silver Supporting Member
Messages
13,965
That's terrible advice and should be removed. My 18 watt will still have 25 volts that way and have most of those volts for some time
25V won't hurt you. As an electrical engineer I routinely touch up to 40V. It's at levels above that where safety comes into play.
 

VaughnC

Silver Supporting Member
Messages
18,012
With the lethal voltages involved, a tube amp is no place to learn electronics. I'd start with a battery powered pedal of some sort, and buy an oscilloscope & signal generator so you can look into a circuit to see what's going on. Whatever...just be careful as high voltage is unforgiving.
 

Eric2R

Member
Messages
101
With the lethal voltages involved, a tube amp is no place to learn electronics. I'd start with a battery powered pedal of some sort, and buy an oscilloscope & signal generator so you can look into a circuit to see what's going on. Whatever...just be careful as high voltage is unforgiving.
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.
 

big mike

Cathode biased
Platinum Supporting Member
Messages
13,293
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.
Really REALLY foolish attitude towards this stuff.
 

Cirrus

Member
Messages
2,429
I'm trying to remember if LoudAF is the same guy who quite often comes into "How do I discharge capacitors" threads and says that stuff.

Thing is, even if it's mostly true, you have to be pretty dumb to come into a thread like this with such a blase attitude because there is a risk attached to being hit by hundreds of volts that depends on various issues - how and where the current passes through you, your health, whether there are any sharp objects to bang your head off when you're thrown backwards off your chair...

I think I'd say that LoudAF is certainly entitled to voice his opinion, but everyone else is equally entitled to say it's a stupid attitude to teach a beginner.
 

mscaggs

Member
Messages
629
I'm trying to remember if LoudAF is the same guy who quite often comes into "How do I discharge capacitors" threads and says that stuff.

Thing is, even if it's mostly true, you have to be pretty dumb to come into a thread like this with such a blase attitude because there is a risk attached to being hit by hundreds of volts that depends on various issues - how and where the current passes through you, your health, whether there are any sharp objects to bang your head off when you're thrown backwards off your chair...

I think I'd say that LoudAF is certainly entitled to voice his opinion, but everyone else is equally entitled to say it's a stupid attitude to teach a beginner.
There are old pilots and bold pilots, but no old, bold pilots
 

VaughnC

Silver Supporting Member
Messages
18,012
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.
Well, yes, while a few hundred volts is unlikely to kill with every contact, it depends on many factors. However, the potential is there...and tinkering with 9v is much safer for the novice than 450v. I knew a guy who got hit with 250v but got killed when he jumped back and hit his head on a radiator. All I'm saying is that electronics novices shouldn't be learning on high voltage circuits...so, we'll have to agree to disagree.
 




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