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.
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.
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.
1 of 1 people found this helpful
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.
1 of 1 people found this helpful
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 !!
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.
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.
Mike F's supposition is testable. The way this PS works, I gather, is to switch in one of three bridge rectifier circuits (hence the twelve diodes) moving the range switch should expose another BR.
Don't worry about breaking anything. Rejoice, your unit is already broken!
Rather than having to find an offset socket or tiny wrench to get your nuts on the power Qs, you can often just use needle-nose pliers.
Hi all again.
I didn't think there would be any humor involved in this electronic detection ! I guess I was wrong ! "Don't worry about breaking anything. Rejoice, your unit is already broken! "
In answer to Mike F's diagnoses......if your referring to the 2nd pic down. If it's the whitish insulated wire behind the diode. I just moved it out of the way. In doing so I didn't notice any heat effecting that wire.
Back to DH.....I have 2 more pics. One of the power transistor I have gotten to. I only did one side too chicken to do both ! I'm glad I did because I can use the other side as reference for replacing the screws, they are not as straight for as I hoped. I dragged out my trusty Multimeter to check the transistor I did get too. I moved the insulation out of the way, I was surprised to see that it's soldered in and not slipped into a holder. So, I tested for simple continuity from the legs of the transistor, and it does have continuity. I know it's not exactly what your asking for as far as parallel but I'm taking it slow, been a long while since I used these principles ! I'm a little bit foggy! But I do get it !
THX, again !
1 of 1 people found this helpful
This is a very straight forward NPN power-transistor. http://www.onsemi.com/pub_link/Collateral/2N3055-D.PDF
There is the mica washer as I predicted. The manufacturer didn't overuse heat-sink compound, which is rare. You should only use enough to fill voids in the metal, otherwise metal-metal contact is better.
If you have a diode test mode on your multimeter, you should see a diode between base and emmiter. You should see another one with a slightly lower drop collector-to-base. The case is collector. High Z in every other two-terminal modality.
These transistors were quite rugged but had low gain. This is a very good choice for a pass transistor. I suspect that the power Q by the op-amps is a predriver for this, probably in Darlington configuration. I've made a fetlington with one years ago, robust as an anchor.
This is your circuit in concept:
Also, proof that I have been hitting the coffee pot to excess.
The amp is differential, the bubble implies inversion. Let's say that I have the reference at one volt. What should happen is that this should cause an output of one volt. Let's say I increase the reference potential to 1.05V. The amp would be imbalanced. The output would go up in value. This would turn Q1 on further (presuming it isn't saturated, et c.) raising the output potential while increasing the potential at the inverting input of the error amp since they are attached. When the inverting terminal rises the delta, .05V, things settle out again, ignoring offset potential.
If the output transistor is good, we can check the reference. If it is good, we are left to conclude, in the context of proper connection, that the op-amp is having a sick day.
I just bought a lot of Slot Car stuff, yes those cars that go around a track. The guy was serious about his hobby he didn't have the usual wall plug in power pack, he used a MG Electronics PS-10AD, 0-20 VDC at 10 Amps. Well this power supply doesn't work anymore. It does turn on and the red overload light stays on. It doesn't put out anything. I used my multimeter etc., nothing. I know that these are made overseas . They did retail for about $200. So the questions is were can I get it repaired OR should I ? I'm in Western MAss.
I did check the fuse , it's fine. I've got a heavy door stop on my hands, it does have a nice handle and cord on it !
Many thanks for info, Ted