power chord connected backwards

[svt4cobra6]

Just got a broken amp from a friend fender roc pro 700, solid state with one 12ax7 tube. Anyhow he said he replaced the switch and it stopped working. Looks like the power wires from the power chord were reversed, corrected this and powered it up, now there is a loud hum and after a few seconds a large resistor close to the output speaker wires starts smoking. Also looks like one large cap is slightly bulged, in addition the board is brown around two large diodes, any ideas, thanks.

[R.G.]

Sure.

1. Get the schematic.
2. Compare the actual wiring to the schematic.
3. Make the amp match the schematic.
4. Get/make a light bulb limiter.
5. Use the limiter to keep things from going up in smoke while you figure out what's wrong.
This last will entail
1. Get the power supply right and stable.
2. From there figure out what else was hosed up when the power cord was miswired.

[Raybob]

Once you go over R.G.'s steps, then with a 3 prong power wire and the green wire to chassis, it shouldn't really matter if you got the black and white backwards.

[svt4cobra6]

thanks for the responses, yep I was thinking since it is AC it may not matter having the black and white wire reversed. I got the schematic and have a good multimeter. In looking at the schematic the components close to the power resister that smokes are the main power transistor, some diodes that are in parrellel to the transistor and a few caps. Also one of the main power supply caps looks like it is buldged, will look at getting the power supply correct as suggested.

[R.G.]

Be a little careful there. Reversing black (line/hot) and white (neutral/cold) won't make a difference with powering the transformer and getting it to run as long as all is correct. But there are reasons why white and black are different. You want to be sure that before you finish repairs that the power switch opens the hot line (if the power switch doesn't open both hot and cold) and that the black line is fused, not just the white/neutral/cold line. If the fuse is in the neutral, and even worse if the neutral is switched but not the black/hot/line, a blown fuse or open power switch can leave the chassis hot to the AC power line and be an electrocution hazard. It's only on the two-wire-no-ground power cords that it doesn't matter. And those have their own electrocution hazards.

Do NOT be casual about how AC power is hooked up. It can be literally a matter of life and death. And lawsuits decades later.

[svt4cobra6]

will double check the schematic as far as the connections to the power switch, also will make sure the hot wire has the fuse as suggested. It did look like the two wires reversed black and white to the switch. Good suggestion as far as safety is concerned.

[Raybob]

Be a little careful there. Reversing black (line/hot) and white (neutral/cold) won't make a difference with powering the transformer and getting it to run as long as all is correct. But there are reasons why white and black are different. You want to be sure that before you finish repairs that the power switch opens the hot line (if the power switch doesn't open both hot and cold) and that the black line is fused, not just the white/neutral/cold line. If the fuse is in the neutral, and even worse if the neutral is switched but not the black/hot/line, a blown fuse or open power switch can leave the chassis hot to the AC power line and be an electrocution hazard.

With all due respect, the only reason there is a difference is if you have a 2 pronged power cord with no ground wire. When the ground wire of a 3 prong plug is directly connected to the chassis, there is not a chance of a hot line from the AC being connected to it.

It's only on the two-wire-no-ground power cords that it doesn't matter. And those have their own electrocution hazards. ..

Actually, if the white and black are reversed on a 2 prong power cord, there is lots of hum and if you touch your guitar when you kiss a grounded microphone, your lips see 120v sparkle.