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Drain Power Caps
http://studentweb.eku.edu/justin_holton/caps.html
Why Drain the Caps?
It is absolutely vital that we drain the filter caps in the DC power supply before working on ANY guitar amplifier. These often hold a charge of several hundred volts, which is potentially deadly. Caps can hold their charge for weeks or longer, even with the power off AND the amp unplugged. It's very important that we learn to properly "drain" this residual voltage so that we may safely work on the inside of our amplifier.
1. What We Need
1. Safety goggles.
2. (1) digital multimeter capable of reading 500 volts DC. Don't have one? The best deal I've found was at http://www.tubesandmore.com, search for part S-Z3220. Great bang-for-buck. The Extech at Radioshack is good, though Flukes are the best general purpose.
3. (1) Xicon 25K 10W Cement Power Resistor for draining residual voltage.
4. (1) pair of pliers with an insulated handle for holding the resistor
5. (1) philips screwdriver to remove the back panel.
2. If you're not addressing a problem that requires the amp to be on, ALWAYS unplug it from the wall. Unplugging from the wall will not drain the filter caps, it actually won't make a bit of difference, but again this is for our own safety. We unplug to keep the amp from accidently turning on while our hand is inside the amp. Don't assume anything's impossible, as that's how people die. Unplugging is the equivalent to "locking out" before working on a downed machine.
Turn the power switch OFF, and place the standby switch in the ON position. Note: This isn't the same as having your amp "on standby." (see picture for clarity) We're hoping the filter caps will drain down some with the standby switch closed.
3. Place the amp on a sturdy table in a well lit room. Be sure to wear rubber soled shoes, and if you have a properly insulated floor mat (made of rubber) standing on it can help prevent the likelihood of electrocution. Remove the back panel of the amp. Once it's gone look around the inside of the metal enclosure, but don't touch anything. (a) See those big grey things with 500V printed on them? (500V and 350V in the Deville) These are the filter caps, and what we need to watch out for. NOTE: There are more filter caps than what are pictured.
Because filter caps are almost always of the electrolytic type, they're "polarized" and therefore have a positive and negative end. (b) We can identify the positive or negative ends by looking for the "polarity indicator" printed on the cap's side. Assuming Fender is still using the same Illinois Capacitors, the line will be black and it'll have arrows pointing toward the negative side. On the cap the positive side will not be marked, though you'll find a "+" printed on the PCB indicating the positive side.
For filter caps the negative side is wired to ground, which is the amp's metal enclosure or "chassis." Caps will only drain when their leads are connected with a resistor or shorted together with anything that conducts electricity—including human flesh. This means touching the chassis with one hand, and touching anywhere that's wired to the cap's positive plate with your other hand, will make your body a path for the caps to drain. Immediately it will discharge several hundred volts which will pass through your heart. If you live then consider yourself lucky. The ground you're standing on may be at the same electrical "potential" as the chassis—especially if you're in the basement or outside. So you could still be electrocuted even if only one hand is working inside the amp! Remember: what makes working on power supplies so dangerous is that you don't even have to come close to the filter caps to be killed.
4. You should have on your safety goggles by now. Inspect your multimeter's black and red probes for frayed or broken insulation (the part you hold). If you find that the insulation is in questionable shape DO NOT use them. Throw them away and go to the hardware store and buy a new pair.
Our first objective is to check for residual voltage with a multimeter. In some cases most of the voltage may already been drained, but NEVER EVER assume the caps have been drained to a safe level without checking with a reliable multimeter. There are many ways to measure the residual voltage contained in the filter caps, but below are the most common. Note that these are also equally valid for draining the caps.
1. Across the cap. (pictured) This is my preferred method, but only if the filter caps are easily accessed.
2. From the positive side of the cap to the chassis. - This is essentially the same as draining "across" the caps since the negative side is at the same electrical potential as the chassis.
3. From the chassis to a pin. This can be done at pin 3 of any power tube, or pins 1 or 6 of any preamp tube. Sometimes, usually in larger vintage amps, the filter caps are located inside a small pan—often called the "dog house." This is fastened to the chassis, and access is normally limited. Often several clothed wires protrude from a hole in the pan, and these are connected to the positive side of the filter caps, or the negative side of the bias cap (if included). The negative leads are usually securely soldered through small holes in the pan. The dog house will be mounted on the same side as the transformers and choke. This method is often used for draining the caps when immediate access is not safely available, or if a proper resistor can not be found. By shorting the chassis to the correct pin the caps will be forced to drain through the plate load of that tube. This prevents sparking from high current. Once your meter indicates that the caps are drained you may unscrew the dog house and check each cap individually if needed.
Set your multimeter to read DC voltage. Adjust the meter's range so that it's above 500 volts. Using only ONE hand, with the other hand far from the chassis (preferably in a pocket), measure the DC voltage across one of the big grey caps. Working with C36, which is behind the input jacks, seems to be the safest choice. You can adjust the meter's range down if you can not get a clear reading. What number did you get? Any DC voltage reading that's less than 10 volts is harmless, so the amp can be safely worked on. If not we must drain off the residual voltage with a resistor until it is safe.
5.
In old electronics books this is usually where they tell you to use a screw driver with an insulated handle to short out the caps. "Shorting" with a screwdriver will almost instantly drain the caps to zero volts, causing sparks from high current. This is acceptable if you know the handle is well insulated (think rubber), you're wearing eye protection, and you don't care about leaving marks on the chassis. If there's several hundred volts stored in the caps be prepared for a nice little spark. It'll most likely leave a black mark on the chassis, and in extreme cases partially weld the screwdriver to the chassis. When draining an industrial power supply, which may use very high capacitance and voltage, never use a screwdriver as the charge can blow a chunk out of it and make it into a projectile. Always use safety goggles, even if you think you look nerdy with them on. Trust me, no one cares.
If you find that you have several hundred volts in your caps you must be extra careful when draining them. Take your power resistor and bend the leads so that you can easily touch a filter cap's leads. Once again, C36 seems to be the safest choice. Use a pair of pliers to hold the power resistor for safety. Always hold the actual body of the resistor and NEVER the metal leads protruding from it (see picture). With your free in your pocket, choose a filter cap and touch the capacitor's leads with the resistor's leads (see drawing for clarity). Hold it there for 20 to 25 seconds.
Filter caps are wired in parallel, so draining one should drain them all, but never assume. Always recheck for residual voltage in EVERY filter cap until there's absolutely no doubt in your mind that all are drained. Anything less than 10 volts is safe, but I prefer to drain mine to 1 volt for peace of mind. (I've been hit with 420VDC before!) Don't be surprised if you find the caps slowly recharging themselves afterward. This phenomena is known as dielectric absorption and is quite normal in electrolytic caps. Rest easy, for the filter caps will not recharge to a lethal level. Once you're certain the amp is safe, I'd like to wish you good luck on your mod or repair!
By Justin Holton
