2xCH3041AHT PSUs problem

[ragis]

Hi folks, i have a problem with two CH3041AHT PSUs they had bulged caps and i recaped them successfully.But now the first one starts after maybe 1min or so..
The second one don't even starts...
I measured them and on the first it has all the outputs like +5, -5, +24.
The big cap 100uf/400v is 330V on both PSUs, but on the second not working the V is lost somewhere around the little filter cap 47uf/50V i measured it and it's 0V and on the other psu(the working one) is 14V
The PWM IC is KA3842 and to go in to switch on mode it needs 16V.
Thanks for your help, best regards
ragis

[budm]

Re: 2xCH3041AHT PSUs problem

"But now the first one starts after maybe 1min or so" So did you replace that small startup/up running cap right next to the SMPS IC1 on this board?
The dead board; What is the resistance reading between VCC pin 7 and GND pin 5?
All 3 rectifier diodes on the cold side are all good?

[PeteS in CA]

Re: 2xCH3041AHT PSUs problem

Concur with budm. That is a flyback power supply. The controller is the KA3842 (Samsung or Fairchild). In all likelihood, the start-up bias for IC1 is provided by a high value resistor (e.g. R4 or R6) charging that cap, and once the circuit is running, a winding on the transformer provides bias. It may be that the start-up cap ESR is so high that it can't provide enough current to start the KA3842. If both are 2008-vintage, just replace the cap on both and see what happens.

[ragis]

Re: 2xCH3041AHT PSUs problem

Thanks guys for your reply i appreciate that,
@budm: KA3842 the resistance between 7 pin (VCC) and 5 pin (GND) is 600R if i reverse the leads(red probe on 5pin and black probe on 7pin) if the leads are red to 7 pin and black to 5 pin i get 1 on my DMM or 0L for all( i test it on diode mode) this is the measurements for the working and not working psus( i think that they are ok)
I didn't replaced the small cap, but i will maybe this week and i will post the results.
For the 3 diodes on the cold side i tested them in diode mode and they all seem good on both psus.
@PeteS in CA: R4 and R6 are good, i will replace the small cap and post the results.
Thanks again, best regards
ragis

[ragis]

Re: 2xCH3041AHT PSUs problem

Thanks, for the help !
As you suggested it was this little bastard 47uF/50V after replacing it on both everything works like charm !
Thanks again, best regards
ragis