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

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

This is actually an improvement. Go for it.

John

You say D311 is this a different transistor from the D313?

The readings that you listed for that D311 sound like a good transistor.

I don't think the diode tester is doing a good job on the 2N3055 s  while in the circuit. If you can get the covers off and take out the two retention screws they should just unplug from their sockets. Then try to run the diode and continuity test again.

John

Sorry, that was a typo.  I retested the D313 and got those readings.

But I ordered a new D313 (now a NTE152)

I put it in when it gets here and get back to you.  If it doesn't fix it then I'll work on the 2N3055s

Sounds good , let me know.

John

I got the NTE152NTE152 transistor in and it tests exactly the same as the pulled D313 transistor.  So I'll put in the new NTE152NTE152 and pull the power transistors to test next...

New Transistor

No Continuity between any pins

Continuity between middle pin and backboard

With positive on pin 1 and negative on pin 2 = .578 V

With positive on pin 1 and negative on pin 3 = .580 V

Old Transistor

No Continuity between any pins

Continuity between middle pin and backboard

Continuity between middle pin and backboard

With positive on pin 1 and negative on pin 2 =  .580 V

With positive on pin 1 and negative on pin 3 =  .585 V

Still a no go for Voltage

Hi Matt,

Good job on replacing the D313. Even though that wasn't the problem you at least have one more thing checked off the list. When you test the output transistors I believe they will be 2N3055. An NTE equivalent is an NTE130NTE130.

Here is a picture of the pin out and the voltage I would expect from a diode check. On these it might also be good to check continuity especially between the Emitter and Collector. The resistance should be high in both polarities. Notice that the Emitter and Base is off center. With the Emitter and Base closer to the bottom of the picture the leads are identified as in the picture.

Based on how the unit is acting I am doubtful that these are going to be the problem but we must be sure. I am also a little at loss as to what to focus on next. Without a schematic I am not sure how the circuit is designed and my usual procedure for testing beyond the basics that we have done usually relies on tracing circuits and reverse engineering. These will be next to impossible to do remotely using only pictures. I will be thinking and you can let me know when you have tested the 2N3055s.

John

Yep, correct on the Power Transistor. Hopefully I won't have to disconnect the blue and yellow wires.  If so, I'm not sure how to reconnect without disassembling the entire back end of the unit.

Yep, correct on the Power Transistor. Hopefully I won't have to disconnect the blue and yellow wires.  If so, I'm not sure how to reconnect without disassembling the entire back end of the unit.

there is no continuity between the base, collector, or emitter.

Diode check

It is showing .015 Volts between the collector and the yellow wire (which I believe is the base in your picture, although I'm not sure if I'm looking at the front or back side)

The data sheet for the 2N3055 NPN power transistors specifies

2N3055 Equivalents

2N6673, 2N6675, complementary pair- MJ2955

Is there something special about complementary pairs vs two NTE130s?

Thanks,

Matt

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.