unloading your amps capacitors

[billboogie]

I would like to clean up the chassis of my Mesa but have read warnings from folks here about unloading the caps first. I don't want to die I just want to clean some cobwebs out of one of my fav amps. could anyone please steer me in the right direction. Also what would be the proper air preasure when using a compressor to blow it out. Any help is much apprieciated.

 

[glassjaw7]

I would like to know too.

 

[moltenmetalburn]

FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)

jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here... double check wit hyour multimeter that they are discharged before poking aroug. I use 18 inch long dual alligator leads.

 

[glpg80]

ok first off, im giving this info - all legal stuff applies here. IMHO never operate inside a chassis if you dont know what you are doing. if in doubt, take it to someone that does.

if you agree to do this on these terms, read on.

take a pencil, wrap a decent sized gator-clip wire around the bottom section three or four times.

on the tip side, connect/clip a resistor, ~6K or so. make sure its 1/2W or larger. on the other side, strap it/clip it to the chassis

plug the amplifier in

warning - amplifier will have 170Vpp AC once its plugged in even though it is off and standby is off - dont think you are safe even though the amplifier is off.

take the resistor clipped on the end thats wrapped around the pencil and touch each pin of the caps. this will bleed off the caps, and the resistor reduces a quick discharge that could cause a spark or burn on the PCB or components around it.

hold the resistor there for about 20 secs, move onto the next one. so on and so forth.

make sure the amps off when doing this!

its important it stays plugged in during this time. that is how it is getting its reference to earth ground, and the chassis is being properly grounded.

take a volt meter and measure the pins in reference to ground once you are finished to make sure you didnt miss one, or that they dont have a charge still.

do this and you will be safe, just unplug the amplifier and you will be fine to handle it afterwards. but do know that all capacitors hold a charge in an amplifier, and non of them are pleasant to get bitten by. depending on the amplifier, you could short a capacitor while the amplifier is plugged in. to know the layout of the power supply section it is best to understand the amplifier.

if in doubt, send it to someone that knows what to do, and is familiar with power supply layouts. if anything, you shouldnt have the boards out to begin with if you are not troubleshooting.

any compressed air will work fine, depending on the amplifier that will not be true though. if there are sensitive MIDI PLC programmed chips inside, then the static could potentially harm them.

do know that some companies will void all warranties if they find traces of someone poking on the boards with volt meters or markings not left by the factory, and no certified repair traces. just and FYI - depends on how much of a douche the person looking at it if/when it ever needs anything is.

 

[glpg80]

FIRST, UNPLUG THE AMP! (But that doesn't make it safe....)

jumper the positive (+) lead of each large cap to GND for several seconds. A jumper with a built-in resistor (10K or so) will help prevent sparks here... double check wit hyour multimeter that they are discharged before poking aroug. I use 18 inch long dual alligator leads.

this will work too for all newer amplifiers that have to meet IEEE grounding requirements with a sticker to verify their location for sale in the US. but older amplifiers might not be grounded, or faulty. therefore its best to use earth ground as a reference point when draining large capacitors for safe measure that you arent floating any ungrounded areas with a potential voltage.

basically, if its a newer amplifier - this will work too.

 

[stratotone]

If you're blowing it out with air, I wouldn't worry about discharging the caps unless part of blowing it out with air also included jamming your hands in the chassis too.

A quick and dirty way that discharges MOST amps is to unplug it but turn the amp and standby on.

You're going to likely reach more into the amp to discharge the caps with a wire than you ever would cleaning it... just compress air clean it, don't reach in and you should be fine. This is of course if you're going to be blowing air across the actual guts of the amp, if you are talking about just cleaning the inside of the back of the amp without removing the chassis from the headshell, just wipe it down with a dry cloth or compressed air, shouldn't be any voltages present there.

Pete

 

[glpg80]

to be honest the dry rag or compressed air idea is best - just no touching at all.

forgot about the standby/power on pete. that will get them low enough that if it does bite, it wont be much at all if anything.

TBH, depending on the filtering of the amplifier, alot of designs are used to drain themselves once the amplifier is turned off. 5150's use that method of design. not sure about mesa's though off the top of my head.

 

[RoidRage]

Most amps I've worked on are discharged to non lethal voltages in 10 minutes overnight you should be fine... If you want to make sure than take a DC volt meter, and check the B-Rail caps

As far as Air Pressure 30-40psi should be fine.. I would not go much higher than that.

Best,

RoidRAGE

 

[billboogie]

Thanks to all for your quick replies . After reading through here I am now confident enough to pull the chassis and blow it out a little without damaging me or the amp. This forum is the best when it comes to support . Thanks again

 

[billboogie]

Reading through different threads I have seen that term used when speaking about loaded caps and that's what made me concerned in the first place. I hope it is an exaggeration because that would be a lousy way to go! Thanks again.

 

[glip22]

Easiest way I found is to have the amp on and off standby and your guitar plugged in. Turn the volume up higher and hit a chord. While the notes are ringing, unplug the power cord from the back letting the cord continue to ring until it goes dead. This seeems to lower the voltage in the caps to a point where it will not hurt you. After I have done this, my voltage reads around 100 or less. Was this already said?

 

[Mudder]

I would also like to mention that I have NEVER heard of anyone dying from the voltage in an amp. Is it possible, probably, has it happened? Not any story that I have read. For anyone that is going to say they knew of someone dying, please provide a link or actual proof of death.

I don't have a link, but I do know that folks have had heart attacks from discharging flash caps. So it stands to reason that it could kill someone.

Of course that doesn't keep me from poking and prodding around, roll the dice I say.

 

[Greazygeo]

I just have the amp on. Play thru the amp and turn the power off, but leave the standby in playing position. Keep playing some chords and the volume will keep dropping. Once the caps run out of juice there won't be any more sound. Pretty easy.

 

[thiswaythatway]

Come one! Aren't any of you old enough to remember Operation. Nothing like working on a big fat naked white guy.