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

Hello Ted,

I will move this over to our Power Management area, as it is a better fit.

Thank you,

Jamie

Hi Ted,

It is probably something simple but you will need to have someone who can open it up and step through the circuit. If you open it up and take some pictures or if you happen to have a schematic we might be able to guess some possibilities. Since there are likely mains voltages (120 Volts AC) at some points inside the case do not mess with it unless you have experience. You might check with the parts suppliers in your area as they might know a tech that can work on it.

John

There isn't anything that I know of around here in the sticks that could help me. I'm going to open it up if I can and stay far away from those capacitors. I did go to an electronics school 25 years ago, didn't enter the field, so I know some....just enough to get me into trouble ! I don't remember how to discharge a capacitor, probably just find a good ground with a thick wire...anyhow.

I'll try snooping around the interwoven , maybe I can find the schematic online. I don't even have a manual that goes with it. I did call MG for a manual. All they really do is import them, nothing else.

Thanks, Ted

Hi Ted,

Likely all the Mains voltage will be on the primary side of the transformer. The caps are probably filtering the secondary voltage and regulated voltage and would therefore have less than 30 or 40 volts on them. I would be glad to look at it for you but I am away from my shop until the middle of April. Book Mark our conversation and if you haven't resolved it by April drop me a reply. I am in Wisconsin so only 1/3 continent away.

John

While 'Jet-Set-John' goes on the haj, maybe I can substitute.  I wasn't able to find the schematic for this baby, but it must be rather straightforward.

Your ideation and caution around the filter caps is well placed.  You can test them for charge using a voltmeter.  Shunting them with something conductive will discharge them.  The only gotcha, which doesn't apply here, is PIO (paper-in-oil) capacitors.  They have so much dielectric soakage that the can charge themselves up after being discharged!  What's all this soakage stuff, anyhow? | Analog content from Electronic Design

I would replace anything with bug-juice in it out-of-hand.  The caps are old.  For all you know they could have PCBs in them.  Dried-up capacitors are a big cause of failure in older circuits.  Causes 'motor-boating' in audio amps, for example.  The other component that can suffer from age are the pass transistors on the back of the unit.

Lets trace through this thing.  This is a pretty good unit, but in the old days power inlets weren't of the safety standards of today.  Make sure you like the strain-relief scheme.  If you don't replace it with modern hardware.

The power comes in from the wall.  Goes through a fuse.  Could be a slo-blo type, or there could be inrush limiters on the main filter cap.  Initially filling the bypass cap is typically a hard time for a power supply.  Then the primary of a transformer.  These can fail, but they are typically rather rugged.  The supply has a range switch.  Probably we can switch out secondaries to place the supply in the proper regime.  This lessens the need for heat dissipation in the pass transistor(s) by lessening the input/output differential.  I think your meters may be hinting at what is wrong.

The fact that you know that this device once worked is advantageous. Now you can test the sub-components confident that if everything is working--you're good.  This device is mostly point-to-point, or at most single-sided PCB.  You should dope out the schematic on a piece of paper.

Next is almost surely a FWBR to feed the filter caps.  You can test the diodes individually in this.

After the filter caps comes the regulator section.  This will have, basically, three parts.  A voltage reference.  You can probably find and test this part.  It can be made of discretes.  An error amplifier.  This compares the reference to the output.  A pass transistor.  The error amplifier modulates the pass transistor using negative feedback to stabilize the system.  If the error amplifier is trying to do the right thing, I'd look hard at the pass transistor.

In a properly designed PS, there are lotsa components that do not operate under normal operation.  There can be OVP crowbars, current foldback limiting, et c.  These will be little circuits hanging out in their individual locales, probably.

Hi all again!

First of all than you for all the courtesy that you have shown me !

I did search the web for a schematic, non to be found. I saw that others had the same luck !

I did open up the MG Electronics Power Supply . I have attached pix of that expedition!

What I found was....No burnt anything, no burnt smell, no heat, no buzzing noise, nothing looks incorrect, it pretty clean in there!

I did see what I that was melted something on the giant transformer, I took a Q-tip to it, nothing. Probably tried glue or sealant of some kind !

Visual I see nothing! I did check the fuse again, it's fine !

So what usually goes first on a power supply ? Just a reminder, the overload light is on, what ever that info is worth !

Thank you all so very much !!

Ted

I gotta make some guesses since we don't have the schematic at hand.  If I'm wrong, it doesn't mean I am a dummy, it just means I'm wrong.  Now, if you would just explain this to my wife. . .

Okay, this thing is really made of discretes.  It is an oldie, but nice and well worth fixing.  This is good, everything is explicit.  The two chips are 741 op-amps, the same chip that governed the Love Controls at my first job.  Take heart.  Notice that the bridge rectifiers are made of discretes.  There is one wire there, that when soldered caused the insulation to bulge from excessive heat.  It will work fine, but it shouldn't have passed Q/A.

I think one 741 is acting as a feedback controller for the supply and the other is a comparator for the overload function.

Going for a quick kill, see if you can pull the covers off the power transistors on the back of the unit and find the part numbers for them.  If they are bipolars, they will be fairly easy to test, if they are Darlingtons it will be a wee bit tougher.

I attempted to take the covers off the back 2 power transistors. Those caps are held on by a lock nut and nut on the opposite blind side. I can't get them off. I had one set of nut and washer fall off that's how I know about connectivity. I haven't ventured into taking off the giant heat sinks in back. I can see the screws. I don't know how things are connected. Are the power transistors just pushed into place and not soldered ? If I attempt to take off the heat sinks will the power transistors come with it and there will be a wires connecting everything together ? I just don't know.

Anyhow...thanks !

Ted

Don't worry, make sure you have a clean, well-lit workspace, there is nothing proprietary or expensive about any of the parts here.  Those look, from here, to be to-218  packages, which have been supplanted by to-220, not to worry.

The transistors are in sockets.  If you remove the screws (being careful to recover all of the washers) the sockets will hang on their wires, and the transistors can be pulled out.  There might be an insulating mica sheet to be careful of.  On the off chance that the transistors are different devices, you might take a picture in order to help us keep things straight.

Can you test with an ohmmeter or continuity tester to see if these two transistor are wired in parallel or not?

One thing to check - second image down, one lead of the diode is against the filter cap and looks like it caused the insulation on the cap to melt when it was soldered.  If that lead is NOT ground and is against the cap, it may have shorted to the shell of the cap (which is most likely ground).  Make sure there is not a short there.

mikey