Need to get MG Electronics Power Supply repaired...or NOT!?

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.

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