GW GPC2405 DC Power Supply Repair - Page 1
Forum section: electronics / repair

author: Adler
author_rank: Regular Contributor
timestamp: November 16, 2012, 03:37:39 pm
content: Hi everyone,FIRST POST!I will start by saying that I am not an EE, just an enthusiastic amateur.I scored a DC power supply from the trash room at my university and I believe it is broken and, since I have no other DC power supplies, I want to fix it.When powered on, it will provide a constant 5 volts from the fixed voltage output but I get nothing from the master or the slave. The gauges do not move either.So I popped it open and poked around insideI tried testing the relays you see there and when I applied voltage to the coil, it did not switch. So I desoldered it and bench tested it and found the same. Put on around 15v and it did nothing.Am I right in assuming that the relays are probably bad? Or am I going about this wrong?Thanks for any help.

author: ElektroQuark
author_rank: Unknown
timestamp: November 16, 2012, 03:42:38 pm
content: It works with a 24v nominal in the coil, 15v could not be enought.

author: Adler
author_rank: Regular Contributor
timestamp: November 16, 2012, 03:48:30 pm
content: Quote from: EdoNork on November 16, 2012, 03:42:38 pmI was worried that might be the case. Unfortunately, I don't have a DC power supplyI'll look around for a way to generate 24v.

author: Monkeh
author_rank: Super Contributor
timestamp: November 16, 2012, 03:57:42 pm
content: Quote from: Adler on November 16, 2012, 03:48:30 pmThree 9V batteries in series will be fine for operating a relay.

author: Adler
author_rank: Regular Contributor
timestamp: November 16, 2012, 04:15:52 pm
content: Quote from: Monkeh on November 16, 2012, 03:57:42 pmGood call, I now know that the desoldered relay is fine. One of the relays still in circuit works fine and the other two do not. Should I desolder them to check them, or can relays be tested in circuit?Thanks for the help so far.EDIT: nevermind, I desoldered aother and found that it was fine. Where do you guys recommend I look now?

author: mariush
author_rank: Super Contributor
timestamp: November 16, 2012, 05:55:41 pm
content: Well, I'd suggest to go step by step measuring AC and DC voltages and see where everything stops.Check if there are fuses for each channel (sometimes they look like resistors)Check the AC voltage coming out of the transformer (the wires in the connector), see if they're close to what's written on the transformer itself.There should be a bridge rectifier for each channel converting the ac voltage coming from transformer into DC voltage, but that may be after the relays.. the pair of relays on each channel link multiple secondaries of the transformer to get low or high voltage.

author: hlavac
author_rank: Frequent Contributor
timestamp: November 23, 2012, 09:07:55 pm
content: Thou shalt test voltages!It would help if you had complete schematics of it.

author: Adler
author_rank: Regular Contributor
timestamp: December 02, 2012, 08:08:00 pm
content: Quote from: mariush on November 16, 2012, 05:55:41 pmIt took me a while to get to all of these.No fuses that I could see. Though they may have just eluded me.I tested the voltages and they seemed to be alright.There is only one bridge rectifier that I could see. You can spot it on the 3rd picture of my first post, on the right.Quote from: hlavac on November 23, 2012, 09:07:55 pmYes, that would be super helpful.After doing all the things suggested I decided it was time to do or die. So, I took it apart fully.I found this hiding behind some wires...Think I may have found my culprit. Little 100uF, 50v cap.

author: Adler
author_rank: Regular Contributor
timestamp: December 02, 2012, 09:55:23 pm
content: Found a comparable cap in my parts pile and went to put it in only to find this.Its a little hard to see in the picture but the traces are coming off of the PCB and are broken in that area and in one other area.Worked some magic and voila.All better.I swapped the cap as well and closed it up.Now it half works. New issues.It is not producing the correct voltage at all. Instead of getting 15v, I am getting 0.015v.

author: Monkeh
author_rank: Super Contributor
timestamp: December 02, 2012, 09:57:29 pm
content: Thou shalt test voltages.Start at the transformer and work your way in.

author: ElektroQuark
author_rank: Unknown
timestamp: December 03, 2012, 09:58:01 am
content: It looks like the big Jamicon capacitor is bad too, it has leaked. Change it.

author: hlavac
author_rank: Frequent Contributor
timestamp: December 03, 2012, 05:40:36 pm
content: This kind of damage is from excessive current thru the trace that makes it so hot it comes off the laminate. Was the bad cap connected to it or is something else shorted out? Look where the damage ends...

author: Adler
author_rank: Regular Contributor
timestamp: December 03, 2012, 06:17:00 pm
content: Quote from: hlavac on December 03, 2012, 05:40:36 pmBoth damaged areas are connected to some caps. I couldn't find any caps like the big Jamicon in my spares so I ordered a couple.