Fear and Loathing on the workbench

[guitron45]

Reading various amp mod sites one can't help get the idea that it's a very dangerous undertaking, sorry, to put your hand or hands [even worse] into and amp chassis. While an electronics engineer friend says it's mostly fear and loathing and CYA concerining the dangers of discharging a large filter cap into you body. He says the caps drain anyway after the amp's been off a while. And you can just check them with a multi meter if you are in doubt of how much dc voltage they retain. He spent 20 years at China Lake fear and loathing center working for the Navy.

Who is right? It now occurs to me, that during biasing an amp his opinions wouldn't hold true since the caps would be recharging anyway if you drained them. So that could be a dangerous time of have both hands in the chassis. What say you fender flailers?

Ron

[Wilko]

Good safety practices would have it that you never have "both hands in the chassis", whether it's on or off.

[ziess]

Yeah, I never have two hands in an amp. Unless it's been fully drained and double checked (and unplugged) then don't do it.

[Wilko]

The thing about two hands in that is wrong is that electricity can/will go through you. If you have two hands in and get shocked, the electricity will go right through one arm, through your chest (heart) and damage tissue, and out the other arm. Even if you don't die, you aren't doing yourself any favors. You are literally "cooking" tissue. little channels of your meat will be dead and/or scarred.

Two hands is bad technique. Period. Whether or not the caps are drained is not really relevant, specially knowing that there times when you need to test hot voltages.

CYA for anyone is rarely an excuse to dismiss a warning.

[ziess]

I agree, I only meant that you should only ever put your hands in to replace components or whatever if you're absolutely sure it's safe.
Sometimes you absolutely can't avoid using a second hand (ever tried to replace a bias pot one-handed?).

[Wilko]

I was just re-iterating.

Bottom line. Use caution.

[Cygnus X1]

Yup.
+1 to Wilko.

NEVER have two hands around a live chassis.
And when draining, keep the the lead(s) clipped up.

I fashioned my shorting wires out f very bright wires so I would not forget to remove them.
I've done that a couple of times.
No real harm done, but it's a d'oh moment for sure!

[Mesotech]

The idea is not CYA, but it's safe practices. If you working with both hands in an amp that has been off and properly discharged without thinking twice about it, then you'll be that much more likely to do the same thing when the amp is powered up and you're troubleshooting or testing voltages. It's when you "aren't thinking about it" that it'll kill you. The goal is to ALWAYS think about it. Always treat every time as if the amp is hot. By doing so, you'll always be thinking, and save yourself the shock (no pun intended).

I remember the time back in one of my Navy electronics classes where the instructor was going to show us the proper way to discharge some 4000v capacitors in a fathometer. He had the unit open and all Navy gear had a failsafe "battleshort" switch that would kill power when the cabinet was open. With power "off", you could cycle the switch a few times by pushing it in and out to discharge the caps. You could also pull the switch to the "battleshort" position to allow the unit to power up even while open. Well, he was about to prove that the caps would discharge by cycling the switch when someone in the class asked a question. So he turned the unit on and engaged the battleshort to answer the question and "forgot" it was still hot when he got back around to touching the shorting probe to the presumably discharged caps. The result was similar to arc welding, with the exception of the smell of burnt flesh and hair. The power in the building browned out, he was stuck to the shorting probe with one hand and the other hand on the chassis. The student beside me went over our table and football tackled the instructor knocking him away from the electricity while I dove under the table to yank the power plug. The instructor lived, but was rushed to emergency. He (and the class) learned a very valuable lesson that day. Never take things for granted, and always work with one hand.

[Jonnda]

Now that is one dedicated teacher.

[Don]

Err on the side of safety. It's not real difficult to discharge caps.

[yankeerob]

I regularly fit 2W 220K resistors across the 1st stage filters... that doesn't mean that I assume they will have discharged as I always check 'em but they do discharge fast - I can usually switch an amp off - go grab a quick cuppa java and come back to find less than a volt left in the caps...

I also use the shrouded probes with a little hook that pops out when you depress them but is shrouded otherwise - this was useful on at least one occasion - where I was checking the plate voltage and the danged wire hadn't been soldered correctly to the tube socket - it came off and shorted to the chassis providing a brief fireworks display and a bit of charring on the end of the probe - as I had 25W ally clad resistor in series between the SS rectifier and 1st stage filter it just blew one end out of said resistor and the show was over but I was glad I was using said probes because they're generally a bit longer in length and I was well outta the way when it happened...

I think the point everyone's making is there's no room for nonchalance when you've got the back off an amp - another safety point that doesn't often get a mention is the removal of all watches and jewelry - there's nothing like a bit of precious metal to increase the potential of arcing! You just have to get into a certain discipline about it and a moderate fear of death or injury is healthy in this circumstance - I don't know how many times I've checked the caps despite 'knowing' they're discharged - this can be annoying when you're up against the clock and you do it day in and day out but you just have to shrug your shoulders and accept it's always better safe than sorry...

[cherrick]

An amp is like your anus.

Try to avoid putting both hands inside at the same time.

[Mike Shaw]

Also, when draining electricity remember to attach the ground first, then attach to the component.

[Doc W]

An amp is like your anus.

Try to avoid putting both hands inside at the same time.

There is some truth in this. I have had many amps that sound like shit.

[Mike Shaw]

LOL! And Fender did use brown 'turd' caps!

[bluetweed]

An amp is like your anus.

Try to avoid putting both hands inside at the same time.

That's going to have to go up in the garage.

[yankeerob]

+1 on that!!! That a 'cracking' bit of advice!!!