HELP! SMPS low output voltage! - Page 1
Forum section: electronics / repair

author: gojunnyo
author_rank: Contributor
timestamp: March 30, 2020, 07:40:31 am
content: I have problems with this Philips FWP3100 mini Hi-Fi sound system because it does not produce any sound at all.I dismantled the said unit, and come across the AMP+Power Board, the said board should have the following voltage rails, 5V, 12V and 32V.(According to the schematic) Unsurprisingly, the 32V rail is to be fed to the amplifier but as I voltage test across the filter caps, it only shows around 1V.Here's what I've tried so far:Connected the 5V rail to the optocoupler (IC700) responsible to feedback "ON" (to turn on the oscillator chip SSC6200)Removed the voltage feedback optocoupler (IC702) to test if it is working, and after knowing it is functional, I returned itResistance across the FB pin of the oscillator chip and the ground is 63.5 Ohms, the same resistance across the other side of the optocoupler (IC702)Visual inspecting all the electrolytic caps, no bulging sightedChanged the TL431 as an assurance maybe it could be the problemResistance across the 32V electrolytic caps is 99.6 Ohms (maybe this could be the problem?)Could it be the feedback circuit causing the problem? or could it be the oscillator chip is bad?Btw the datasheet of the oscillator chip (SSC6200) is japanese, and I am a noob at looking at specifications.Any help would be much appreciated!Here's the link for the service manual, it contains the schematic diagram of the said unithttps://elektrotanya.com/philips_fwp3100_05_55_sm.pdf/download.htmlHere's the link for the oscillator chip responsible for driving the bigger chopper transformerhttps://datasheetspdf.com/pdf-file/822984/Sanken/SSC620D/1

author: xavier60
author_rank: Super Contributor
timestamp: March 30, 2020, 08:26:09 am
content: 63.5 Ohms is too low.I have occasionally found spiked FB pins on SMPS controller ICs.

author: gojunnyo
author_rank: Contributor
timestamp: March 30, 2020, 10:31:37 am
content: Thanks for the reply! So its broken then? and If it is, any recommendations of substitute ICs, I cannot find these specific chips anywhere

author: PKTKS
author_rank: Super Contributor
timestamp: March 30, 2020, 10:36:15 am
content: If you have a ~30V proper PSU with auxiliary5/3/12V whatever to replace whole board..remove board and power via controlled external PSUIf your "LOAD" is not faulty you can diagnosis"ONLY" the power sectionIt may eventually require to strip some power rails...Whatever. You need to discard faulty load from faulty SMPSAnd you have 2 supervisors with 3 opto coupled sensorsPaul

author: gojunnyo
author_rank: Contributor
timestamp: March 30, 2020, 11:35:23 am
content: Sadly, I do not have any ~30V PSU lying around, and the AMP and Power supply section lies on the same PCB. Although I already tried powering the 32V rail with a 19V laptop charger, the amplifier worked but not to its full potential. My goal is to find the bad component to be replaced. I hope it would be worth the hassle.

author: PKTKS
author_rank: Super Contributor
timestamp: March 30, 2020, 11:49:27 am
content: Quote from: gojunnyo on March 30, 2020, 11:35:23 amTwo UPS batteries in series would do the trick about 24V.But you still need some CC/CV bench gizmo to workAnd probably "CUT" some tracesPaul

author: gojunnyo
author_rank: Contributor
timestamp: March 30, 2020, 03:58:31 pm
content: Thank you for your inputs!Due to quarantine protocols, all electronics shops are closed and the original IC is not readily available, Can I use pwm oscillator IC from other smps circuits? (forgive me if its a noob question, I don't have a vast experience on electronics yet)Also I want to utilize the existing NPN Mosfet and Transformer so if I were to design my own +32V smps, where do I start?Any help would be much appreciated!

author: gojunnyo
author_rank: Contributor
timestamp: March 30, 2020, 06:08:35 pm
content: Quote from: blueskull on March 30, 2020, 04:06:01 pmSir, assuming if I don't die, can you help me design a driver circuit for the original MOSFET and Transformer? Not just for repair actually, I wanted to know more!

author: wraper
author_rank: Unknown
timestamp: March 30, 2020, 06:17:54 pm
content: Quote from: gojunnyo on March 30, 2020, 10:31:37 amOrder on aliexpress or ebay. Search for sc6200

author: xavier60
author_rank: Super Contributor
timestamp: March 30, 2020, 10:50:39 pm
content: Quote from: gojunnyo on March 30, 2020, 03:58:31 pmI should have looked at the schematic earlier. There is a zener from the FB pin to ground. Has it been ruled out?

author: gojunnyo
author_rank: Contributor
timestamp: March 31, 2020, 10:16:27 am
content: Quote from: xavier60 on March 30, 2020, 10:50:39 pmThe schematic mentioned a zener diode, but there is no zener diode on the actual PCB

author: xavier60
author_rank: Super Contributor
timestamp: March 31, 2020, 10:43:22 am
content: What appears to be the SMD version,SSC620S, is abundantly available on ebay and Aliexpress.Fakes are always possible, I don't suspect it though.Maybe there is a good reason for the zener to be present.

author: gojunnyo
author_rank: Contributor
timestamp: March 31, 2020, 12:20:27 pm
content: Quote from: blueskull on March 31, 2020, 10:32:10 amThank you for enlightening me how the IC works, I am stuck here in Philippines and shipping time normally takes 1 month as I inquired, but if I were to design, any tips where to start? or the IC to start from? ICs that are readily found. I'll just remove the transformer and NPN and start building from there.

author: gojunnyo
author_rank: Contributor
timestamp: April 01, 2020, 11:51:57 am
content: Thank you all for your inputs! I will update you all whatever happens. I hope the risk would be worth it as much as it provides valuable experience. Keep safe!