30V/3A power supply now has display on steriods? - Page 1
Forum section: electronics / repair

author: Rodast
author_rank: Contributor
timestamp: May 17, 2018, 07:26:02 pm
content: Hello All,I am using one of those entry level 30V/3A power supplies marketed under various model and brand names.https://www.amazon.de/Wentronic-Stabilisiertes-Labornetzger%C3%A4t-Ampere-Display/dp/B001C6FQF0This makes it pretty hard to search for information others might have posted.For the purposes I was using it so far it served me quite well until the display started to go crazy.In set mode with the load switch turned off the voltage value is sometimes dancing -/+ 20 digits.At the same time the amps are ranging from 1 to 2A.It is now impossible to set anything using the coarse selectors and unthinkable using the fine selectors.At the same time the output seems to be working correctly as an attached multimeter displays a constant 20V output or whatever I set.As a workaround I am able to tune the voltage to the desired value with this (output turned on + multimeter attached) setup.But actually I am hoping that someone could hint me to what is wrong with this device.Maybe someone had a similar issue with one of those power supplies in the past and has an idea what to look for.I am also happy to take the power supply apart but from my first investigation there was nothing obvious to be seen insideand as only the display portion seems to be misbehaving I concentrated on it.Though the front cover containing the display and electronics for it is pretty crowded and not very service friendly.Here is a video showing the issue in real time:Thanks for any suggestions

author: coromonadalix
author_rank: Super Contributor
timestamp: May 18, 2018, 01:17:06 am
content: bad connections, bad soldering in the display supplybad contacts of displays ic's in sockets ??

author: Rodast
author_rank: Contributor
timestamp: May 18, 2018, 11:53:30 am
content: Hello All,thank you for the feedback.So I have taken my PSU apart again and had another look around.As you can see from the pictures the actual display portion and the one with the controls and outputs on them are two seperate boards.I have removed and reseated both IC-7107 LED display drivers without any improvement.There are two connections going tothe display board with the two wired ones being the power and the three wired ones being the data input.Removing the data input cable gives me a constant 0.00 reading on both V and A displays.This makes me wonder if the issue could be with the control board which is feeding the display board.Closely inspecting the components on both boards I do not see anything out of the ordinary.I have so far disregarded looking at the high voltage board as the output is stable one set using the multimeter workaround.So I would assume the issue is not with the high volatage board.Any suggestions how to go from here?

author: coromonadalix
author_rank: Super Contributor
timestamp: May 18, 2018, 08:46:28 pm
content: j4 on the display pcb is the ac voltage powersupply ...j2 are the inputs, one for voltage i think its the red one, one for current must be the blue one, and black for the groundICL7107 a pretty standard dmm ic's, specs and schematics available on google ...tons of solder residues to remove ...if you remove j2 : are the displays stable and display zeroes ?? you can put an dmm on the pins when connected to see if the variations are read the same on the display and an external dmmif yes your problem may be on the second pcb ...the pots may need to be cleaned and lubricated ??