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

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 !

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.

Then again, I have a theory:  I have two existential fuzzy fax at hand.  From the WWW.  1)  The schematic is unpublished.  2)  There are a lot of dysfunctional versions of this unit on the market.

Here's my guess:  The other transistor on the case is identical and wired in parallel.

1 is probably because our hero may be a little chagrined.  2 Indicates design flaw.

My guess is that our hero took a flyer for market reasons.

Hogging over time may have forced one of your Qs to fail open.  This would account for your over-current light.

My consequent guess is that a pair (possibly re-biased, maybe not) of power mosfets or lm395s instead of the 3055s or whatevs that were in there would tighten this boat up nicely. 

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Or, all of this could be wrong.  Those with alternate theories that are parsimonious and fit the facts as we know them currently are free to come forward.

The ball is in your court, Bucko.  Time for some empiricism!

I was questioning whether problems with the outputs would trigger the overload circuit. My first simple check would be to use my ohm meter to look back into the output of the power supply to see if there is a short. Make sure the positive probe is on the positive terminal and the negative probe is on the negative. This could be as simple as a shorted filter cap.

John

Continuing my observations: This is a voltage regulated power supply and the only current regulation appears to be the overload trip when the current exceeds specifications. We know the front end of the supply is working as we are getting power for the overload light. If the output transistors were shorted we would get full output voltage with no regulation. Since we have no output voltage and an indication of an over current situation the first place to look seems to be where and which components are capable of giving us a high current situation at the output when no load is attached. The test that I mentioned in my previous post should allow us to test for a full or partial short. If that is the situation it will be necessary to go through the components that are across the output and isolate and test until the culprit is found. The other possibility, though less likely, is that the overload sensor circuit itself has failed and is reading a false overload and shutting down the output for no reason.

John

Thanks Don and John for the suggestions on how to tackle my dead power supply problem.

The problem I have now is that the power supply is a my kitchen table opposite of were I have coffee and read the paper in the morning and it's apart somewhat and looking sad!

The other problem is that the both of you are far above my pay level in electronics theory etc. I sort of know what your talking about but it's been awhile !

Right now the power supply is apart were I disassembled the one power transistor via Don's theory . So my situation is put it back together , and investigate and investigate John's theory ?

I would just like to be able to figure out what the problem is a repair it, if it's within my capabilities. I do have a multimeter, solder sucker , soldering iron and if we can figure out what is really the culprit those parts are easily accessible via FEEbay ! 

Many, many thanks to the both of you, especially Don , for his humor and making me chuckle a bit !

Ted

One thing to not rule out is that it is not actually an overload condition, but a failure of the overload detection circuit that may be shutting it down.

mikey

Hi Ted,

Based on what you have told us about your experience level I believe you are correct to reassemble the supply so that it isn't more complicated to fix than necessary. Let us know what you find with the tests I proposed.

John