Elenco XP-720K problems | +15 Volt Rail Problems - Page 1
Forum section: electronics / beginners

author: bigmikeisasasquatch
author_rank: Newbie
timestamp: May 31, 2020, 07:33:11 pm
content: Hello all! First time poster and new to electronics and enjoying everything that I have learned so far! Because even blue smoke is a good teacher, if it does not happen all the time.I was needing a bench top power supply and I picked up the XP-720K Kit. I wanted something that I could build and learn from before I picked up a proper one.I got it all together. Turned it on, and no blue smoke. Was checking the Voltage across the different posts. AC 6.3 is reading fine on both posts. Checked the 5V DC and it is fine. Checked the -15V DC post and it is fine and adjust fine on its POT.When I check the +15V DC Post it is reading at 26.64 when the POT is turned to the max. When I turn the POT down it wants to blow Resistor R2 on the board. (Diagram is attached)I replaced the 150Ω resistor with a 180Ω resistor. (That is the resistor that they use in the version of this power supply that is not a kit.)I started to check the Voltages at the different points in the circuit for the +15V. I started at the Transformer input and that is what it is outputting is the 26 V. I checked the voltage at every point in that portion of the circuit and it is showing the 26 volts. It is showing it at POT, all pins on the LM317T, and all the positive sides of all the Caps.I checked all the soldering points on the bottom of the PCB and there were no shorts. I checked it for any problems with flux I noticed that there were some flux left on the bottom, I cleaned it with some isopropyl alcohol and a new tooth brush and got a all cleaned off dried it with a paper towel and a can of air to get anything that might have come loose off. I then let it dry for an hour for good measure.After that I checked the voltages again, still getting 26V on the +15 side and all other posts are correct. I burned up another Resistor testing the POT for the +15 side.I checked all the Diodes and Caps to see that they are the correct orientation for the board and they are. Before I put anything on the board I tested all the Caps and Diodes and all were working correctly before soldering everything.At this point I do not know what the problem may be.Any help would be grateful! I have a feeling that it is something that is obvious that i am not seeing.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: June 01, 2020, 03:12:26 am
content: I have another version of basically the same power supply. You probably have a blown 317T regulator chip. Note that its pinout or rather its connection to circuit is different from the 337T. Check very carefully around the legs of the regulator chips to make sure you have the 317 connected properly and no shorts in that area. If I were there I'd buy a handful of the regulator chips and replace the 317T just on spec. (I even keep some spare regulator chips taped to the case inside of my Elenco supply so I'll know where to find them when I need them.) They are cheap.Also, just nitpicking, but the wires soldered to the banana jacks should be soldered around the little spindle shaped thing on the end of the stud, not up around the nuts. The idea is to keep the threads free of solder in the event that you need or want to replace a jack. Also it takes less heat to solder to the spindle area. This won't be your present problem so just leave that alone, but for your next project....

author: floobydust
author_rank: Super Contributor
timestamp: June 01, 2020, 04:30:07 am
content: Check you got the Vreg IC pinouts correct - LM337 has a different pinout than the LM317.edit: pic looks wrong. "W" goes to the middle pin of the LM317, so "W" and "S" flipped; out-in.If a Vreg IC shorts, it will want to smoke the potentiometer, so don't turn it down if it's doing anything silly.Note these Elenco XP-720K are missing six protection diodes- which makes it easy to damage the voltage reg IC's. If you add the diodes, the PSU will be fairly tough.https://www.eevblog.com/forum/beginners/mpsa70-pnp-transistor/msg2620812/#msg2620812

author: bigmikeisasasquatch
author_rank: Newbie
timestamp: June 01, 2020, 08:19:40 am
content: Quote from: alsetalokin4017 on June 01, 2020, 03:12:26 amThanks! I had a feeling that might have been the problem with the 317t. I checked all the pins on it b4 I posted and all pins were showing 26 V. That seemed a bit of to me.I also figured that out after Soldering the AC Side. At the time I was trying to rush bc I had to do something else. I just stuck with what I was doing. I might go back and fix that.Again thanks!Quote from: floobydust on June 01, 2020, 04:30:07 amI will check the pinouts as well! Thanks!