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My newly aquired BF Champ has the original 2 prong plug. Has a 'ground' hum. Noise goes away when I touch metal on my guitar. I switched the polarity at the plug, but it's still hummin'. I want to change to a 3 prong plug. How/where do I ground the ground wire on the new cord? Do I just buy an extension cord, cut it and use that? Or is there something better? Thanks.
You should be able to find a round cord and new strain relief that will fit without too much trouble. An easy way to ground the green wire is to put a ring terminal on it and 'double nut' it onto one of the power transformer studs. Just make sure you have a good crimp connection or solder joint and the stud, nuts, etc. are clean. Attach it to the stud closest to the power tubes if possible.
Most older Fenders have the fuse on one side of the AC line and the switch on the other. With a 3 prong, it's a really really good idea to rewire them to have both on the hot side of the line(in household AC circuits, black is hot), fuse first, then the switch. If you're replacing the switch, a DPDT can be used to switch both sides of the line - that may make wiring a little eaiser too.
You should either remove the ground switch cap or replace it with a new UL approved AC rated cap made for this purpose. With a 3 prong conversion, the cap does very little and if you plug into a power strip with noise filtering(these have caps on both sides of the line) the ground circuit cap in the amp does nothing at all. If you want to retain it's function or occasionally plug into old circuits, replace the switch with a center-off type and leave it off when plugged into a 'good' circuit.
Be real careful in there - if you don't discharge the filter caps, there's lots of volts just waiting for you on the standby switch terminals, not to mention the tube sockets etc.
Thanks for the great info. A few more Q's. The sockets for my tubes are 'screwed' to the chassis. Can I ground (with ring terminal) under the screw, or double nut on the long chassis screw (close to the power cord)? Could you explain a little more about the fuse rewireing? How to wire to the fuse first than switch? Kinda lost there. My Champ has the 'block' up/down on-off switch on the face panel...the DPDT switch is the 'toggle' type used on the rear? I don't know if I can change the face type switch. OK, safety...discharging the caps...is that unplugging the amp (no standby on my Champ) and let it sit a few minutes? Just wanna be safe, not shocked. I'm gonna give the amp a face lift, Blonde Tolex and wheat grille...the whole amp will be taken apart, I'll have a good chance to rewire and inspect all the guts. Thanks again.
OK got my Champ layout diagram and I see what you mean about the switch, etc.
You can ground the green to a tube socket screw, but a transformer stud is better - should the transformer ever short internally you're not relying on too many mechanical connections. The 'other' way to do it is to solder it to the chasis, but that requires a really big soldering gun or torch and it's hard for some people to get a good connection.
You definately want the fuse on the hot side of the line(black). With a 3 prong conversion if the fuse is on the neutral(white) side and you have a short, you can still have a hot circuit to the green earth connection and the fuse will offer no protection at all. Moving the on/off switch over is a good idea for the same reason. Turning off the switch should kill the hot-AC going into the amp.
The way the Champ is currently wired, there's one power cord wire running to the fuse and another to the switch. Two black wires run from the transformer to the other terminals on the fuse and switch. What you want to do is run the new black power cord wire to the fuse and then run a new wire from the other fuse terminal to the switch. Leave the wire running from the switch to the transformer alone. Connect the other transformer wire that used to go to the fuse to the new white power cord wire. You could use a wire nut, but solder and heat shrink tubing would be much better. You don't have a ground switch, but there should be a cap soldered between one of the fuse terminals and the chasis. You should remove it.
The Champ circuit won't automatically discharge the caps like some larger Fenders do. It uses a 3-section cap mounted right next to the circuit board. Assuming it's stock, there will be 3 terminals sticking thru the chasis between the transformer and the board. Leave it off overnight and then short each of these to these terminals to the chasis with an insulated screwdriver. Some Champs that have been recapped may have a different arrangement as these 3-section caps can be hard to find.
If in doubt about how to do it, take it to a tech. The charge for this type of conversion should be pretty reasonable(and he'll have the right parts). Even though it's only a Champ, there's 300+ volts in there when it's on.
Hi Teleologist. Thank you very much for getting all this technical info for me. Once I have the amp apart, I'll see what I'm up against. I have enough soldering experience to do the job. I'll see when I'm looking at the wireing if it's over my head. 300 volts! Wow.
You can find a nice printable schematic and the layout diagram for an AA764 BF Champ at
http://www.fenderholic.com/
3-prong conversions are also explained in the Amp Mods section, but for larger Fenders with ground switches.
Also check out the Fender Amp Field guide at
http://www.ampwares.com/ffg/
and if you're really into it, follow the Ampage link in the Links section(lots of other good links too). Maybe you can find a diagram of the conversion.
Hey, I know the Ampwares site...I'm ordering my 'Blonde' conversion goodies from Mark. Thanks for the links. I'm suprised Radio Shack told me they no longer carry replacement cords or chassis grommets. A electronic supplier has the stuff I need. If you don't mind, I'll try post a pic of the rewire so you can see if it's done right. Thanks.