GW-INSTEK GPD-3303S Repair/Troubleshooting - Page 1
Forum section: electronics / testgear

author: FrankChannel
author_rank: Newbie
timestamp: April 23, 2025, 11:29:04 pm
content: I have a GW-INSTEK GPD-3303S DC power supply that is malfunctioning. I'm hoping to revive it. Channels 2 and 3 are in an overload condition despite having no load connected.I found this posthttp://magicsmokecircuits.com/repairs/2020/03/27/gwinstek-gdp3303s.htmlwhere the author was able to repair their unit by replacing some diodes across the output rails. I was hopeful this would be my problem, however, those diodes appear to be functioning fine in my unit. I can't actually find any evidence of a short circuit on either of these channels.I checked all the fuses (there are many), and none of them appear to be blown. There are also many relays. I checked their coil resistances and those seemed fine as well.Any thoughts of where to turn next? I might re-assemble the supply and try updating the firmware if there are no suggestions for things to poke around with internally.

author: Nanitamuscen
author_rank: Contributor
timestamp: April 24, 2025, 03:07:58 am
content: What current is set? In my device, if you set a low current, the unit will be over-current.

author: srb1954
author_rank: Super Contributor
timestamp: April 24, 2025, 09:07:45 am
content: This is not an issue. My GPS-3303S behaves in exactly the same manner and it can be easily fixed by ensuring that the current limit is set to at least 3mA.A lot of power supplies, not just the GPS-3303A, exhibit this behaviour if the current limit is set at zero. Some power supplies even oscillate between an overload/no overload indication if the current limit is set too low even when there is no load on the output.This behaviour is most likely down to offset voltages and/or noise in the current sense amplifier and you also need to bear in mind that there is always a small internal drain on the PSU output from the output voltage sensing divider and this generally has to be allowed for when setting the current limit. Only the most exotic PSUs compensate for this small internal load allowing accurate current limit settings down to the lowest levels.

author: picburner
author_rank: Frequent Contributor
timestamp: April 24, 2025, 09:32:13 am
content: Looking at the photo, channel 2 and channel 3 are overloaded while channel 1 is not.For channel 2, what was said in the previous answers may apply, but channel 3 has a fixed limitation of 3A and is currently overloaded without load.

author: FrankChannel
author_rank: Newbie
timestamp: April 24, 2025, 10:20:28 am
content: I had already verified that the current limit for Channel 2 was turned up quite high. I don't think this is the issue. Also as picburner pointed out Channel 3 is in overload too, and the current limit for that channel is fixed internally.

author: picburner
author_rank: Frequent Contributor
timestamp: April 24, 2025, 03:11:22 pm
content: The service manual for your power supply can be found atElektroTanya.With the help of the manual it should not be difficult to locate the faults.CH3 is probably the least difficult part to repair because it includes fewer components.To repair CH2 it can be helpful to compare it with the working CH1 because it is practically identical.

author: FrankChannel
author_rank: Newbie
timestamp: April 24, 2025, 03:15:04 pm
content: Thanks for the pointer---I had already found that service manual before posting. Beyond checking the diodes, fuses and relays I'm not sure where to go next unfortunately. It's also hard to check anything with the unit powered on because I had to remove several cables just to access the front of the control board.

author: MathWizard
author_rank: Super Contributor
timestamp: April 24, 2025, 06:37:02 pm
content: After work I might make some of the circuit in a simulator. To see what those op-amp circuits are doing, I'd suggest you do the same, if you familiar with any simulators, or try on a breadboard if you have more PSU's, or even try calculating some of it. The exact parts shouldn't matter too much, and you can leave out the caps, to get a feel for the DC operating point. And then you can see the general behaviour of the op-amps, when the current /voltage on the sense resistors increases.If the basic's are good, like the voltage rails for all the IC's, etc. Then I'd check the voltage across the large 5W output sense resistors, like R1261/62 on page on page 81 of the schematics. Then check the output of U104, at a glance IDK what it's doing, but I think it's acting as a comparator, and will be either high or low, so +/-15V (or as close as it can get to the supply rails, so maybe +/-14V). Any little part around there could fail, and throw off the circuit, like C120 could be shorted, or just a solder joint breaks and some resistor like R1282 is no longer in circuit.Then that U104 signal goes to some sort of OR gate, and maybe it has a problem, and the comparator circuits work fine.You could check a lot of stuff unplugged, especially if you start removing PCB's, unhooking everything, and even removing large caps, it's time consuming, but I'll do it sometimes like when I don;t have a schematics and make my own, (and check the caps with an ESR meter at the same time). And I'd solder on lots of test leads, and even drill holes in the case, for powered testing.Ok so U104 is not used as a comparator, but it will kick-in and limit the output current as set. I started making a model, but there's ADC's, DAC's and an MCU meant to control it. So far I can't get the loop to regulate current or voltage, just w/ more opamps.