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

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

Discovery, by following the wires I figured out there are actually a total of (4) 2N3055 power transistors (not 2) hidden in all the heat sinks.... the blue and yellow wires are soldered to the transistor leads so removing them completely will be a chore. 

Here a pic of the transistor tucked away behind a set of heat sinks...  the picture is actually upside down so I could read the description.

I believe that the transistors are not soldered in but rather plugged into sockets. Once you have taken the screws out I believe that the transistors will unplug. At least I hope this is the case. If the wires are indeed soldered to the pins then you will have no choice except to unsolder them. The reason that they use matched pairs is so the transistors, which all vary slightly from one another, work together and share the load equally. If the transistors are badly mismatched the burden of the work will fall on the weaker of the two devices and it will fail more quickly. Keep in mind that unless there is a short between the Emitter and the Collector these transistors are not likely to be the cause of the problem with the power supply.  I am still trying to figure out what the next step is.

John

Hi Matt;

I think your best bet for continuing will be to see if you can find, locally, an old retired radio TV service technician to look over your shoulder and help trouble shoot the unit. I really can't think of any way to direct you to the next steps remotely. When you finally crack this puzzle please come back and post about it as my curiosity is killing me.

John

The are definitely soldered and it would be a major pain to take it all apart to completely isolate the transistors from the unit in order to test them.

There are also the rectifier diodes, which I didn't know what they were at first because they tested like capacitors while on the board.

I can desolder one end of each diode and test them before I completely remove the transistors

Solder joints on one of the transistors.  Hardly any wiggle room to work with and the back end of the unit would have to be dissembled in order to reconnect everything.

Hi Matt,

As long as there isn't continuity between the yellow and the metal case of the transistor they are probably good and I would not unsolder them. The diodes that you show should be able to be tested in the circuit using your diode test. I would expect about 0.5 Volts on each of them in one polarity only.

John

The rectifier diodes test at .403 V with the black lead on anode side. When I reverse the leads, I get a reading of about .5V and it starts going up to 1V where I'll get a .0L reading (overload)

The 0.403 Volts would be normal for a Schottky Diode. Can you read the numbers on the diode? We can look it up to verify this. The voltage you are seeing on the reverse polarity is likely just seeing the capacitors charging and sounds normal for in circuit measuring.

John

I can make out N5402 with an NB below it. Maybe there's a 1 before the N, but I'm not certain.

Yes that would be a 1N5402 which has a 3 Amp current capacity and can tolerate 200 Volts. The some of the ones in your power supply are probably being used in parallel to accommodate the higher current of the power supply. If you were to take them out of the circuit I would expect the diode test to be closer to 0.55 Volts as these are not Schottky diodes. Your test however has convinced me that they are not the problem.

John

I sent an email to MG Electronics asking for a schematic, but I doubt they will respond as they have been unresponsive so far.

I sent an email to MG Electronics asking for a schematic, but I doubt they will respond as they have been unresponsive so far.

A schematic could make all the difference.

I also got in the analog multimeter like you have.  I still need to test the capacitors....

That is excellent news. The first thing that I want you to do once you get it set up is to put it on the Ohms RX1 scale. Next take your digital meter set to volts and read the voltage across the probes from the analog meter. Note which of the two analog probes your red digital meter probe is on when it indicates a positive voltage. We are determining the polarity of the voltage that is being used by the analog meter. It may not correspond to the colors of the leads. For example on my analog meter the black lead actually has the positive polarity. Let me know if the black or the red lead of the analog meter has the positive voltage so  are testing with the analog meter I can better understand what is going on. Use the analog meter to go back and test the 1N5402 Diodes. You should find that the meter needle will deflect towards zero when the positive lead of the analog meter is on the side of the diode that is away from the marker line and it will not deflect when the probes are reversed. This is the indication of a good diode.

John

I'll be able to get to that on Monday.

I just heard back from the CEO of https://www.professormotor.com/  where I purchased the unit.  They said that MG Electronics had refused to honor returns and warranties on over 100 hundred units that they had purchased.  He has no details on the schematic but said 'most power supplies of this type fail by the output transistor or the rectifiers going bad.'  He will scour the shop and look for potential parts.

Well this didn't take too long.  I have to cross the leads to get positive voltage (black/negative digital MM to red/positive analog MM and red/positive digital mm to black/negative analog MM)

My first test I was the top left diode and it deflected immediately and then went back to zero and stayed there. It repeated tested at zero as did the rest of the diodes.

All diodes tested the same.

That looks good. The leads of the meter have a positive voltage on the Black and a negative voltage on the Red. The meter allows you to apply a 3 volt voltage to any component and the meter tells you whether a current is flowing or not. If you test a large uncharged capacitor and you put the Red lead on the side with the line (negative) and the Black on the positive lead of the capacitor the meter will deflect and then gradually return to zero. This is the effect of the capacitor charging. When it is empty it draws a lot of current and as it fills the current flow lessens, You can also test transistors with the meter . Put the black on the base and move the Red to the collector and emitter. If it is an NPN transistor you will see a nearly equal deflection. Now reverse the leads with the Red on the base and you should see no deflection. There may be a little deflection between the Emitter and Collector depending on the polarity. If the meter deflects all the way this is indication of a short and may indicate a failed transistor. Any full deflection on diodes in both polarities will be a likely bad diode. You can check the smaller capacitors as well. On the smaller capacitors you may need to got to the RX10 or RX100 scales in order to see a small momentary deflection. For testing most components the RX1 scale is the best. If the meter deflects all the way to zero while testing any of the components except an inductor it is a sign of a potential problem. Sometimes it is necessary to take a component out of the circuit but most of the time you can get a pretty good test result right in the circuit.  Do some experimenting and testing of components. Always look at the component in both polarities. Sometimes other components in the circuit will affect your reading in one polarity more than the other. Good luck and ask questions as needed. If I had to go on a service call and I could take only one instrument with me it would be the little analog meter.

John

Everything I've tested looks good, but I haven't gone through and tested procedurally, so must have missed something or I'm testing incorrectly. 

I got in touch with MG Electronics.  They just buy these as black boxes from China and resell - no schematic available. Service guy said he's never opened one up.  He suggested to follow the voltage from the outlet to the front of the unit and vice-versa.   Maybe it could be the front terminals and to test for voltage from the inside, and to test the potentiometer on the output.  Other than that, could be just some small component that went out somewhere down the line...

M

Hi Matt,

I would like to have you test the potentiometer on the front. With the power turned off, use the resistance setting on the digital meter and test the resistance from the two outside terminals. Next put one probe on an outside and middle terminal and slowly turn the control from one end to the other. You should see a gradual change from zero to the reading that you got on the two outside terminals.

Next let's measure voltages on the LM741 integrated circuits. Turn on the unit. Turn the potentiometer of the unit fully CCW. Put your digital meter on volts. Place the black probe on pin 4 Take voltage readings on each of the other pins 1,2,3,5,6,7,8 and write them down. Now turn the potentiometer fully CW and repeat the taking of voltages. Do this for each of the two LM 741 ICs.

Let me know what the voltages are and I will try to interpret them. Remember the numbering of the IC pins as described in a previous post.

John

One more thing to check. There are a lot of brown disk capacitors on the main board. Use the GMT analog meter set on RX1 and test across each of the capacitors when the power is off. You may see some needle movement. We are looking for situation when the needle deflects all the way to zero ohms.

John

Here are the diode tests. Notice I set the R value to X10