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

The diode test isn't going to tell us much on the power transistors. You could try to test them with the analog meter. They could be hooked in parallel and if one was bad but not shorted you would not see it.

John

Voltage Test for large capacitor (both measured exactly the same)

at 0-7V = 16.50V

at Mid-mark = 24V and falling

at 7-14V = 26.09V

Mid-mark = 35V and falling

at 14-20V = 36.7V

LM741 IC Tests

Top Left IC

Pot at Full CCW

1 = 5.2mV

2 = 6.22V

3 = 6.33V

5 = 4.8mV

6 = 17.71V

7 = 18.33V

8 = 35mV and falling past 20mV

Pot at Full CW

1 = 4.9mV

2 = 6.22V

3 = 6.33V

5 = 5mV

6 = 17.71V

7 = 18.33V

8 = 35mV and falling past 20mV

Bottom Right IC

Pot at Full CCW

1 = 9.6mV

2 = 6.33V

3 = 6.98V

5 = .6mV

6 = 18V

7 = 18.33V

8 = 30mV and falling past 20mV

Pot at Full CW

1 = 10.1mV

2 = 6.33V

3 = 10.20V

5 = 2.5mV

6 = 18V

7 = 18.33V

8 = 96mV and falling

Hi Matt,

Good Job. The only one that looked suspicious to me was the bottom right with full CW the pin 3 did not look right but we will wait and perhaps check it again later. I think this information will be helpful. It looks like the range selector switch is picking a voltage tap from the transformer to feed to the output transistors so that they do not have to over regulate and get too hot. I believe the intermediate settings on the switch are just off positions that is why the voltage is dropping.

Back to the output transistors. From the drawings I could see that the yellow wire went to two of the output transistors. Does it go to the same spacial pin on each of the 4 output transistors? Do the other wires from each transistor happen to tie back to the same connection point? If possible draw a diagram of the output transistors and show where the wires go.

Let me know what the results of using the Analog meter to test the output transistors is.

I might ask to anyone who has been monitoring this process, particularly, genebren  if they have any insights or if they can propose any other tests we can run to try and figure out why this unit seems to have voltage to all the power and control circuits but doesn't put out any power. I haven't yet seen any indication of an overload circuit that might be clamping off bias to the output transistors. I expected some variance in the  2 and 3 pins of the LM741s with the potentiometer turned from one extreme to the other but except for the anomalous reading on pin 3 in the full CW position nothing seems to be changing.

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

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