Need to get MG Electronics Power Supply repaired...or NOT!?

Thank you John for the detailed instructions.

I will try to attach pictures that I took today.  I think I have a bad diode to start off with. (a camera is a good inspection tool as it turns out)

I unplugged the unit and performed the diode tests, the LM741 IC tests, and wrote down the measurements in the pictures below.

I shorted the capacitors and tried to test them but the readings were all over the place, I'm not sure what was going on?

I haven't tested the transistors yet as the power transistors? in the back of the unit are difficult to get to, unless I could test them from inside the unit?

Thanks for your help,

Matt

Hi Matt,

Nice pictures and helpful. The diode that you point out may be a concern. I could not tell from the picture the actual condition of the lead. It should probably be replaced in any case. Make certain you get the number off it and replace it with an equivalent. Just because they look alike on the outside there are many different types that have different purposes.

In order to get a voltage reading on the LM741 you will have to have the unit plugged in and turned on. Be careful not to let your probes slip and short anything together.

Below is a picture of the analog meter that I would take with me if I was going to check out your components on the board. Analog meters work better than digital for this purpose.

The line on the diode is to the right. As you can see the picture on the left has the red lead to the left  and there is no reading on the meter. The picture on the right the red lead is on the right side of the diode and this time the meter deflects. Incidentally I have the meter set on RX1 scale for the test. This indicates a good diode. When the polarity of the test leads are one way we see some continuity and when hooked up reverse polarity we see no continuity. Transistors can be tested in much the same way. To a meter like this a transistor looks like two diodes that are connected at the Base lead. Now the base lead can be any of the three. You can google a data sheet for the transistor that you are going to test and it will tell you which lead is the Base.

A standard transistor will have one of these two configurations for testing with the meter. Put one lead of the meter on the Base and then you should have either continuity or no continuity to each of the others. Now put the other lead on the Base and you should now see the opposite of what you saw in the first test. Finally test both polarities of the leads that are not the Base. You may see a slight movement of the meter in one polarity. If the meter swings all the way down to zero on the scale you have a shorted transistor.

You can perform this test on the output transistors without taking them out of the unit. From your pictures I can tell that the blue wires go to the Bases of the transistors and in case you want to know the red wires go to the Collectors and the Yellow wires go to the Emitters. One these transistors you should pay close attention to the polarity test between Emitter (Yellow) and Collector (Red) and these transistors typically fail shorted between Emitter and Collector with means very low resistance.

You may need to get yourself a small Analog meter if you are going to make these tests. I am not sure it can be done with your digital meter as their ohms measurements are sometimes made with voltages that are too low to work for these tests.

If you do get an analog meter I will show you how to tests capacitors with it too.

John

Hi Matt,

Nice pictures and helpful. The diode that you point out may be a concern. I could not tell from the picture the actual condition of the lead. It should probably be replaced in any case. Make certain you get the number off it and replace it with an equivalent. Just because they look alike on the outside there are many different types that have different purposes.

In order to get a voltage reading on the LM741 you will have to have the unit plugged in and turned on. Be careful not to let your probes slip and short anything together.

Below is a picture of the analog meter that I would take with me if I was going to check out your components on the board. Analog meters work better than digital for this purpose.

The line on the diode is to the right. As you can see the picture on the left has the red lead to the left  and there is no reading on the meter. The picture on the right the red lead is on the right side of the diode and this time the meter deflects. Incidentally I have the meter set on RX1 scale for the test. This indicates a good diode. When the polarity of the test leads are one way we see some continuity and when hooked up reverse polarity we see no continuity. Transistors can be tested in much the same way. To a meter like this a transistor looks like two diodes that are connected at the Base lead. Now the base lead can be any of the three. You can google a data sheet for the transistor that you are going to test and it will tell you which lead is the Base.

A standard transistor will have one of these two configurations for testing with the meter. Put one lead of the meter on the Base and then you should have either continuity or no continuity to each of the others. Now put the other lead on the Base and you should now see the opposite of what you saw in the first test. Finally test both polarities of the leads that are not the Base. You may see a slight movement of the meter in one polarity. If the meter swings all the way down to zero on the scale you have a shorted transistor.

You can perform this test on the output transistors without taking them out of the unit. From your pictures I can tell that the blue wires go to the Bases of the transistors and in case you want to know the red wires go to the Collectors and the Yellow wires go to the Emitters. One these transistors you should pay close attention to the polarity test between Emitter (Yellow) and Collector (Red) and these transistors typically fail shorted between Emitter and Collector with means very low resistance.

You may need to get yourself a small Analog meter if you are going to make these tests. I am not sure it can be done with your digital meter as their ohms measurements are sometimes made with voltages that are too low to work for these tests.

If you do get an analog meter I will show you how to tests capacitors with it too.

John