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Trouble shooting your power supply part 1

First off, I am writing this as if the person interested in fixing their own power supply, know's enough to be afraid of electricity and respect it. You are on your own and I will not be responsible for any accidents due to carelessness and lack of safety.

Unplug the unit. Take the cover off the unit. Do not touch the inner parts of the unit because there could be lethal voltage there. Find at least a 1k ohm resistor valued at least 5 watts and connect it across the filter capacitors to bleed them out. Remove the resistor when there is no trace of voltage on the capacitors any longer.

Let us just say that you had full secondary voltage at the output terminals. We will refer to this as an over volt situation.

There are several things that can cause this. The most common fault is an emitter to collector short circuit. This situation causes a bypass of the voltage regulator circuit.

So we have to pull the pass transistors off one at a time, being careful not to bend the pins and to watch where the placement of the pins are in relation to the top or bottom of the TO-3 transistor and socket. Also keep an eye on the placement of the thin mica insulation material. it is easy to break it. If you do, your local electronics store will have them.

If you are lucky enough to have a power supply that has been built with transistor sockets, it will be easy to take them off one at a time to test them. If the wires are soldered directly to the emitter and base pins of the transistor, we have to take care as not to heat up the solder connection too much. It doesn't take a lot of heat, if we have the proper equipment.

Don't worry about the white sticky substance. It is a heat conducting silicone material. When re-assembling the transistors to the heat sink, you might want to have some of the silicone grease to put on the surface of the transistor and some on the heat sink where the transistor sets.

Some of the larger units have the same type of transistor as a driver from the regulator circuit to the pass transistors. It will also need to be tested.

I like to use an old D'varsnal type meter. You know. The one with the old meter movement in it. I just never took the time to get comfortable testing with a digital meter. I use a digital for checking voltages, of course.

Once you get the transistor off of the heat sink, set it on the work bench with the pins facing up. Position it so that the pins are nearer to one end of the transistor. We will refer to this as the top of the transistor.

The following test is done for the NPN type transistors, which most of the commercial manufacturers use.

Ok, the pin on the left, with the pins facing up and toward the top end, away from you, is the base. The pin on the right, is the emitter. The case of the pass transistor is the collector. Using your meter, put the black lead on the base pin and the red lead on the emitter.

The meter should give you about a half scale reading on the 1k scale, plus or minus. I cannot give you an exact reading because it seems that none of the transistors will read exactly the same. This means that the transistor is fine. If there is no reading at all, the transistor is junk and it is open. If the meter slams against the peg with movement, the transistor is shorted. Either one of the later situations, pitch the transistor in the trash.

Next, leave the black lead on the pin on the left, the base, and drop the red lead down onto the case of the transistor, which is the collector. Once again the readings of the meter should be the same as the readings in the previous paragraph.

Testing with a (Digital) DMM:**

NOTE: Analog and Digital meters behave quite differently when testing nonlinear devices like transistors or diodes. Most digital meters show infinite resistance for all 6 combinations of junction measurements since their effective resistance test voltage is less than a junction diode drop (if you accidentally get your skin involved it will show something between 200K and 2M Ohms). The best way to test transistors with a DMM is to make use of the "diode test" function described below. For both methods (analog or digital DMM), if you read a short circuit (0 Ohms or voltage drop of 0) or the transistor fails any of the readings, it is bad and must be replaced. This test discussion is for OUT OF CIRCUIT transistors *ONLY*.

Set your meter to the diode test. Connect the red meter lead to the base of the transistor. Connect the black meter lead to the emitter. A good NPN transistor will read a JUNCTION DROP voltage of between .45v and .9v. A good PNP transistor will read OPEN. Leave the red meter lead on the base and move the black lead to the collector. The reading should be the same as the previous test. Reverse the meter leads in your hands and repeat the test. This time, connect the black meter lead to the base of the transistor. Connect the red meter lead to the emitter. A good PNP transistor will read a JUNCTION DROP voltage of between .45v and .9v. A good NPN transistor will read OPEN. Leave the black meter lead on the base and move the red lead to the collector. The reading should be the same as the previous test. Place one meter lead on the collector, the other on the emitter. The meter should read OPEN. Reverse your meter leads. The meter should read OPEN. This is the same for both NPN and PNP transistors. As noted, some transistors will have built in diodes or resistors which can confuse these readings.
** Thanks to Gregory Beat, W9GB

In the next writing, we will go on from here as we are not done with the testing.

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