Trying to repair a INSTEK GPR-30H10D 0-300V DC power supply? No output. - Page 1
Forum section: electronics / repair

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: August 19, 2015, 12:00:46 am
content: Bought this power supply broken as figure could save some money, if it can be repaired. Issue with it is no output. So did check output stage and is fine, no shorted output's. Then check driver stage and is also fine. Then pulled OP-AMP 'U205" and power supply started to work, was able to get full voltage control, but no protection, or current control as OP-AMP was removed from circuit. At least it tell me power supply section isn't the issue as it worked fine and was regulated with no unusual noise and wasn't drawing excessive current. So assume OP-AMP maybe bad and replaced it, then same issue no output, or voltage control at all again. Tested voltage going to OP-AMP and notice it was at 30 volts DC on that rail, but seem to be within spec, as data sheet for OP-AMP say it can handle 44vdc max. Checked capacitors ESR and all check good and check diode in circuit and all are good. So stuck on this one, as don't have the schematic diagram and try finding one with a service manual with no success.Any help from someone familiar with this power supply would be highly appreciated.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: August 19, 2015, 12:08:39 am
content: Here picture showing board and that it works with that OP-AMP U205 removed from circuit.

author: singapol
author_rank: Frequent Contributor
timestamp: August 19, 2015, 02:45:15 am
content: Check the other 2 opamps and circuit with adjust pots, most probably there is a relay to switch to high voltage and protection circuitry. Measure opamp plus/minus inputs...they should have equal voltages if they are in good condition.Measure the resistors...the big ones too.Good luck.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: August 19, 2015, 08:12:26 am
content: Issue is with input of U205, doubt the OP-AMP as was changed. When I bypassed protection power supply works, but no current control or constant current and red C.C. light stay on. So enable protection circuit, but disconnect input to OP-AMP U205 and C.V. light come on and then power supply work without current control. This tell me amplifier is fine and OR gate is fine and voltage comparator works fine. Issue has to be within the input side of that OP-AMP. did check resistors and all within tolerance, checked ceramic capacitor and tested good. Did test reference voltage source and seem off I did check diodes in second rectifier circuit that provide the reference voltage and were tested good same with q101 and q102. Don't have a service manual, so taking a shot in the dark on this one. I did also swap out OP-AMP U202, which is the reference amplifier. I don't suspect it the issue as won't work with voltage control ether. Narrow it down to current input side of current comparator OP-AMP.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: August 19, 2015, 09:12:03 am
content: C212 is bad 301j ceramic capacitor. It right to the current controls and input of OP-AMP. Have to order it, as don't have that value.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: August 19, 2015, 03:35:42 pm
content: Found the issue, turn out to be simple issue, bad current pot. It was crack and broken on the PCB. Figure it out after pushing up on the panel that hold the pots in and it started to work if you push force on it. So though maybe cold solder joint on that board. It turn out the post was completely broken. as when removing board it fell apart with no force.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: August 19, 2015, 04:05:21 pm
content: Just rig in a pot for now for testing and it works now, just have to wait for new one to show up. Problem is solved

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: September 05, 2015, 08:14:41 pm
content: Did fix the no output problem with this PSU, but notice another issue and that is only getting anywhere from 272vdc to 280vdc max output. It not the outputs as check every one and tested good and well within spec to data sheet, so not the issues. Even swap them around to make sure. Did also check all diodes in that circuit for each output stage and all of them tested good. Did also check all taps on transformer and getting 50 to 54 VAC on each output as they all have a bridge rectifier and getting 68vdc to 71vdc out on all after bridge depending on line voltage, so is also not the issue. Check all resistor and well within tolerance, even pulled some out of circuit and no issues could be found. Check every components in second board smaller board and all tested good and outputs are good. Tested both NPN transistor in amplifier circuit and both tested good and seem to be getting proper voltages. Check other diode on board and not a single bad one. At this point at a loss with this thing.Getting 72 VDC on all drivers, so is not the issueHere the issue only between 280vdc to 270vdc on output max, depends on line voltage. This is suppose to be a 300vdc power supply? It not op-amps, or reference that the issue.Picture of main board.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: September 05, 2015, 08:18:50 pm
content: Already try finding a schematic and service manual and can't anywhere. Only found manual which give you a breakdown and calibration steps, that about it.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: September 06, 2015, 12:18:59 am
content: Going to suspect, one, or two outputs are failing. They all tested good, but notice when power supply under load max voltage drops when heated, then when cooled goes back up. It isn't thermal paste as removed old stuff and re applied new stuff when putting the outputs back on heat sink. So do think one, or more is going bad and breaking down under load. Only thing can come up with as check everything else and appears no other issues.

author: jitter
author_rank: Frequent Contributor
timestamp: September 06, 2015, 07:34:12 am
content: As these PSUs can be pretty old, I was going to suggest looking at the state of the electrolytics, but then I saw the datcode 0826 on the pcb... this unit isn't that old.Maybe this PSU has been used hard and long.I'd probably check for ripple voltages under load with a scope, cold and hot. Perhaps there's something on the way out, as you say.Or if this doesn't find the cause, I might get the PSU nice and warm and then go around with small bursts of freeze spray on individual components.And if that didn't do it I might "torture" the unit by setting the output to a voltage under load that causes the most internal dissipation and cover it with a blanket.Please note that this procedure is not reccommended. Even though the design of the PSU should prevent mishaps (thermal overload protection and stuff like that), better not leave it unattended. Use it as a last resort if all efforts fail. The heat stress that this causes might take the failing component(s) over the edge, hopefully not with catastrophic results.

author: Shock
author_rank: Super Contributor
timestamp: September 07, 2015, 06:20:24 pm
content: What is the current set to? Can you put it on max and reproduce the test. As well, make sure it's not your temporary fix causing issues.

author: OldSchoolTechCorner
author_rank: Frequent Contributor
timestamp: September 08, 2015, 06:24:49 am
content: I replace the OP-AMP and the broken current potentiometer. So that work now, but started the other issue afterwards. I narrow it down to a one failing output, it was tricky to find, as tested good out of circuit, only that one fails when loaded. Others test fine. Will update once I get the replacement. I think temporarily disabling the current protection and pulling a 1 amp, could have kill it. Plus someone was in it before, as bought it used on eBay broken and had lack of thermal paste on a couple of the outputs and a few to much thermal paste and a mess. Had to completely remove every one and put new isolation pad and fresh thermal paste. That why I was shock one was bad, as they all tested good out of circuit at rated specs from datasheet I pull on them.