Power supply repair  - Page 1
Forum section: electronics / repair

author: hpibmx
author_rank: Regular Contributor
timestamp: August 15, 2020, 06:26:55 am
content: Hi everyone,I’ve got these two power supply unit in my company which they put them aside but it’s weekends and I have time to go around it and get it fixed perhaps by your helpSo once you turn them on they both have out put but adjustable knob isn’t changing them.I’m just a mobile technician for those who don’t know me, but I’m sure with your help I can get this thing running.Thank you all.

author: t1d
author_rank: Super Contributor
timestamp: August 15, 2020, 11:31:38 am
content: Quote from: hpibmx on August 15, 2020, 06:26:55 amFirstly, you will need to give us much more information...1) Is the output just that the display shows some voltage, or are you actually getting that voltage reading on the connector plugs with a multimeter?2) You need to try to find (on the internet)a) the schematic for the unit (even a schematic for a similar unit can be helpful,)b) a repair manual for the unit, andc) the operator's manual.3) With the units disconnected from the wall power socket, open the units, drain any capacitors, do a visual inspection for damage (leaky capacitors, burned parts, etc.) on the top and bottom of all PCB boards and take lots of pictures of any damage and the general layout of the circuitry.With that information, we would be glad to try to help you.

author: hpibmx
author_rank: Regular Contributor
timestamp: August 16, 2020, 04:18:42 pm
content: Quote from: t1d on August 15, 2020, 11:31:38 am1) yes I do read what it shows on my meter,2) I have found a similar schematic3) visual inspection nothing was obvious bottom and top sidethank you for guiding me

author: t1d
author_rank: Super Contributor
timestamp: August 16, 2020, 06:27:38 pm
content: This is good progress!1) Please post the schematic, here, or give us the link to where we can find it. This is the key to getting started on finding the fault(s). If you know what the basic difference is between your units and the unit for the schematic, please tell us what the difference is.2) Please tell us about your skills and resources... You are a mobile technician... I am sorry, but I do not know what skills that includes... Do you work with configuring cell phones on cell phone services? Or, do you actually repair cell phones, radios, etc? What repair tools - soldering iron, hot air machine, etc. - and what test equipment - oscilloscope (you said you have a multimeter) - do you have, or have access to?I look forward to your answers and helping you make more progress.

author: t1d
author_rank: Super Contributor
timestamp: August 16, 2020, 06:35:49 pm
content: Do you know how long it has been since these units were last used? I am wondering if there might be a small battery that retained their programming in memory and that the battery may have died... That is just a wild guess. Again, the schematics will tell us what to do next...

author: hpibmx
author_rank: Regular Contributor
timestamp: August 18, 2020, 01:54:06 pm
content: Quote from: t1d on August 16, 2020, 06:27:38 pmThanks for your follow up,1) unfortunately the schematic I got was the wrong one.2) I’ve got everything I need I guess hot air station, solder, meter that’s what I need for phone repair I have good background of electronics, I don’t have oscilloscope as of nowMy Repare skills are mostly everything in iPhones and iPads.

author: hpibmx
author_rank: Regular Contributor
timestamp: August 18, 2020, 01:56:20 pm
content: Quote from: t1d on August 16, 2020, 06:35:49 pmVery good guess, I think it’s been really long time I think 2 years.But I ran diode mode readings on the pins and I’ve checked I have out of 5.4v but still not changingAnd my potentiometer is good I’ve measured it.

author: t1d
author_rank: Super Contributor
timestamp: August 19, 2020, 03:01:19 pm
content: Quote from: hpibmx on August 18, 2020, 01:56:20 pmWell, not having the schematic, or at least one for a similar model, will make things a little more difficult... But, not impossible. Continue to search for it, until you have it. Searching doesn't have to be your main purpose, or delay investigations, but keep after it. Have you located the manufacturer and contacted them, to try to get the schematic? Sometimes, they will share it with people that are doing repairs.I need to take a moment and explain to you what we need, to be able to communicate... We need to be very precise in how we describe things, to each other... And, you need to give great detail about what you are doing... I will use your comments, as an example..."Very good guess, I think it’s been really long time I think 2 years.But I ran diode mode readings on the pins and I’ve checked I have out of 5.4v but still not changing "You are not particular about how the battery is related to the pins, if at all... Meaning, did you find a battery, or a capacitor, that would power the setting memory, while the unit is off? Is this the pin that has 5.4V? If not, what has 5.4V on it? You said that the main output has the same voltage reading on the multimeter as on the displays... So, please explain this to me..."And my potentiometer is good I’ve measured it." You say you measured it... Did you discover that these pots are analog, or digital. I am guessing analog, but I should not be guessing. If analog, what method did you use to test them, what were the readings and are the pots the same in both units?Don't worry, we will get a rhythm to how we communicate. But, do give it some thought.We are going to start testing inside the unit.Electroshock, death, explosion and damage are possible. If you are not sure you can manage the testing correctly, get a professional to do the job. You assume your own risk. I am not responsible for anything.Now, back to the two pots... I'm still assuming they are the analog type... We are going to test them thoroughly... I want you to take these readings, on both units... Make notes...Looking at the pot with the stem pointed at your face and its pins pointed down, we will call the pins 1, 2 and 3, from left to right.1) With the power off... Look for the pot's resistance value written on the body of the pot. It would look something like "B10K." "B" is the type of pot. "10K' is the resistance. What did you find? Check both; they might be slightly different.2) What is the resistance across the whole pot? This is the multimeter resistance reading from pin 1 to pin 3. Is this resistance approximately the stated value of the pot? If not, we have a bad pot.3) Now, take a resistance reading from pin 1 to pin 2. Write down the value. Take a resistance reading from pin 2 to pin 3. Write down this value and add it to the pin1/2 reading. Does this approximately equal the stated pot value?I really don't expect to find that the pots have failed, but we need to start somewhere and this will tell us if the pots are good.Warning: The next test is taken with the power on...I am guessing that the pots are controlling the base voltage of a transistor, or the gate voltage of a MOSFET. I am also guessing that they are configured as voltage dividers with pin 2 as the input, pin 3 as the output and pin 1 tied to ground.If the pots look like they are wired as I described... For each pot, read the voltage value from pin 2 of the pot to ground. Tell us the readings. Are they close to the same, or are they significantly different?We will stop there. Take your time and be extremely careful. Get us this information and we will move on.I am going to give you a little homework (grin)... I want you to watch some YouTube videos on how power supplies work. I think that for the age of yours, they all worked in a fairly similar manner... Yours looks to be a linear (uses a transformer... is not switch-mode,) analog unit with a digital display on the voltage side.The number one failure, IMO, in electronics are the electrolytic capacitors. Even if they have not bulged, or have not leaked, they can dry up and go bad.Do some research on cap failures. We are going to need to find a way to test your electrolytic caps, later. Generally, for a unit this old, I would change them all out with a high quality brand, just as a matter of maintenance.That's it, for now...

author: hpibmx
author_rank: Regular Contributor
timestamp: August 23, 2020, 01:08:13 pm
content: Quote from: t1d on August 19, 2020, 03:01:19 pmThank you so much for taking your time to help me out! and sorry for late reply.Yes I will try to do some research for manufacturer, and if I find them I'll request for it, if not, as you said its gonna take time but not impossible of course with your help.I have noted the parts u mentioned, I'll go thru them again, unfortunately I dont have ESR meter to measure my good looking electrolyte caps.. so I guess i have to skip that for now,i have measured my potentiometer and yes its 10k.Blue to Green Blue to Yellow and Green to Yellow before and after power!I have change one the potentiometer with another 10K and i saw some changes on my out put voltage, ranging from 6.8v to 15v. i could change it yes.then i thought of adding some load to see if my ampmeter is working too, i added this motor but the moment i started to run and i saw my ampmeter moved but then i got short buzz, then my output started to show 25v fix with no changesand i have measured my potentiometer again and readings are the last one in the picture.I have measured my voltage regulator (L7805C2T) input and output which was 12v in and 4.9v out before and after the short buzz sound.thank you again for reading this and helping appreciate it.

author: t1d
author_rank: Super Contributor
timestamp: August 23, 2020, 02:36:56 pm
content: A potentiometer is equivalent to a resistor divider network. For a 10K pot, the full resistance of ~10K can be measured across the two outer pins, pin 1 and 3. Then, the measurement reading from pin 1 to pin 2, plus the measurement reading from pin 2 to pin 3 must add up to ~10K. If not, there is something wrong with the pot.Because your pot readings are not adding up as expected and the circuit performed differently with a new pot, you will need to disconnect the pot's wires and test them out of the circuit. The remainder of the circuit may be effecting you readings.After you know that you have good potentiometers, you will want to take the voltage readings on pin 2. Take one reading with the pot dialed to fully off and another with the pot dialed to fully on.Testing the pots first is something of a sort cut. It is just my guess as to what may be wrong.After testing the pots is completed, we will take a step back and start a more organized investigation. Meaning, will follow the voltages within the circuit along their paths, until they stop. Wherever the voltage stops is likely the problem, or very near the problem.As for your caps, yes an ESR meter is the better way to test a capacitor, because it can see more problems than just low capacitance. But, do not discount the value of taking a capacitance reading with your multimeter. It can at least find caps that are bad, due to low capacitance.

author: hpibmx
author_rank: Regular Contributor
timestamp: August 25, 2020, 02:56:36 pm
content: Quote from: t1d on August 23, 2020, 02:36:56 pmI’ll be back again on my voltage readings and testing my caps with my multimeter.Thank you 😊

author: hpibmx
author_rank: Regular Contributor
timestamp: September 08, 2020, 02:52:27 pm
content: Sorry for late reply, but I have measured all my caps no short was found although my power resistor right beside my output pins turns so hot but the value is good which is 200ohm,What else should I do sir?

author: ledtester
author_rank: Super Contributor
timestamp: September 08, 2020, 04:40:21 pm
content: I think the best approach is to get a schematic.The HAIRUI website has a contact form at the bottom of the page:http://www.hairuisolderingstation.com/I would simply politely explain your situation and ask for the schematic. The 1502AD seems to be an older model and they advertise themselves as "focusing on soldering station" so they just might help you.Some observations and questions...The power supply seems to have a 5v / 15v switch. I think this means the output range is selectable between 5v and 15v -- is this right?What happens if you set the 5v/15v switch to 15v? What does the voltage read? Does the voltage knob change anything?One way the output could read 30.3V on a 15V supply is if the pass transistor is bad. The pass transistor looks like it is bolted to the back of the case.

author: t1d
author_rank: Super Contributor
timestamp: September 08, 2020, 08:42:23 pm
content: ledtester is correct about finding the schematic. It is going to be difficult for us to tell you exactly what to check, point to point, without it.The method for finding a problem without a schematic is to start at the beginning of the circuit (back of the unit) and check for voltages along the chain of components. The point where you stop finding the voltages that you would expect is likely the component that is the problem, or near to it. I can tell that ledtester is more knowledgeable at this than I am.Finding a component that has excess heat on it is another trick. A hot component is likely the component that is the problem, or near to it.But, reaching into a live circuit is dangerous. The trick for that is to turn the circuit off, drain the capacitors and brush a little isopropyl alcohol on the components on one section of the board. Then, turn the circuit back on and look to see where the alcohol evaporates first. Repeat, from section to section, until you find the spot where the alcohol is flashing off quickly. This point is where the heat is.This method has dangers, too. Alcohol does not have a flame that you can see. If something in the circuit sparks and ignites it, you may not know it. So, only do small sections and look, do not reach.It may take awhile for you to find the problem, but do not quit. You are making progress, when you find out what isnotthe problem. And, you are learning from your efforts.If you can not acquire the schematic, then post lots of pictures of the circuit board. Make the pictures form a chain of the circuit, so that we can see the components in detail, from the beginning of the circuit to the end.

author: hpibmx
author_rank: Regular Contributor
timestamp: September 10, 2020, 04:25:15 pm
content: I would really appreciate it, thank you,It was not changing at all I was reading 13v if I’m not mistaken but then after connecting a load not shoes 25.4v and not changing at all.And the know was changing before I added a load.I’ll check again closely the bloated transistor again.

author: hpibmx
author_rank: Regular Contributor
timestamp: September 10, 2020, 04:29:30 pm
content: Yes sir you’re right, I’ll use that to find any other hot component, aside of my power resistor.I hope we can get the schematic so I’ll be able to use it.

author: hpibmx
author_rank: Regular Contributor
timestamp: March 02, 2021, 06:02:03 am
content: Hi,just a quick update, I've fixed them both by replacing my power transistors,works completely fine now just that output current has been limited, any ideas for that please?thank you.