i've been searching around for a general how to amplifier repair thread and i wasn't able to come up with one (please let me know if i missed it). that being the case.... here goes!

first off, i don't know what the hell i'm doing, so i picked up a book called "how to service your own tube amp." it's a great book, but it's left me with many questions. my first round of questions deal with capacitors:

#1 - i know i can measure the stored charge of a capacitor with a multimeter. i'd like to open up an amplifier and first off check to see if the caps are holding a potentially lethal charge. i have a multimeter than can read up to 1000 volts DC and 600 volts AC. can i use it to measure a potentially lethal cap? if so, do i clip the negative probe of my multimeter to the chassis and then put the positive probe to the positive terminal of the cap (and of course, keep one hand in my pocket at all times)? or do i need to put the negative probe to the negative terminal and positive to positive (which makes more sense to me, but i don't want to accidentally kill myself)?

#2 - how do you properly discharge a capacitor? the book mentioned above recommends a 1k 10 watt resistor with alligator clips. where exactly do i clip these clips? it seems to me that one clip should go on the positive and one clip should go on the negative, but the book words it in a way that makes me wonder if that's so (i think it says something about putting the clips "across" the terminals).

lastly, can anyone recommend another good book on amplifier repair?

thanks!

RE #2: I may have the same book as you (or similar) if so there is a big section in the middle with pretty pictures of how to make the gadgets he describes in the book. If you don't have the book I do then I'll scan mine when I get home (providing I do before Scott breaks out some knowledge here).

phildotidiot wrote:#2 - how do you properly discharge a capacitor? the book mentioned above recommends a 1k 10 watt resistor with alligator clips. where exactly do i clip these clips? it seems to me that one clip should go on the positive and one clip should go on the negative, but the book words it in a way that makes me wonder if that's so (i think it says something about putting the clips "across" the terminals).

lastly, can anyone recommend another good book on amplifier repair?

thanks!

I think that I remember hearing that if you have the amp plugged in and power off but flip the standby switch to on it should discharge the caps to ground but I may be way off on that, Scott or TMidg? I believe you could clip one side to a ground post in the amp and the other side of a wire with alligator clips to either side of a cap(unless it's an electrolytic, then you go to the negitive side) with a resistor in between and that should do it.

Kevin O Connors "the ultimate tone" books are supposed to be excellent. Gerald Webers books are pretty darn good but he is off the charts hokey. his video is very informative but he shows an obseen amount of chest in it, and apparently leather pants are an appropriate clothing option for amp repair, I had no idea.

I found this link very helpful :
http://geofex.com/ampdbug/ampdebug.htm
best k.

Ryan Owens wrote:RE #2: I may have the same book as you (or similar) if so there is a big section in the middle with pretty pictures of how to make the gadgets he describes in the book. If you don't have the book I do then I'll scan mine when I get home (providing I do before Scott breaks out some knowledge here).

the book i have is called 'how to service your own tube amp' by tom mitchell. i'm not sure i'd call the pictures 'pretty' per say... hehe... but if it turns out you have a different book i'd be interested in seeing some of the pictures you're talking about. thanks!

notimeforlove wrote:I think that I remember hearing that if you have the amp plugged in and power off but flip the standby switch to on it should discharge the caps to ground but I may be way off on that, Scott or TMidg? I believe you could clip one side to a ground post in the amp and the other side of a wire with alligator clips to either side of a cap(unless it's an electrolytic, then you go to the negitive side) with a resistor in between and that should do it.

i think there's a couple ways to reduce the power in the caps... but i think they still store some charge. i think the other thing to do is pull the plug when the amp is on. but i'm not too sure about any of this either. comments by anyone who does know would be appreciated.

keeno wrote:I found this link very helpful :
http://geofex.com/ampdbug/ampdebug.htm
best k.

bookmarked! thanks

As far as discharging caps in general-
Turn off amp and let it sit for about 20 seconds or so. If there are bleeder resistors they will be draining the voltage away pretty quickly. If you measure a cap after a minute or so, and there is still voltage (more than 5% of charged), then there are may ways to go about it. A safe and slow way is clamping a suitable resistor across the terminals. You can use ohms law to find out the size of the resistor with a mind on the wattage it can take. Example: If you want to discharge a cap that has 200VDC on it and you only have 1/2 Watt resistors, the lowest value your resistor should be is 80kohm (really use higher than 100k). W=V*I and I=W/V so I=.5/200= .0025, then V=I*R, 200=.0025*R, R=200/.0025=80000

Two tools amp technicians keep handy are a well insulated clip lead with a well insulated resistor in the middle. This is a like a 10W-1k resistor, totally insulated with some alligator clips on the ends. Don't try to clip it to the capacitors, you'll slip and shock the shit out of yourself or die. Clip it to tool #2 always from the ground.

You'll probably use what you have around. Since you are not applying a continuous load, you just need a resistor that can handle the surge generated from the discharge. Wirewounds are the best solution here since they are least likely to create an 'open circuit' when they're overloaded. Using a 1/2W resistor for this serious job, would be a mistake imo.

The other tool (#2) is a big screwdriver 'Chicken Stick" that is left shorting the capacitors to the chassis while working. Keep your clip lead attached too for that matter. Shove this bad boy under the capacitor leads and up against the chassis to keep shit OFF even if power is accidentally applied. You may blow fuses when you forget to remove it, but it's better than lighting yourself up.

Keep one hand behind your back, brother.

Ptay

Most of oldschool down and dirty guitar techs in the back of the guitar shop will just jam tool #2 up against all the capacitors causing a large explosion of atomized screwdriver molecules ... people say not to do this because it's bad for the capacitors, but I would argue against doing it because it's more-bad for your lungs, eyes, and ears, to say nothing of the screwdriver.

Of course, after using your resistor, this is the only sure way to discharge the caps and keep them discharged, and you will want to be in the habit of doing to everything you ever open up to work on transistor, tube or otherwise.

Maybe it's just the amps I've worked on, but I always just short them out with a screwdriver as described above. It makes a little sparky spark, but no crazy explosion or anything. I guess it's not really advisable, but it has worked fine for me.

ebeam wrote:It makes a little sparky spark, but no crazy explosion or anything.

You are fukkin lucky. I've had screwdrivers welded to cap screws.

ebeam wrote:I guess it's not really advisable,

Uh... yeah.

ebeam wrote:but it has worked fine for me.

Use a light bulb. Put a 40-60 watt lamp in a pigtail socket.