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

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.

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.