PITA Led sign SMPS - Page 1
Forum section: electronics / general

author: kolbep
author_rank: Frequent Contributor
timestamp: May 24, 2013, 10:27:40 am
content: Customer gave me an LED Sign to Fix. Piece Of shite Chinese junk.Anyway, problem Was That It Needed a crossover Rs232 Cable.Was Just a Out To Drop off With Customer, When Refused To Start Up.Smps board Is 200w 5v, But Is Only Giving Out 2v, Loaded Or Unloaded.Strange Thing Is, Unplug Ac, Then Abou 1.5 Second Later, u Hear the Transformer hissing, And the voltage goes Up To About 4.5v Before the Caps Discharge, Which Is About Another 0.3 Seconds..Have Paralled Another Mains filter Cap, With no Change.Uses A Ka7500 D4iver.Any Ideas where to LookThnks

author: Precisiontools
author_rank: Regular Contributor
timestamp: May 24, 2013, 10:42:04 am
content: Got an ESR meter? Check all the electros, particularly any small capacity units on the primary side.

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 24, 2013, 11:16:31 am
content: PTC?Look around for short circuits or hot devices or scorch marks.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 24, 2013, 01:19:14 pm
content: Nope, no esr meter.Will Just have To Swap out caps and seecant See any Shorts. resist Across Output Is 47ohms.pic attached.Thanks

author: HackedFridgeMagnet
author_rank: Super Contributor
timestamp: May 24, 2013, 01:39:27 pm
content: I meant components gone short, just in case you misunderstood.

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: May 24, 2013, 02:24:23 pm
content: Look for a low voltage electrolytic on the mains side of the PSU and replace it.There will probably also be a resistor from the + of this cap to the + of the main HV reservoir cap - check this hasn't gone up in value.

author: SeanB
author_rank: Super Contributor
timestamp: May 24, 2013, 04:15:47 pm
content: Check C71? near the power switchers, it is part of the start up oscillator. Check as well all the resistors, they tend to go open circuit. Check the transistors and diodes are not short circuit either.I have used an ATX supply in place of these, just wired it permanently on and used the 5V rail. Easier and cheaper to replace when it dies.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 24, 2013, 06:12:26 pm
content: Quote from: SeanB on May 24, 2013, 04:15:47 pmGetting an ATX supply that can do 200W (40A) on the 5V rail will be tough. The higher power supplies put the extra headroom onto the 12V rail(s). I picked a few random supplies on newegg ranging from 250W to 1600W and the 5V rail was anywhere from 14A to 25A.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 24, 2013, 06:30:13 pm
content: HiReplaced C11 (the one near the choppers)Still the SameThe KA7500B is getting 10v Constant.I have gone through all the resistors, diodes and semi's, and all looks in order.I then removed the 51Ohm Power resistor between the 5v and Ground Outputs. The Supply then went up to just over 5 Volts, and stayed there, Until I tried to reconnect the resistor, then it drops to 2v again.

author: kfitch42
author_rank: Frequent Contributor
timestamp: May 24, 2013, 06:41:10 pm
content: 5V/51Ohm < 100mA.So either:A) That 51Ohm resister is not 51Ohms anymoreB) Some problem you can't identify (yet) means the supply can't handle even the small stabilization load.Googleing for200W 5V power supplyor40A 5V power supplyboth return a number of relatively inexpensive options. Including this guy which look almost the same:http://www.dhgate.com/wholesale-led-power-supply-5v-40a-200w-for/p-ff808081374f8f82013775d2b42d40b4.html

author: kolbep
author_rank: Frequent Contributor
timestamp: May 24, 2013, 07:07:01 pm
content: Tried replacing the 51Ohm with 68Ohm 1/4w (Just as a test),Nope. Did not get above 2v.So should be something with regulation then?P

author: SeanB
author_rank: Super Contributor
timestamp: May 24, 2013, 07:30:31 pm
content: Check the brown film cap between the big caps and the rectifier diodes, it might have self healed to near nothing.

author: kolbep
author_rank: Frequent Contributor
timestamp: May 24, 2013, 08:49:36 pm
content: ThanksWill check when I have time.Just busy finishing Editing and authoring a DVD of a wedding,then tomorrow I have a Production at a High School to video....When will I ever get time just to play around.....

author: envisionelec
author_rank: Frequent Contributor
timestamp: May 24, 2013, 09:29:01 pm
content: It's not regulating, so I'd be looking at the small caps around the PWM IC if the output caps are OK. The hissing means the switching frequency is dropping and goes discontinuous which causes noise from the transformer to be audible.The transformer core could be cracked.

author: David_AVD
author_rank: Super Contributor
timestamp: May 24, 2013, 10:39:36 pm
content: The 51R resistor will be for minimum load when the output is disconnected. Leave it there.Most likely one or more of the secondary capacitors has gone low capacitance or high ESR. Just replace them all and be done with it.

author: kolbep
author_rank: Frequent Contributor
timestamp: June 03, 2013, 09:18:49 am
content: Hi SeanBThe Film Cap is 225JI do not have anything the same.Can I go up or down a bit in value? or is it a critical part?ThanksQuote from: SeanB on May 24, 2013, 07:30:31 pm

author: SeanB
author_rank: Super Contributor
timestamp: June 03, 2013, 05:43:35 pm
content: That is 220nF 5%, check it for capacitance, and as suggested before replace the output caps, and try it again. If it is low in value use something close to the value, from 200nF ( 2 100n in parallel) to 330n with a 400V rating. As you do get regulation with no load resistor change the OHL caps on the output, or add another across them for a check.