Korad ka3005D  Power supply display LEDs  issue - Page 1
Forum section: electronics / beginners

author: lots o tots
author_rank: Contributor
timestamp: May 19, 2022, 10:12:42 pm
content: Hello,I tried to replace the on/off LED to a green one with a similar voltage rating in my Korad power supply ( two-year-old). I managed to replace the LED, but in the process the display LEDs no longer works properly when in operation. I hypothesize that I may heated up some component too much as I tried to use solder wick with my soldering iron.... I was struggling with that ( was not soaking)... never again.the issue is all left panel LEDs (4 of 5) except for the "lock" LED don't appear to work when the unit is powered on. If I remove the display module, I can individually test all LEDs from my multimeter, and they appear to work fine In diode mode. also, I tried continuity checking for the traces near the one LED that was replaced. I can't seem to find an issue. no obvious burning Marks anywhereI was wondering if anybody had any ideas about what I should do, what I should test, or what could be wrong to cause this issue. my skill set for this type of thing is beginner.thanks

author: ledtester
author_rank: Super Contributor
timestamp: May 20, 2022, 01:33:43 am
content: This post contains a zip file with schematics for the KA3005Pbut I imagine the display board would be similar to theDmodel.https://www.eevblog.com/forum/repair/korad-ka3005p-power-supply-calibration/msg1109803/#msg1109803Also, fwiw, this post contains some written from scratch firmware for the KA3005D:https://www.eevblog.com/forum/repair/korad-ka3005p-power-supply-calibration/msg1283374/#msg1283374In the display board schematic I would look at the NPN transistors. Q4, Q5, Q6, and Q8 control the digit columns and Q7 controls one bank of single LEDs.

author: lots o tots
author_rank: Contributor
timestamp: May 20, 2022, 04:06:26 am
content: I did more digging. I noticed that the negative terminals on all the functional LEDs (6 of them) have good continuity between them.the (4 troublesome LEDs that is not properly connected to the circuit) also had good continuity in their negative terminals, however the negative terminals between the set of (6)six and (4)four are not connected to each other. even with an NPN On one of the banks, shouldn't All negative terminals ( 10 LEDs) have continuity?I'm wondering this because if I damage a trace ( to ground), I'm wondering if I could bodge the ground pins of the LED sets together?

author: ledtester
author_rank: Super Contributor
timestamp: May 20, 2022, 05:31:43 am
content: I've attached the schematic of the display board that was included in the "3005p.zip" file that you can find at the first link in my previous post.You'll see that one set of 5 LEDs is directly connected to ground whereas the other set of discrete LEDs are connected to a transistor.I would first try to determine if this schematic accurately describes your display circuit. If all of the LEDs in the second group do not light up then perhaps the transistor Q7 is damaged and needs to be replaced.

author: lots o tots
author_rank: Contributor
timestamp: May 20, 2022, 06:21:49 pm
content: I don't think it's exactly the same for two different reasons.the resistor values are 330 ohm next to the LEDs, and 1K ohm next to the NPN transistor,Which is not what the 2013 schematic showsI traced each resistor identification(R) to the LED(D) using continuity check ( diode/resistor number in the list below). the identification does not line up with the LEDs in the schematic Or the schematicd made by djeddittD1:R3D5: R9D3:R8D10:R12D4:R16Not all five LED in one bank have an issue, onlyfourof them. I also tested all 5 NPN transistors in circuit. They all appeared to drop .7 V, and reversing the leads for all five is OpenL. is it possible at least one failed somehowEven when passing this test?I not sure what to do from here

author: ledtester
author_rank: Super Contributor
timestamp: May 20, 2022, 07:32:18 pm
content: From your pics the board looks like it is basically the same schematic even if the resistor values are not quite right.Which LEDs are failing, the ones you replaced or the old ones? Could it be that you put the new LEDs in backwards?Whenever you have multiple failures start looking at what those components have in common -- that's very likely the culprit.

author: MikeK
author_rank: Super Contributor
timestamp: May 20, 2022, 08:08:32 pm
content: Obvious test: Remove the LED that you replaced and recheck.

author: lots o tots
author_rank: Contributor
timestamp: May 20, 2022, 08:42:01 pm
content: I only replaced one LED, the green one. when testing with my multimeter, all (10) of the LEDs are functional. it's only when I hook it up and circuit is an issue. all the LEDs that are not lighting up are all in the left column except for the one at the very top which works.I tested the traces nearby the green LED, there was no issue that I can find.I was having a heck of a time removing the original LED, as I used at soldering wick. It did not work well. will never do that again

author: ledtester
author_rank: Super Contributor
timestamp: May 21, 2022, 12:26:21 am
content: Is it possible there is a broken trace on the board? For example, if there was a break at the red X then D4 would work but not the other LEDs in the same bank:Otherwise my next step would probably be to hook up the board to an Arduino and write some code to exercise the display. The way they are using the shift registers and transistors are pretty standard.There are a lot of Arduino/595 shift register tutorials on the web, e.g.:https://lastminuteengineers.com/74hc595-shift-register-arduino-tutorial/

author: lots o tots
author_rank: Contributor
timestamp: May 21, 2022, 01:02:31 am
content: Quote from: ledtester on May 21, 2022, 12:26:21 amI would like to try that. The only issue is that the silkscreen LED # does not match up to the schematic if the silkscreen is correct. out of the four (4) inoperable LEDs, half are attached to the top display and half are to the bottom. ( #5#,3,#10,#4)Thanks for the link for the Arduino shift register testing. I guess I can give this a shot

author: ledtester
author_rank: Super Contributor
timestamp: May 21, 2022, 01:18:12 am
content: You should draw up your own schematic. The one in the PDF file has some obvious errors in it but is still useful for understanding what the general structure of the board is.

author: lots o tots
author_rank: Contributor
timestamp: May 22, 2022, 01:21:53 am
content: I am not sure what the obvious errors are As I am a beginner level. I was just starting to set this up ( Last-minute engineers) and realize that my chip is SMD. Not sure how it could test this in its installed condition.Is the easiest thing to do just to replace both chips and all 5 NPNs? I wish there was a way to test this just by powering up the right pins the port

author: radiolistener
author_rank: Super Contributor
timestamp: May 22, 2022, 02:11:54 am
content: As I remember one people has replaced that LED with green one and after some time he caught shift register IC malfunction with 19V on the output terminals when output is off. Which require IC replacement, after that it worked ok. At a glance LED replacement should not be related with IC malfunction, but who knows...

author: ledtester
author_rank: Super Contributor
timestamp: May 22, 2022, 02:33:10 am
content: Quote from: lots o tots on May 22, 2022, 01:21:53 amThere is, but you have to have an understanding of how the board is supposed to work. Basically you shift data into the 595's and then activate one of the transistors. This will light up a particular "column" of LEDs according to the data in the shift registers. You then shift different data in and activate a different transistor to light up another column. So at any one point in time at most 16 LEDs can be illuminated. But if you cycle through the columns fast enough it appears to your eye that LEDs from all of the columns are showing at the same time. This is called multiplexing.This is a pretty good explanation of how a 595 is used to multiplex 4 7-segment digits using an Arduino:https://simple-circuit.com/arduino-7-segment-74hc595-shift-register/You can find more information and tutorials by using search terms "multiplexing 7-segment LED display shift register". If you've never done any Arduino programming or used 7-segment displays before it can be a steep learning curve, but it is a very commonly used technique to drive LED displays.

author: lots o tots
author_rank: Contributor
timestamp: May 24, 2022, 01:51:58 pm
content: Sorted!I went back over the traces again. And found one trace that was not Connected. it was hard to see as the trace was hiding under some components. Since all the negatives were supposed to be connected from what I can see, I bodge wired two of the other LEDs so all 10 are connected to each other.thank you everyone for helping!