power failure

Guest · Posted: Tue Jul 19, 2005 5:12 am Post subject: power failure

what's the cause for letting no power in the secondary side of an SMPS power supply. there is power in the primary side but no power in secondary. The SMPS has a transistor , controller IC and an autocoupler . I guessed one of those may be faulty which are in the primary. Anyway please help me what can be the cause.

Al_Dodge_Bundy · Joined: 05 Apr 2005 Posts: 93 Location: Netherlands

There could be two failures:

1) There is a secondary short. (most of the times the PSU will make a high pitch/wisling sound)

2) The primary side is not "starting". This could be to small dry-out capacitors. If no sound comes out of the PSU, just replace all the small caps in the primary PSU circuit.

Good luck,

Mark

Guest · Posted: Tue Jul 19, 2005 7:42 am Post subject:

dear mark thanks for help. I have seen those 2 faults you wrote.

1. In one Tv i got sound (fast tick tick sound ) the power is ok but there is no any sound or picture. the tick tick sound increases or decreases when i change volume. so this could be a short in the secondary , but where is the short to happen. what i have to check or change.

2. And the other fault in a reciever it have power in the primary but nothing in the secondary.

Help me for those faults plz

Guest · Posted: Fri Jul 22, 2005 2:55 pm Post subject:

give me some idea & Help me for those faults plz

Al_Dodge_Bundy · Joined: 05 Apr 2005 Posts: 93 Location: Netherlands

Hi,

Just a few questions,

1) Start by checking which cicruit is failing.
E.g. Do you have power on the HOT and is this a "clear" high voltage puls.

2) I'll gues you have 150 or 300 volt on the main cap, but do you have a pulsing high voltage on the main transitor in this primairy cicruit?

Grtz Mark

Guest · Posted: Thu Jul 28, 2005 2:19 am Post subject:

On the 1st question. Yes I do have power on the hot but nothing happens.

On the 2nd question How do i have to check if i have a pulsing high voltage on the main transitor in this primairy cicruit?

Al_Dodge_Bundy · Joined: 05 Apr 2005 Posts: 93 Location: Netherlands

Again a few new questions:

item 1) Do you have high voltage on the CRT? And if so, when turning up the screen adjustement slightly on the flyback trafo, do you get a light gray picture, with flyback lines trough it?

item 2) You could measure that with a osciloscoop (beware of the ground connection of the primairy side. It's a "live" circuit!!!!)
If the psu is not starting, and you do have voltage over the main cap. start with replacing als the small capacitors in this primairy cicruit (should be only a few). Sometimes these caps dry out to the heat generated by the main transistor in this cicruit!

Good luck,

Mark

Guest · Posted: Tue Aug 02, 2005 11:40 am Post subject:

dear mark, thanks for the help, so

1st Item. I am not seeing any difference, but when power is on i can see the power led on, (power is ok ), and fast tick tick sound indicating a short. that's all.
2nd Item. I have changed the capacitors there but nothing happens, i changed the transistor again nothing. So can it be the autocoupler ( with 4 legs, 2 in primary & 2 in secondary ) Can it be ?

so give me some idea

TVlad · Guest

Check ur secondary side for short circuits like rectification diodes, or

Change the small start up cap on primary side, would be something like 1, 4.7, 10 or 47 micro farard electrolytic type. or

If the main chopper transistor is not short circuit and has not blow fuseable resistor's change the control IC.

The Optocoupler should be ok as they don't go very oftern.

have fun

Guest · Posted: Wed Aug 03, 2005 6:05 pm Post subject:

so it could be the control IC, can u give me some ideas about that IC's function & it's cause's when it is faulty.

And in the 1st Item the secondary side is shorted but which parts are more likely to be shorted in the secondary side.