New power supply PS305D not regulating and running over voltage and current - Page 1
Forum section: electronics / repair

author: Tronto
author_rank: Contributor
timestamp: June 12, 2015, 05:31:28 pm
content: Hi EEVBlog;I could use some help if you can spare some time. Not really sure what the process on this forum is, so apologies if I get this wrong.I have a brand new Longwei branded PS-305D-M 30V 5A bench power supply.So when the PS started at first, it was ok for a few seconds then the smell of magic smoke and then it showed 49.3V and 3.5 or 6.5A depending on the Hi/Low switch.Now on startup, it is ok for a second then ramps right up to the 49V 6.5A.There is a burnt out resistor at R144who's value I do not know(68?, thanks gman4925). This resistor is one of two that feed non inverted input 1 to an LM324N.There is a resistor adjacent to the LM324N in a spot marked D107. Not sure if label or resistor is wrong. The resistor is 150K. Suppersready has a diode at D107 on their board, but gman4925's board has a resistor like mine.I've been working through the system slowly trying to understand each part. I'm also using photos to make an image of the traces with components.Power to the board comes in at the bottom left and gets turned into +27 and --27V at the rectifier. Power then runs through an L7815CV and L7915CV which should regulate to +15V and -15V. From there power goes to power the LM324N quad op amp and HA17741 op amp. There is a short in between +15v and -15v somewhere.The power pins 4 and 11 show a short (0.5 ohms) on the LM324N. When I test a different LM324N out of circuit there is no connection between the power pins.I have replaced R144, L7915CV, D107 and the LM324N. Before powering on, the 15v system had a healthy resistance between + and -. Powering on produced the same symptoms as brand new and for 30 seconds everything worked perfectly until R144 burned out again and a short appeared in the 15 volt system.Thanks for reading and I appreciate any comments.EDIT: Higher Res images herehttp://imgur.com/a/YUEIGEDIT2: The big power transistors 2N3055s never get warm. R106 150? is starting to burn up. I've added to the Imgur album an image that I have been working on for my use understanding the circuit.EDIT3: Schematics from Longwei are attached.EDIT4: Found a PNP A1015 in a spot for an NPN transistor at Q104. Anyone know what is supposed to go in Q104? Schematics don't have values on them.

author: gman4925
author_rank: Regular Contributor
timestamp: June 13, 2015, 01:58:09 pm
content: I have the same power supply.R144 is 68?

author: Tronto
author_rank: Contributor
timestamp: June 13, 2015, 06:27:38 pm
content: Thanks for posting a pic of your control board.I see you also have a resistor in D107 (just under the LM324).I'm still having a heck of a time figuring out the problem.

author: IanB
author_rank: Super Contributor
timestamp: June 13, 2015, 06:35:10 pm
content: If there was smoke when you turned it on it suggests a short somewhere. Presumably the unit passed a smoke test in the factory before it was shipped, so it would be an idea to check for shipping damage. Did anything come loose or get bumped out of position during shipping? Did any component leads touch something they shouldn't touch?(Note: too late now, but it is a good idea to open up and inspect items received before turning them on just in case there has been anything bumped or disturbed during shipping, especially if the the item was not carefully packaged.)

author: Tronto
author_rank: Contributor
timestamp: June 13, 2015, 06:40:20 pm
content: Was packed well, no damage to box or power supply.I did not find anything loose or shorted inside. Went over the pcb carefully looking for bridges or leeds touching and there were none.There are no missing or extra screws.Thanks for the suggestion.Quote from: IanB on June 13, 2015, 06:35:10 pm

author: mikerj
author_rank: Super Contributor
timestamp: June 13, 2015, 08:58:24 pm
content: The soldering on the TO220 voltage regulator closest to the edge of the PCB looks very poor, check this for dry/broken joints.

author: Tronto
author_rank: Contributor
timestamp: June 13, 2015, 09:44:19 pm
content: Yeah, it looks poor in the picture, but it's actually ok.Quote from: mikerj on June 13, 2015, 08:58:24 pm

author: jaycee
author_rank: Regular Contributor
timestamp: June 13, 2015, 10:23:09 pm
content: I take it you checked that the pass transistors are not shorted ?

author: Tronto
author_rank: Contributor
timestamp: June 14, 2015, 01:50:36 am
content: Quote from: jaycee on June 13, 2015, 10:23:09 pmWell, I'm narrowing it down.Power to the board comes in at the bottom left and gets turned into +27 and --27V at the rectifier. Following the positive side through the L7815CV gives +15V out, but following the negative through the L7915CV and I get +15, rather than -15.Either the L7915CV is dead or I have a short somewhere.UPDATE: the power pins 4 and 11 show a short (0.5 ohms) on the quad op amp LM324N. When I test a different LM324N out of circuit there is no connection between the power pins.

author: suppersready
author_rank: Newbie
timestamp: June 14, 2015, 09:26:19 pm
content: Hi Tronto,I can confirm that in my PS305D D107 is a diode and is a 1N4148.

author: Tronto
author_rank: Contributor
timestamp: June 14, 2015, 09:28:09 pm
content: Quote from: suppersready on June 14, 2015, 09:26:19 pmThanks suppersrady.

author: Tronto
author_rank: Contributor
timestamp: June 15, 2015, 07:43:36 pm
content: This morning I replaced a 4.7 uF cap that looked off. Actually replaced all the 4.7 uF caps.Also replaced both L7815 and L7915 and put a proper diode in D107.The system ran fine but the output voltage ranged from 0.1 to 0.3 volts rather than 0-30 V. Also, R144 was getting really hot, so still haven't found the problem.Reference voltage off of pin 1 of the LM741 is good (compared to the schematic) as is the voltage at C108.

author: Tronto
author_rank: Contributor
timestamp: June 16, 2015, 12:05:37 am
content: I just confirmed that U103 which is a TL431A is good by pulling it and using this circuit on a breadboard.http://www.electro-tech-online.com/attachments/tl431_voltageregulator-01-gif.74473/

author: Tronto
author_rank: Contributor
timestamp: June 16, 2015, 12:44:49 am
content: Quote from: gman4925 on June 13, 2015, 01:58:09 pmHey, could you please confirm the value of R144. The manufacturer told me 680 ohms so I just want to double check.Thanks

author: gman4925
author_rank: Regular Contributor
timestamp: June 16, 2015, 03:21:01 pm
content: Quote from: Tronto onYesterdayat 20:44:49I'm 90% sure it was 68 ohm, though I measured it in circuit.I read it as BLUE GREY BLACK GOLD GOLD off the markings as well, which matched.If you look at the pic the fourth band is definitely not black.If I remember I will take another look tonight.

author: gman4925
author_rank: Regular Contributor
timestamp: June 17, 2015, 01:37:51 pm
content: Checked again, it's 68 ohm in my one.

author: Tronto
author_rank: Contributor
timestamp: June 17, 2015, 05:59:56 pm
content: Quote from: gman4925 on June 17, 2015, 01:37:51 pmIs your PS working?I'm now a bit confused. Guess I'll ask Lonwei to confirm the value as they told me 680 ohms in email.

author: gman4925
author_rank: Regular Contributor
timestamp: June 17, 2015, 06:01:56 pm
content: Mine works fine, though I've not really tested it much past 1A.

author: Tronto
author_rank: Contributor
timestamp: June 18, 2015, 12:04:45 am
content: I found a PNP A1015 in Q104, which according to the schematic should be a NPN transistor. Hmm

author: gman4925
author_rank: Regular Contributor
timestamp: June 18, 2015, 12:33:24 am
content: Q104 is A1015 in mine too

author: Tronto
author_rank: Contributor
timestamp: June 18, 2015, 12:35:43 am
content: Quote from: gman4925 on June 18, 2015, 12:33:24 amthanks so much for checking. The schematic sure looks like it should be a NPN.Edit: I think the schematic just has the wrong symbol Looking at the connections PNP looks right.

author: broxsoft
author_rank: Newbie
timestamp: December 24, 2017, 01:23:49 am
content: Tronto, It has been awhile since there was a post here. Did you fix the power supply? Moreover, someone says they have a schematic for it? Any chance I can get a copy? I have just started having problems with my PS and it has the Longwei PS-303D-M board. Thanks.

author: jaycee
author_rank: Regular Contributor
timestamp: December 24, 2017, 12:00:45 pm
content: Looking back at this thread, and having seen many similar PSU's around.. I would only ever be tempted to buy one of these for around the £40 mark - simply to get the case and the transformer. The rest I would gut and replace with my own control circuitry, and a proper heatsink on the back!!

author: higopi
author_rank: Newbie
timestamp: February 24, 2018, 08:56:52 am
content: Hi TrontoI have EXACTLY the same issues with a new power supply that I purchased. Did you manage to resolve it ?Thanks

author: smooth_jamie
author_rank: Newbie
timestamp: April 08, 2018, 08:32:28 pm
content: Hi All. For anyone still looking, I had the same problem and I fixed it very easily after testing/probing a few diodes.I did the obligatory visual check. No blown caps or obvious signs of burning so started checking components starting at the DC output. Used the diode checker to test all the diodes and found a 1N5399 rectifier diode failed to short circuit.Replaced with a 1N4007 and this solved my problem. From memory it was D108. It is right next to the two output wires going from the main PCB to the output binding posts

author: dijkmane
author_rank: Contributor
timestamp: September 25, 2018, 06:00:47 pm
content: hello,for future reference.i had this unit with same problem, giving full rectified main input to output, not beeing able to adjust it.48V on output leads.also R105 is getting very hot.schematics as posted here where usefull.found one output transistor, a 2N3055, in full short.after replacing both ( to keep same brand/batch ) its working again.found mistake on PCB print, there are 2 C104 on layout, one should be C111.unit is lacking fuses, i will build in mains and secondary fuse for more safety

author: cosmin1
author_rank: Regular Contributor
timestamp: March 14, 2019, 06:27:42 pm
content: Hello.I have the same power supply and replaced the voltage indicator with green one. Problem is, now the current indicator is much brighter than the voltage.Can you give me some idea how can i increase the current through the voltage LED indicator or decrease it through the current indicator?

author: syedhussain_alishah
author_rank: Newbie
timestamp: October 09, 2019, 07:06:21 pm
content: Hi,Dear you have full photo of this pcb??