Help for open frame dual output PSU - model FSP012-2P01 - Page 1
Forum section: electronics / repair

author: Cavia
author_rank: Newbie
timestamp: April 05, 2023, 04:02:07 pm
content: Hello,I'm trying to repair an old (retired) open frame dual output PSU: model FSP012-2P01.This model is commonly found inside old Extron devices (i.e. 201Rxi, DVI-RGB 150, and more). Since Extron does not provide any schematics (as per company policy), I need some experienced soul that can help me debug the problem (I also contacted FSP asking for the schematics of this retired device, but I got no answer so far).Let's jump into some details. The model FSP012-2P01 (produced by FSP) is a dual-output PSU with the following outputs: 5V at 1.5A and -5V at 1A.Note: Regarding a possible substitution with a similar device, the only candidate I found is a Meanwell PD-2505 which unfortunately does not fit into the Extron case. I may keep the power supply outside the Extron in a 3D printed case and route the cables to the logic board, but I would like to keep this option as the last solution.The faulty PSU is showing the usual ticking [tik tik tik] cyclic-reset behavior. I've already replaced all the capacitors, checked the diod+high resistor (measured 15kOhms) line next to the transformer and looked for shorts. Could not find the problemAs I'm not an electrical engineer (or expert), I'm a bit lost now. Any help would be greatly appreciated. If needed, I can setup a video call with anyone who wants to help me live (I have a streaming position at my bench).Here are some pictures of the PSU.Thanks all.

author: voltsandjolts
author_rank: Unknown
timestamp: April 05, 2023, 04:25:46 pm
content: Check as many passives and diodes as you can in circuit.Then I would probably try replacing Q10 and U30 if you can get new ones. What are they?

author: wraper
author_rank: Unknown
timestamp: April 05, 2023, 04:29:57 pm
content: You say replaced all caps, but did you replace small electrolytic cap on primary side, just in case? Check if diodes on secondary side are not shorted.

author: wraper
author_rank: Unknown
timestamp: April 05, 2023, 04:34:08 pm
content: Quote from: voltsandjolts on April 05, 2023, 04:25:46 pmQ10 would simply short and blow the fuse when fails. Also does not look like UC3842 issue. I suggest checking its datasheet for typical schematic. This also could be an issue with resistor on it high voltage power input or faulty diode from auxiliary transformer winding.

author: FIXITNOW2003
author_rank: Regular Contributor
timestamp: April 06, 2023, 06:44:31 am
content: some info hereFSP012-2P01https://goughlui.com/2015/07/12/salvage-teardown-extron-dvi-rgb-150-iluv-usb-charger-nvidia-9600gt-oem/

author: Cavia
author_rank: Newbie
timestamp: April 06, 2023, 06:23:57 pm
content: Hello@wraper, @voltsandjoltsDiodes report:All diodes work as intended.@voltsandjoltsQ10 is aFairchild SSS4N60B600V N-Channel MOSFETU30 is aSTMicroelectronics UC3842BPWM ControllerSchematics are attached.

author: Cavia
author_rank: Newbie
timestamp: May 09, 2023, 04:01:05 pm
content: Hello, some updates.I've replaced Q10 and U30 with new parts. The PSU is still cycle-resetting.Interesting behavior: I didn't work on this circuit for a long time. When I took it again, I immediately replaced Q10 and tested the PSU (first cold boot in a long time).The PSU started perfectly (no reset) and it was outputting steady 5V DC and -4.7V DC. As soon as I turned it off, and on again, the PSU was showing the issue again and never restarted correctly.Not sure what to do next. Any help?Thanks.

author: Cavia
author_rank: Newbie
timestamp: May 12, 2023, 11:04:22 pm
content: Updates:Following some suggestions on Youtube, I measured the DC voltage across the diodes in the secondary part of the circuit: D60 is carrying 5V (ofc it's oscillating due to the circuit reset), while D50 measures 0.330V. The transformer is outputting on all 5 pins 87.1V AC.In the feedback circuit, the DC voltage across the input pins (1,2) of the optocoupler PC20 shows a fluctuating 1.0-1.1V, while the output pins shows an unreadable DC voltage (random values).If anyone can spot the problem, or a direction to debug it pls let me know.Thanks.

author: Cavia
author_rank: Newbie
timestamp: May 15, 2023, 01:47:05 pm
content: Updates:I've replaced the Optocoupler PC20 (a classic model 817A), and the Programmable Shunt Regulator IC30 (model KA431A). Still same issue. I'm not sure if the problem is in the feedback area, or the secondary part (after the transformer).This is challenging

author: jd213
author_rank: Newbie
timestamp: May 19, 2024, 05:21:13 pm
content: I have the same PSU in my Extron RGB 160 xi that recently stopped working.Replaced all the caps (all of which were cheap brands, around half of the caps were way out of spec when tested out of circuit).5V rail seems to be working but not the -5V rail. Some of the resistors and diodes didn't test right in-circuit, will try testing them out of circuit later and report back.

author: jd213
author_rank: Newbie
timestamp: June 01, 2024, 11:42:47 pm
content: All of the resistors and diodes seem fine when tested out of circuit. I believe the transistors (Q10 and IC30) are also fine since my TC1 component tester recognized them.Would anyone have a guess as to what else could be the problem? Should I test the film/ceramic capacitors?Not sure how to test U30 (UC3842B) or PC20 (marked L0421 817A, not sure what kind of component this is). Are either of those responsible for the negative voltage?Thanks in advance.

author: Chris56000
author_rank: Frequent Contributor
timestamp: June 02, 2024, 06:17:18 pm
content: Hi!. . .PC20 is a Sharp PC817 Opto–Coupler!. . .The best way to debug any power supply based on the UC384x series is simply to measure the d.c. voltage between pins 5 and 8 – pin 5 is power ground and pin 8 is the Reference Output, connected internally to the protection circuits within the chip!. . .If the voltage between pins 5 and 8 is 0.5 V or less, either the chip is locked out due to an under–voltage lockout, a short within the chip itself or across the supply decoupling between pins 5 and 7!. . .A pulsing voltage indicates the chip is tripping due to either excess current, most likely a defective output rectifier diode or capacitor, (open–circuit or high e.s.r. output capacitors also cause tripping!), or (rarely!) short–circuit turns in the power switching transformer!. . .Loss of, inadequate negative feedback from the output side can be due to PC20 (Sharp PC817) going low–emission, will also cause the chip to persistently trip, substitution with a new replacement will prove this!. . .If the voltage between pins 5 and 8 is 5V approximately and the board is not producing power, then the components between pin 6 (output) and the gate (usually the left–hand leg) of the power switching MOSFET may be defective – a defective MOSFET can destroy these components , commonly a resistor from 22–100 Ω and a protective zener diode of about 15 V 400mW between gate and source of the power MOSFET.. . .Dry–joints where the transformer lead out wires are soldered to the bobbin/PCB pins is another common cause of persistent tripping or failure to start, particularly if that lead–free muck has been used to make the original board, as is very likely – removing the transformer from the PCB and resoldering the leadout wires where they connect to the bobbin pins is the cure for this!. . . Rectifier diodes do sometimes short out as soon as power is applied and then promptly test OK when power is removed – binning the lot and fitting suitable replacement types will eliminate this possibility – the "UF" series is suitable as replacements in most cases!Chris Williams

author: Harry_22
author_rank: Frequent Contributor
timestamp: June 02, 2024, 08:21:43 pm
content: Hi!The PaintNET diagram will help you.

author: jd213
author_rank: Newbie
timestamp: June 04, 2024, 07:45:36 pm
content: Quote from: Chris56000 on June 02, 2024, 06:17:18 pmAh, thank you!! Looks like it is under 0.5V, but not pulsing. I'll try replacing UC3842B and PC817A then, doesn't look like they will cost much.QuoteOh yeah, I've dealt with lead-free solder before... thankfully this board is from 2005 and seems to use leaded solder.QuoteHmm, when read in-circuit, D21 read 150 both ways and D50 read at 43 both ways, but the readings were the same both with and without power to the board. Should I assume they're bad?Thanks again!

author: jd213
author_rank: Newbie
timestamp: June 04, 2024, 07:47:03 pm
content: Quote from: Harry_22 on June 02, 2024, 08:21:43 pmYes indeed, that came in quite handy when testing the board when powered on! Thanks a lot!

author: jd213
author_rank: Newbie
timestamp: February 24, 2025, 12:47:21 am
content: Unfortunately the -5V rail on this PSU is still faulty (unstable output less than -1V), even after replacing the caps, UC3842B, optocoupler, and diodes that seemed like they might be faulty. Didn't find any other components that were faulty (even out of circuit or under load), so sot sure what the problem is.

author: reodor
author_rank: Contributor
timestamp: February 24, 2025, 07:54:47 am
content: Since the positive rail is working, that does not leave many components left as suspects. From what I can tell, the positive output is the one that is used for regulation feedback, and the negative is simply an additional winding on the transformer that gets rectified by D50 and then smoothed and filtered by some passives.Diodes can fail in a way where they test OK on the multimeter, but still fail when they see higher current or voltage. Have you already replaced D50? If it still does not work, it could be a transformer problem.

author: wraper
author_rank: Unknown
timestamp: February 24, 2025, 12:08:20 pm
content: Quote from: jd213 on February 24, 2025, 12:47:21 amReplacing those makes no sense. If other voltage is OK, then problem very obviously is on secondary side, and not in feedback loop, it's not even used for negative rail. There's only a few parts responsible for this. -5V winding, diode, a few capacitors. Frankly, astonishing you could not fix it in 2 years, as it should take like 5 minutes to repair.

author: jd213
author_rank: Newbie
timestamp: February 24, 2025, 03:31:18 pm
content: Quote from: reodor on February 24, 2025, 07:54:47 amYeah, have replaced D50 and D60. Was starting to suspect the transformer myself, don't see any burns or anything on it though. Guess I may as well get a new PSU if the transformer is bad.

author: jd213
author_rank: Newbie
timestamp: February 24, 2025, 03:37:40 pm
content: Quote from: wraper on February 24, 2025, 12:08:20 pmRight, another poster mentioned those parts, so I just threw them into a digikey order, they were cheap enough.I personally haven't been working on this for 2 years, I just piggbacked off of a previous poster's thread. I've just been tinkering on this a few times since last year as part of my soldering hobby, I don't have much experience with PSUs.

author: wraper
author_rank: Unknown
timestamp: February 24, 2025, 04:47:49 pm
content: Quote from: jd213 on February 24, 2025, 03:31:18 pmAre you sure you did not put D50 in opposite direction? Winding and series inductor can be easily checked with multimeter, they should measure shorted. Anyway, you should measure the voltage right after the diode, maybe there is some solder joint cracked after it.