To remove the PC Board Assembly from the AD50, first remove three jack nuts (1 red and 2 black) on the rear panel.

Also, pull off the Power Level control knob, then remove the control nut and washer. 

Next, remove the two black screws, shown here, that attach the heat sink to the bottom of the chassis

Then, remove the four silver screws, shown here, that attach the circuit board assembly to the chassis.

Two ties hold a ribbon cable to the fan.

Cut the nylon ties and unfold the wires, so they're long enough for the PC board assembly to slide out of the chassis.

Remember to install new cable ties when you reassemble the amp.

There's enough slack in the cables for you to flip over the assembly, as shown here.

Use care moving the circuit board assembly.  Five small integrated circuit (IC) pins are all that hold the heat sink to the circuit board.

Below, is the schematic diagram of the AD50's output.  R48, circled in red, is the power resistor that broke loose from its solder points.

The resistor (right) limits the IC's output current in case of a short circuit in the speaker wiring.

The 0.47Ω 5W resistor is in series with the speaker and can get quite hot when the amp is working.

The resistor stands off from the circuit board to keep the board from burning.  Surrounded by air, the resistor can dissipate the generated heat.

The body of the resistor is prone to swaying, however, making broken solder points a potential problem in Vox AD50 Valvetronix amps.

To strengthen the solder connection, scrape additional resist from the printed circuit trace, exposing more of the copper.

With an increased soldering surface, the connection is stronger than new.