I got a couple of those Athearn powerpacks that they have made zillions of. Nothing fancy but I bought them mainly for use in testing. One is fine but the second one has me stumped. The DC output is always at full power no matter where I set the reostat. Has anybody else had this issues? Any ideas on the fix?
I'd try jumping around the rheostat and see if the output is the same. If the rheostat is bad you MIGHT be able to change it out, but getting another rheostat might be a problem. Don't destroy the board in the process.
Without having a schematic of the circuit I would avoid jumping around anything. Also, from the second picture there appears to be a transistor in the circuit. The transistor is more likely the faulty component. Try measuring the resistance across the emitter and collector leads of the transistor and see if it's shorted.
Looking at your photo it appears that it is a simple electronic power pack with a power transistor providing the variable DC output. You indicated that the pack provides only a constant full voltage output. While the potentiometer (“rheostat”) might be bad, it’s more likely that the Power transistor is defective. The transistor is a very common TIP122 NPN Darlington transistor which sells for about $1.00. The transistor is easy to replace. Most Radio Shacks stock a TIP120 transistor, ($1.99) which is an acceptable replacement for use in this pack.
