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

Hi Matt,

I would get the 2N3055 as opposed to the NTE 130s if I had the choice.

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

You will also need to get some of these:

https://www.newark.com/nte-electronics/tp0001/thermal-insulators-thermo-pad/dp/79M5626?st=to-3%20insulator

Which can be used without thermal paste. If you want to reuse the current mica insulators, if they are in good condition, you will need to put a thin layer of thermal paste on the contacting surfaces. Be sure to install the insulators before soldering the wires to the pins..

If you want to save on shipping you might check to see what your electronic store resources are in your city. I would be surprised if you couldn't find a local store that would have these two items though what you save in shipping might be offset by higher component prices.

John

jw0752  wrote:

 

H...

 

If you want to save on shipping you might check to see what your electronic store resources are in your city. I would be surprised if you couldn't find a local store that would have these two items though what you save in shipping might be offset by higher component prices.

 

John

I do that here. In Brussels there's a shop called Elak. They have loads in stock. And I can order anything from element14 / Farnell.

Because they order from there, my components are a tiny part of their order.

They have a markup. For most orders lower than what I'd pay for shipping.

And I prefer to spend the extra money there rather than on FedEx/UPS - who try to make my life miserable whenever they spot the opportunity.

Well, I spent all day replacing four 2N3055 transistors....it wasn't easy

and...

Success!!!

BTW

I tested the old 2N3055 transistors, the continuity is the same as a new 2N3055.  There is no continuity between any of the terminals

But when I run a diode test, a new 2N3055 tests at .486 volts with negative on the emitter, and the positive on the base.  .OL in any other test configuration.

The old 2N3055's test at .OL in every test configuration.

Hi Matt,

Hurray!  Well done.  I am curious what the theory is on why these failed.  If I remember correctly it was working one day for you and the next time you turned it on it wasn't working.  Maybe John has a thought.

Frank

Congratulations Matt,

You stuck with it and you won the battle. Your diligence in taking good measurements and notes as well as useful pictures made all the difference. I hope you stick around the forum and continue your adventure in electronics. Don't hesitate to share your projects. The next time someone needs one of these fixed you are the go to expert.

John

John, thank you very much for your knowledge and ability to communicate it!

Matt

Hi Frank,

I think that the problem comes from the paralleling of the (4) 2N3055 output transistors. When the unit is used at High current and low voltage the 4 transistors have the challenge of getting rid of the extra power that is not being used. In a perfectly balanced system they would each carry 1/4th of the load and all might be well. Unfortunately they are not perfectly balanced and the one transistor that insists on taking more of the load gets hotter and hotter until it fails. In this case it seems that they fail open Base Emitter which takes the most ambitious transistor off line and transfers the load to the remaining three. One by One they fail until we are left with the situation that Matt had. The designer realized that this was a potential problem so they put emitter resistors on each of the Outputs as this helps to balance the load. This are the stretched out coils that we see on one circuit board. I am sure that they help but apparently not enough to prevent failure. Since we have seen two of these units with the same problem on the forum I would guess that these failures are a design problem.

John

I thought that might be it.  It is a common problem in parallel systems whether they be splitting fluids or electrons unless there is some sort of active control.

If you open one of the old transistors, you will most likely find that one of the bond wires has melted inside.

If you open one of the old transistors, you will most likely find that one of the bond wires has melted inside.