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

Here is one oddity I noticed.  It looks like the yellow lead to the center terminal of the pot was in contact with the terminal to its right as pictured.  So much so that I thought it was welded to both.  But I pushed it with some tweezer and it separated as pictured in the last two photographs

It's difficult to test because I don't have three hands, but it seems like the R value between the two outside terminals on the pot is 1.6 kOhms.  The value between the red outside lead and the center yellow lead ranges from .9Ohms to 1.6k Ohms.

Hi Matt,

The readings on the potentiometer appear to be normal and acceptable. I would not have been surprised if the yellow was connected to both terminals. This potentiometer is being used as a resistive trimmer in either configuration. I am waiting to see what voltages you measure on the leads of the two LM 741 ICs.

John

Hi Matt,

Here is another experiment I would like you to try. There are two large capacitors on the board with the diodes. I would like you to set your digital meter to DC volts. Turn on the power supply and measure the voltage across the capacitor leads in each of the settings of the range control on the front panel. We may get different voltages and we may get only one voltage. This may tell us something about the actual circuit design. Do this test for each of the two filter capacitors. Note the lead that is closest to the line on the side of the capacitor is the negative. Let me know what you find out.

John

The diode test on the power transistors yields .010 volts between the base and the emitter in both polarities while the transistor is in circuit.  All four power transistor yield the same results

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?

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?

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 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

I wanted to thank you for your patience and thoughtful explanations!   I really appreciate it!! 

Hi Matt,

The voltages that you provided tell me that the Output transistors are likely failed open. I just need to confirm that the voltage on the blue wire was 0.6 Volts?

I recommend that you bite the bullet and remove one of the output transistors so we can test it and verify that it is bad. Don't order replacements without talking to me first. Once it is out use the analog meter to run the same test we just ran on them when they were in the circuit.

With 36 volts on the collector and 36 volts on the base the emitter should also be at approx 36 volts not 0.6 volts as this is an NPN transistor.

John

Confirm, I'm reading .6V between the blue wires and the negative banana plug

36V between Red and the negative banana plug

36V between Yellow and the negative banana plug

Actually, it's -.6V

Hi Matt,

Here are some thoughts on the Output Transistors. If the wires are long enough you might try pulling the wires through the holes in the heat sink and then desoldering them or just clip them off and make new connections to the new transistors. I believe that the coils of wire on the circuit board are emitter resistors. One blue wire should run to one end of each coil and the other ends of the coils should be connected to the red banana jack on the front panel. Confirm to me that this is the case. Otherwise we may need to install our own emitter resistors to help the outputs avoid thermal runaway. When you remount the new output transistors you should either use a teflon insulator or if you are going to use the original mica insulators silicone heat sink compound needs to be used. We want the heat from the 2N3055s to easily transfer to the heat sinks. I saw that Newark has some reasonably priced 2N3055s

https://www.newark.com/multicomp/2n3055h/bipolar-transistor-npn-60v/dp/78M7081?st=2n3055

Get yourself at least one extra just in case. Also you should be able to find the teflon heat sink insulators on Newark. I still want to see the results of your out of circuit test of the 2N3055 before we go ahead but I had some time so I jumped ahead to these ideas on replacing the 2N3055s.

John