Bleeding capacitors

[vortexunion]

I recently purchased a 30 yr old Univox 1226 head. It's got a nice clean sound and is exceptionally loud. However, I'm noticing some hum and think it might be time to change the filter caps. I know these can store high voltage and want to avoid a burnt finger.

So does anyone know of the best way to bleed the caps before messing with them? And do I need to bleed all of them or just the ones I'm going to replace?

[earth tones]

Check out this site, amongst many others that are out there. Supposively, this is a very important step to take prior to probing around electronics, especially amps. Before I resoldered a filter cap on my amp I simply used a digital multimeter to measure the caps, as is explained in the article. Since the caps had little charge, no draining was needed. Hopefully, you will save time there as well.

http://studentweb.eku.edu/justin_holton/caps.html

[theBlubberRanch]

And I thought I was the only one that did this.

[meblumen]

I was totally expecting to open the thread and see someone say BFS (big fucking screwdriver)

Anyway, just take some insulated aligator clips, a resistor (something like a 5watt 100ohm resistor should do the trick), clip one end of the aligator clips to the chasis and then the other end to the resistor. Make sure they are properly insulated and then carefully touch the resistor to the filter caps, holding it against the + lead for a few seconds and it should be discharged. If you have a DMM which I recommend if your working on amps, you can then double check to make sure the cap has been drained. Good Luck and be safe.

[soundguy]

youre not avoiding a burnt finger, you are avoiding death. you might be better off to take your critical advice on a subject like this from someone that can SHOW YOU IN PERSON just to avoid any misunderstanding on your part from something you read on the internet, which you know, is always true in the first place.

Thats my advice, feel free to disregard it.

dave

[psychicoctopus]

how effective is it to just unplug the unit and leave the power switches on?

[Scodiddly]

how effective is it to just unplug the unit and leave the power switches on?

Depends on the circuit design, but as a rule I wouldn't expect it to be safe. Good shop technique includes not making risky assumptions. For me it's second nature to double check the amp is unplugged before touching the insides, even if I'd just stepped away from the bench for a moment.

[thearnicasync]

Many designs already incorporate a bleeder resistor in the power supply. Your amp being 30 years old, it probably doesn't!

The alligator clip method above sounds right to me, the idiot, but you may want to puts somthing insulated (like electrician's pliers) between your hand and the alligator cap, if you actually decide to try to do this yourself.

Also, do check with a voltmeter after bleeding to make sure it's gone. I've done this the same way twice (or so I thought) and one time it worked and for some reason the other time it didn't.

Make sure to read as much as you can about this. Don't get into a hurry, or get pissed, or impatient and destroy yourself.

kb

[hulahalau]

Let's be a lttle safer:

1. Use a resistor with a higher resistance value, on the order of 10K. it will take longer to discharge, but then again, the reistor won't burn up. 100 ohms is pretty low and you may get a surge and a large burst of heat 9and burnt fingers, not to mention adverse effects on nearby components).

2. Wear safety glasses, just in case you short a capacitor and it explodes.

3. Decoupling resistors or coils will slow the dicharge of capacitors ina CRC or CLC configuration. So do the safe thing and discharge ALL resistors.

4. Capacitors, especially big electrolytics, are subject to dielectric absorbtion (DA), where the capacitor film does not release all stored electrons at once. Other than bad sonic effects, DA also contributes to the unpleasant effect of an apparent "recharging" of the capacitor after it has been discharged momentarily. You discharge the cap, remove the capacitor, and a minute later touch the terminals with your hand and get a big shock!!!!! So, to be safe, keep the discharge resistor on for 5 minutes.

[Milkmansound]

hmm - just use a screwdriver and short the leads to the chassis one at a time. Should be a loud snap and some sparks. That'll get it done. I have done this to thousands of capacitors and TV tubes with no problem, and its a fun thing to show someone. "Hey, check this out!" SNAP!

[meblumen]

hmm - just use a screwdriver and short the leads to the chassis one at a time. Should be a loud snap and some sparks. That'll get it done. I have done this to thousands of capacitors and TV tubes with no problem, and its a fun thing to show someone. "Hey, check this out!" SNAP!

YESSSSSSS!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
I was waiting for someone to post this, didn't you see my comment above?!? Anyway, the screwdriver works, looks cool as hell but when the tip blows up and all the little metal bits shoot up at you or in your amp you won't be a happy camper. What Dave said about getting somebody to show you how to do all this stuff in person is huge. A while back I had no clue how to work on tube amps and was pretty much scared shitless to pop the hood and tinker around. However, I got someone to show me the ropes and while I'm no tech, his advice on safety combined with my electronics knowledge has really benefited me. I think even if you have to pay to take it to a tech to show you a few things it will benefit you in the long run and maybe save your life.

[Milkmansound]

nah - stand on the floor with no shoes on with a ground wire tied to your ankles and discharge it with your tounge.

Just kidding - DO NOT DO THAT!!!

Well, unless you are suicidal.

Did anyone happen to see the mythbusters where they put a dummy in the bath tub and throw appliances in with it and measure the amperage across the fake heart? Tube amps will kill you. It does not take a lot of current through the old ticker to stop it and end your life.

As for the screwdriver method - just make sure to use a nice large flathead with a super clean tip. You can even solder an allogator clip to it and clip the lead onto the chassis of the amp. That is how I discharge CRT's - I have a special tool that my former military tech friend called a "Jesus Stick" Cause if you do not use one you get to see Jesus. I'm a Jew and I still have one of these - its essential for working on electronics.

[vortexunion]

Thanks for the suggestions. Does anyone know of a reputable tech in the DC/Baltimore area who's willing to show an electrical novice? Reading Geoff Farina's articles on tubes & caps as well as Aspen Pittman's Tube Book have peaked my curiosity so now I've got to do it. Actually, one of the reasons I bought the Univox is because I don't want to use my Marshall as a project.