MRC Power Pack Suddenly Stopped Working - Need Help - Model Railroader Magazine

Matthew,

It's been a few years but I'm inclined to think that the potentiometer (the round gizzie with the turning shaft) should be easy to check. My notion is that the two outboard tabs that are soldered to the PC board are the opposite ends of the coiled resistance winding inside and the offset tab in the center connects to the wiper arm. If that is the case there are two things you can check if you have an ohmmeter to test resistance.  What follows is best done AFTER you have isolated the potentiometer by removing it from the circuit board.  This will avoid the possibility of phony resistance readings that could be sensed through the board's circuits.

First, place the two probes of the meter across the two outer tabs. That should measure the total resistance of the winding itself. If it's an old analog meter with a face plate and a needle to read the value, start with a range on your meter set to 10-20K ohms or more and work down (that's to avoid whacking the needle against the fulll range stop if the value of your pot winding is more than the range you've set on your meter - not a big issue for a meter with adigital readout). As previously suggested, look for something around 3000 ohms. You will not get zero if your meter is on the 10K range or so. That would be a dead short which would not produce the results you describe and is a virtual internal impossibility if your pack has ever worked properly. If you get no reading at all, even on the highest range, the winding has opened up altogether. Your results indicate that such a break would be near one end or the other.

Second, attach one probe to the center (wiper) tab, and the other probe to either of the outer tabs. Rotate the shaft clockwise. If I'm guessing correctly, you will get no continuity at all when you turn the pot full range and you are connecting between the center tab and the winding tab to the left.  When bridging between the wiper and the right hand winding tab (as the shaft is facing you) you will get nothing on your meter until you've turned the shaft almost fully clockwise.  If at that point you suddenly show just a few ohms (say less than 100), you've found the point where the winding has opened up.

If either of these two results show up, your potentiometer (pot) is bad and should be replaced.  Frankly, this kind of defect is rather rare but not entirely unknown.  And as it's the easiest to check, I'd give it a shot first.

If it is bad, there's likely some figure stamped on the pot's casing that indicates its value.  You can probably get a replacement at an electronics parts store - if you can find one nearby these days!  Otherwise, you'll have to roll your eyes and fork over for one on-line.  If so, I hope you have Amazon Prime and have already coughed up for the postage!

This is just one possible fix.  There may be other issues.  I'd think if your power transistor was bad (I'm guessing it's the one beside the pot that is screwed to the aluminum housing being used as a heat sink), you'd have gotten no action even with your pot turned up all the way.

Here's the bottom line: you will have to judge just where to draw the line here.  As Aristotle (?) once said, "Time is money."  All the more so when you don't have the right tools, parts, or sense of adventure at hand.  There's more than one reason why MRC doesn't provide schematics for these units - or make them easily accessible.

Good luck! 

John