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

Reconfirm Lower RIght LM741 IC  with Pot at full CW between Pin 4 and Pin 3

10.16V

I believe all the transistors are wired the same, except the ones on the left side have two yellow wires, the transistors on the right have one yellow wire.

The wires get lost in a bundle, I can't follow them.  It seems they end up at the board and potentiometer.

But each transistor yellow terminal has continuity to all other yellow terminals, each red terminal has continuity to all other reds, each blue terminal has continuity to all other blues.

How should I test the transistors with the analog multimeter?

John,

I have been following your steps in helping to solve this issue with great interest.  I wish that I could be of more help, but my days of being a technician and doing any real heavy device debugging are sadly for behind me.  I tend to fix broken equipment by buying a replacement.  To that end, my search skills have have found a couple of replacement power supplies (same brand - one for 99.00 and free shipping).  Let me know if you need any details.

I am (and always have been) in absolute awe of people who can fix broken gear and save it from the trash.  I seem to have a phobic thing that prevents me from opening up anything.  With a schematic and plenty of time, I have a fairly good chance of figuring it out.  But my running 'blind' skills are severely lacking.

Best of luck guys!  I will continue to follow along, and who knows, maybe I will come up with some way to help.

Hi Matt,

I tried following the thread and it's hard to follow, but it got me wondering, does slot car racing need such a supply?

It seems overkill, and I can see that it could fail through accidental misuse, or even damage the slot-cars if set accidentally to too high a voltage.

Probably a few amps is enough, if it is a 'scalextric' style slot car racing system.

HI Shabaz,

My problem isn't my purpose for having the supply.  My problem is that it doesn't work and I'd like to fix it.  I do not do slot car racing.

Thank you for the thought, though.

Matt

Hi Gene,

I'm just trying to fix this one for fun, I realize I can buy other power supplies.  I also wouldn't purchase another MG Electronics power supply out of principle now, they have neglected to return emails, neglected to provide contact information on their website, and flat out refused to honor warranties to their distributers.

Matt

John,

As you well know this isn't my area of expertise.  Too bad there is no schematic.  Some thoughts on things I didn't see but may have missed:

• have all of the electrolytic caps been checked
• can the transformer be isolated and checked
• if the caps, diodes, transistors, assorted ICs, and transformer are good then is it worthwhile to go through the circuit with an iron and touch up all the joints to make sure all the connections are good

Frank

Thanks for the input Frank, Shabaz, and Gene. I will keep your suggestions in mind.

John

Hi Matt,

Let's Continue:

Test the output transistors with the Analog meter by choosing one. Since I believe they are all in parallel we will look at all of them at the same time. With the unit unplugged and turned off. Put a meter probe on base of transistor I believe it is the yellow wire and test to the other two. Note meter readings. Switch probes and test in the other direction. Now leave out the base (Yellow) lead that was common in the first test and test both polarities between the other two ( Emitter and Collector).

Next test. Still using the Analog meter with the unit off. Test to see if there is connection between the negative of the big capacitors and the black (negative) banana plug jack on the front of the unit. Test to see if there is a connection between the negative of the large capacitor and the #4 pin of each of the LM741 ICs.

Now with the unit still off test to see if there is a connection between the Blue wires of the power transistors and the RED positive Banana JAck on the front of the unit. If not is there a connection between the yellow wires and the Red Banana Jack?

With the unit turned on and using your digital meter set on DC volts measure the voltage between the Black binding post on the front of the unit and the Collectors of the output transistors (Orange? RED? Wire I can tell the color clearly) Try turning the range selector on the front. We should see the same voltages that we saw when we tested the large capacitors.

With the unit turned on and the black lead of your digital meter still on the black banana jack check the voltages on the yellow and blue wires to the power transistors.

Let me know what you find.

John

You are getting some really good practice in using your meters.

Hi Matt,

I just wanted to let you know that I feel that we are making real progress. I have a preliminary concept of how they are making this circuit work. We hopefully are only a couple steps away from zeroing in on the source of the problem. You are doing an excellent job of interpreting my measurement requests. I understand the #3 pin reading on the lower Right LM741 and I believe we are good on the circuit through this 741.

John

If I'm measuring Resistance, then it's 15 Ohms between Yellow (base) wire on different transistors in both polarities (at X1)

Between Red to Reds it's also 15 Ohms

Between Blue to Blues it's also 15 Ohms

Between Red to Blues there is no reading

Affirmative continuity between negative on both big capacitors and negative banana plug

The continuity between the big capacitors and the LM741 is screwy.  It seems to change...

Negative continuity between negative on both big capacitors and #4 pin on each of the LM741 ICs, except that there was continuity for a second

Positive continuity between negative on both big capacitors and #2 pin on each of the LM741 ICs, and then there was no continuity

Intermittent continuity between the positive on both capacitors and some of the pins of each of the LM741 ICs

Positive continuity between the top row of rectifier diodes on the non-line side and the #2 pin on each of the LM741 ICs, then negative continuity

Negative continuity between the bottom row of rectifier diodes on either side of diode and the #2 pin on each of the LM741 ICs

Positive continuity between each of the blue wires on the power transistors and the Red Banana Jack

Negative continuity between each of the yellow wires on the power transistors and the Red Banana Jack

16.5V between Red wire of power transistor and black banana plug, then turning the front range selector

26.14V

36.9V

Black banana plug to yellow transistor wire (all were the same)

36.7V

Black banana plug to blue transistor wire (all were the same)

.6V

If I'm measuring Resistance, then it's 15 Ohms between Yellow (base) wire on different transistors in both polarities (at X1)

Between Red to Reds it's also 15 Ohms

Between Blue to Blues it's also 15 Ohms

Between Red to Blues there is no reading

Affirmative continuity between negative on both big capacitors and negative banana plug

The continuity between the big capacitors and the LM741 is screwy.  It seems to change...

Negative continuity between negative on both big capacitors and #4 pin on each of the LM741 ICs, except that there was continuity for a second

Positive continuity between negative on both big capacitors and #2 pin on each of the LM741 ICs, and then there was no continuity

Intermittent continuity between the positive on both capacitors and some of the pins of each of the LM741 ICs

Positive continuity between the top row of rectifier diodes on the non-line side and the #2 pin on each of the LM741 ICs, then negative continuity

Negative continuity between the bottom row of rectifier diodes on either side of diode and the #2 pin on each of the LM741 ICs

Positive continuity between each of the blue wires on the power transistors and the Red Banana Jack

Negative continuity between each of the yellow wires on the power transistors and the Red Banana Jack

16.5V between Red wire of power transistor and black banana plug, then turning the front range selector

26.14V

36.9V

Black banana plug to yellow transistor wire (all were the same)

36.7V

Black banana plug to blue transistor wire (all were the same)

.6V