Repairing a Cenco 33031 Power Supply
by Laurence Rowe and Eric Myers
We use the Cenco 33031 Low Voltage Power Supply (Figure 1) for several of our electricity and magnetism labs. It can provide variable DC up to 6 Volts at up to 5 Amps, and it can provide a steady AC voltage at around 22 VAC.
Unfortunately, our students sometimes exceed the 5 Amp DC limit and damage the supply, rendering it useless. The problem is almost always the main power transistor. We have even tried adding a 5A fuse in the DC output circuit, but the power transistor blows out much more quickly than a fuse. We are currently testing the use of 4A fuses in the hope that they blow before the transistor, but in the case of a short circuit this small difference will probably not matter; the power transistor will blow first.
Presented below are step-by-step instructions for replacing the power transistor. Another component that might need to be replaced is the voltage regulator, so further below we also include instructions for that. But when the DC fails, the first thing to try is replacing the power transistor.
Before you get started, you will need the replacement part(s) and some tools:
- Phillips screw driver
- Soldering iron
- 2N3055 power transistor (Figure 2)
- Thermal paste
- Replacement mica heat shield for 2N3055
- LM317T voltage regulator (optional)
While it is also optional, it can be helpful to use a plastic compartment box to hold the screws and other parts during disassembly. If you don’t have a plastic box, an egg carton works just as well.
Replacing the Power Transistor
- Remove the 8 small case screws (see Figure3), gently lift the cover, and slide it to the right.
- Remove the 2 screws holding the heat sink to the bottom of the case (shown in Figure 4). Be careful not to bend or break the leads on the voltage regulator, which is the black chip with 3 leads which is attached to the heat sink. Those leads can break off easily.
- Start with the screw at the far “plain” end of the transistor – the one without a wire attached(see Figure 5). Remove the nut, washer, lock washer, and screw.