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

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 John,

Here is the reading for the LM741 ICs

I tested the UTC D313 NPN Epitaxial Planar Transistor

When the negative is on the base, I get no continuity between the base and either the collector or the emitter

When the positive is on the base, I get no continuity between the base and either the collector or the emitter

When I test the collector and the emitter, I get no continuity, but I hear an electrical sound from the transistor.

I tested the CW7812 Transistor

This is a strange one

When the negative is on the base, I get no continuity between the base and either the collector or the emitter

When the positive is on the base, I get continuity between the base and either the collector or the emitter for 1 SECOND, then no continuity

When I re-test with the negative on the base, I get continuity between the base and either the collector or the emitter for 1 SECOND, then no continuity

and so on....

When I test the collector and the emitter, I get no continuity, sometimes a bleep of continuity, then nothing

When I reverse the polarity of the leads, I get no continuity, then a bleep of continuity, then nothing

and so on....

I tested the 2N3055 NPN power transistors from the internal leads as you suggested

A

When the negative is on the base (blue), I get no continuity between the base and either the collector (red) or the emitter (yellow)

When the positive is on the base, I get continuity between the base and either the collector or the emitter for about 1/2 of second

Then if I reverse the leads I get continuity for about 1/2 a second

When I test the collector and the emitter, I get no continuity, but I hear an electrical sound from the transistor, and my meter starts displaying changing numbers

B

The exact same as A

Here is another angle on the corroded lead on the questionable diode

Thanks again for your wonderful help!

Matt

PS If you don't mind, could you point out in a picture the internal parts that are lethal?  Is this only when the power is on?

Thanks,

Matt

The high voltage danger is from the power wires that come into the unit, go to the fuse, the switch and then to the primary of the transformer. Than should be all.

As I mentioned the 7812 is not a transistor but a voltage regulator. The 12 Volts that you measured on the one LM741 indicates that the 7812 is probably working OK. Go back and review my instructions for testing the 7812 and you should be able to verify that it is OK. You will expect a voltage of about 15 to 18Volts on the left lead and 12 volts on the right one.

The lead of the diode looks OK. The color that you see is just left over flux from the soldering.

The readings on the D313 do not sound normal. I do not know how your continuity tester works for this test compared to my analog meter but the readings you got make me suspicious. Putting the positive lead of the tester on the base should give continuity to the emitter and collector. If you want you can remove the D313 from the circuit and have it tested outside the circuit. If you don't have somewhere locally where you can get it tested you could send it to me. A good substitute for this transistor is a universal replacement from NTE called NTE152NTE152. It is an NPN power Transistor.

John

7812 voltage regulator test results:

w/ Power off and writing facing me

Black lead on the center pin, read lead on the left pin = 1.2mV and it trickled down to 0

Black lead on the center pin, read lead on the right pin = .5mV and it trickled down to a negative number

w/ Power On and writing facing me

Black lead on the center pin, read lead on the left pin = 30V

Black lead on the center pin, read lead on the right pin = 12.04v

I re-tested the D313 for continuity (after the unit has been turned on and off)

There is NO continuity between the base and the emitter or collector

There IS continuity between the emitter and the collector

I retested the corroded diode and it is ok.  The corrosion was giving me a bad reading previously but I scraped it off and the diode shows a voltage reading of .519v in one direction and nothing in the other.

All sounds good and normal except the D313. We need to test that one out of circuit and verify if it is good or bad.

At some point I mistyped Read when I meant to type Red. I mention it so that we don't start a new convention for what is the correct nomenclature.  Sorry about that. I always try to proofread but at 1:30 AM I may miss things.

John

I took out the D313.

There is no continuity between any of the legs in any configuration

I am going to order a replacement,  according to the data sheets, the NTE152 is similar but has different voltage (90v min and max) while the D313 is 60v min and max)

Does that make a difference?

Thanks,

Matt

I ran another test with the D311

With my multimeter set to the diode tester...

With the negative lead on the base, I get no reading touching the emitter or collector

With the positive lead on the base, I get a .578 reading touching the middle pin

With the positive lead on the base, I get a .580 reading touching the right pin

I ran another test with the 2N3055 NPN power transistors

With my multimeter set to the diode tester...

When either the negative or positive is on the base (blue), I get a reading that starts out .03 and rises to 1V between the base and both the collector (red) or the emitter (yellow)

When either the negative or positive is on the collector, I get a reading of about .010 V between the emitter