You said you measured voltage. Just use the same meter to probe the circuit path.
Check for voltage and also use the resistance scales to see if a component is defective.
Since it blew up at one time, You must have had too heavy a load or the 3 amp power supply had too high a voltage.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
With two post, I can see you are new here. Welcome.
Never post your email address in a message. Hackers use software to scan the Internet for an address like this.
Once in a while I see the same from a new member in different forums who are not aware their email is hidden from people who do not belong to the forum.
Anyone can see the address without being signed in.
Members here can PM you when they are signed in. Standard procedure.
If you rebuilt it with all new parts, make sure you have the same or true equivalent parts as the original (not that there are many). And make sure you have all the wires exactly as shown in the article, especially the orientation of the transistor. And that you haven't fogotten the insulating bushings on the transistor so it doesn;t short against the case.
The transistor used, at least witht he minimal heatsinking of the metal case, cannot handle a 3 amp dead short across it. That'w probably why your old one blew up with a 3 amp power supply, under a short circuit condition on the rails that whole 3 amps could now go through the transistor. With a 1.5 amp power supply, only half the current is available, and the transistor can handle that.
If wired exactly as showm, it should work fine, assuming the transistor is good. It's unusual for a resistor to be bad, but under a high current short it could have burned that as well, in case you aren't using a new one. It also needs to be excatly the value specified int he parts list or the circuit will not work - too large, especially an order of magnitude or so if you misread the color bands, would cause it to behave like you are experiencing. If in doubt, put your meter on ohms or the 1K range and measure - it's supposed to be 100 ohms, or 0.10K If it's 1K or 10K, the circuit will not work. And the diode is critical to keep the loco motors from damaging the transistor - make sure that is installed the correct way around, if not, the cicuit will not work (the diode will be a dead short if wired backwards).
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.