Simple repair of a power supply based on the UC2845

I'll say right away that I hadn't originally planned to write this article, so it turned out a bit rushed and spontaneous, but it may still be useful.

So, here's a power supply in the familiar case many of you will recognize.


You can see that the PSU has clear signs of use. It's been through a lot over the years :)


Warning: dangerous voltage may remain inside the power supply for some time after it has been unplugged. Before touching any live parts, it's best to wait about 5 minutes.

Let's take the PSU apart. Since these photos were taken after the repair, I'll mention that the inside was quite dirty; it's already been cleaned in the photos.
It's best to start any repair by cleaning, then always inspect the components and circuit board for obvious damage.


Next, unscrew the power transistors and the output diode assemblies.


This PSU has a thermal fuse fitted into the mounting bracket for the output diode assembly—a very useful feature.


After that, unscrew the screws holding the circuit board in the case. There are usually four, but sometimes there's a fifth, located near the center of the board.


The power supply is built around the fairly well-known UC2845 PWM controller, link to the datasheet.
Cheap power supplies of this wattage are usually built around a TL494, but this manufacturer chose a slightly different approach.


The IC comes in two package variants. Ours is in a DIP-8 package, so the pin number isn't shown in parentheses.

First, let's check the IC's supply voltage.
Typical faults in PSUs built around this IC include:
1. The IC's supply capacitor has dried out
2. The high-voltage transistor has failed and taken the IC out with it
3. The resistor that provides the IC's initial startup has burned out
4. The resistor supplying the IC during normal operation has burned out. It's usually in series with a diode connected to the transformer's auxiliary winding.

The input filter capacitors are visible on the right, and the resistors supplying the IC are to their left.
In my case, the IC had only 2.5 V on it.


Let's check the datasheet: this IC's startup voltage is about 8.4 V, so it won't start.
This can happen if the IC itself has failed, or if one of the components connected to it has failed.


The first simple test is to connect a resistor of about 150 kΩ in parallel with the existing IC startup resistor.
If the original resistor is faulty, the PSU will start up. If not, we'll see how the supply voltage has changed.


In my case, nothing changed. The voltage rose slightly, to 2.8 V, and that was it.
The typical circuit for this IC is very simple: a resistor from the 310 V supply, then, once the PSU starts, power from an auxiliary transformer winding.
In our case, the resistor is fine, but the supply voltage is too low.
But this circuit is missing one more component: a protective zener diode on the IC's supply rail. Sometimes it fails short, but in this case there's no short on that rail.


All right, let's power this circuit from an external power supply. Warning: do this only with the PSU under test disconnected from the mains!!!
With the normal 12–15 V applied, everything looks fine: there's no short, and the IC's built-in 5 V regulator is producing the required voltage.


Switch off the power, lower the voltage to 5 V, and apply it again. That's when we notice a small detail: the current draw is about 8 mA. That's not right, since the startup resistors can supply only 2 mA, so the voltage can't rise to the required level.


The zener diode was the first suspect. And I was right: it isn't shorted, but it has a high leakage current.
The zener diode is rated for 16 V, but at 5 V the current is already 9 mA, rising to about 10 mA at the startup voltage.


This little rascal.
Ideally, replace the Zener diode with one of the same voltage, but I only had 15 V ones at home, and that’s an acceptable substitute too.
As for the power rating, it’s better to use a higher-rated one. They’re usually available in 0.5 W (shown in the photo) and 1.3 W (larger) versions.


Replace the Zener diode and power up the PSU—everything works perfectly. It’s best to test it through an incandescent bulb.
For low-power PSUs (5–50 W), use a 15–25 W bulb; for more powerful ones, use 40–100 W, or sometimes 150 W.
When switched on, the bulb should flash and then go out; that means the input capacitors have charged. If it stays lit, there’s a fault.
Do not connect a load to the power supply in this setup.


That’s it. Put the PSU back together. The Zener diode costs around 5–10 cents; the rest is usually what you pay for the technician knowing which Zener to replace :)



That’s all. If you have any questions, write to me. Next time, I’ll try to explain things in more detail and in a more orderly way.