Samsung HP-S4253 Power supply repair

[cedward138]

Re: Samsung HP-S4253 Power supply repair

The pair against the upper heatsink are Q401, Q402 They bear the following: Toshiba k3911 .631

The pair against the lower heat sink are U501, U601 Marked 7M0880 G31
Hope this helps

[vinceroger69]

Re: Samsung HP-S4253 Power supply repair

theres about 13 transistors/fets on all the heatsinks resolder them all and try powering on the board.

[cedward138]

Re: Samsung HP-S4253 Power supply repair

The K3911 are FET'S according to Google.
Resoldering joints soon

[selldoor]

Re: Samsung HP-S4253 Power supply repair

If it dont work here is a test for mosfets
Mosfets

If you have a manual range multimeter, set it to 200 ohms (two hundred). This is for 3 pin ICs only. You can test a mosfet (Q, IC designation) "in circuit" by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any reading is less than 30 ohms you might have shorted mosfet. Remove from circuit and repeat the tests to verify.

[cedward138]

Re: Samsung HP-S4253 Power supply repair

Resoldering did not breathe new life to the board.
The mosfets read open/infinite between all pins all directions while in circuit.
Though unscrewed from heatsink, mosfets are so firmly bound to it that I'm not sure they will come off in one piece for retesting out of circuit.

[selldoor]

Re: Samsung HP-S4253 Power supply repair

Mosfet Test is only for shorts so they are ok

[cedward138]

Re: Samsung HP-S4253 Power supply repair

I set the meter on 1KV and probed various (random) points on the board. Seem to get a range of voltages (5, 15, 18) within the area marked hot but nothing on the cold side.

Hope this sparks some idea for someone

[cedward138]

Re: Samsung HP-S4253 Power supply repair

I have a working identical power supply in another tv.
With guidance, I may be able to check corresponding components if that will help narrow down the problem(s)

[selldoor]

Re: Samsung HP-S4253 Power supply repair

Obvious thing, though you might not want to do it would be to try the working PSU
in the non-working machine.

[cedward138]

Re: Samsung HP-S4253 Power supply repair

Actually, the psu came from the other machine which has sound but no picture. I've started a separate thread for that problem.

What I meant was that I could take voltage measurements at specific components of the working and bad psu for comparison if that would help. Not a board swap.

[selldoor]

Re: Samsung HP-S4253 Power supply repair

Problem is that the boards are interactive so in the set we are testing it in it may not be working due to a problem on any of the other boards having a problem, causing a protection circuit to switch it off.
You are in the enviable position of being able to try a different psu to see if
the other boards are working correctly.

The alternative would be to sit down and physically check every component
on both boards to try and find the dud ones. I realise that it may come to that anyway but it would be a great step forward to know the fault is actually on the board and not elsewhere.

What is the part number on U404 - top of the board just below the yellow transformer

[cedward138]

Re: Samsung HP-S4253 Power supply repair

There is a cap directly in front of it so one of them has to come off to get the number. Just give me a few minutes

[cedward138]

Re: Samsung HP-S4253 Power supply repair

Its KA7815
Btw it was 1 of 2 components that desoldered during use

[stj]

Re: Samsung HP-S4253 Power supply repair

that's a voltage regulator.
if it got hot enough to desolder itself then either it's fucked, or something it's connected to is shorted out.

[selldoor]

Re: Samsung HP-S4253 Power supply repair

Voltage regulators (U designation) can be bad. They can be shorted or output the incorrect voltage. The test described below only tests the former.

If you have a manual range multimeter, set it to 200 ohms (two hundred). You can test voltage regulators "in circuit" for shorts by (power off and unplugged)

a) black on pin 1- red on pin 2 - record ohms
b) black on pin 1- red on pin 3 - record ohms
c) black on pin 2- red on pin 3 - record ohms

If any ohm reading is less than 30 ohm you might have shorted voltage regulator. Remove from circuit and repeat the tests to verify.


If it is out - test the solder points for shorts as well

[cedward138]

Re: Samsung HP-S4253 Power supply repair

I had to remove it to see the part number. I tested it as described while it was out of the board and all test reported "open"

[stj]

Re: Samsung HP-S4253 Power supply repair

well the center pin & tab are ground,
the rightmost pin is the output - and should be 15v
hence the part number ending in 15

[cedward138]

Re: Samsung HP-S4253 Power supply repair

I'm getting -ve 48 to -ve 56 on pin 1 (fluctuates repeatedly) and -ve 40 to -ve 46 on the other pins. Black lead connected to RTN peg on board.

Found ?transistor on cold side (Q252) with 13.4 v to center pin and 0 v on the other pins. Part number not written or may have 'burned off' the component. no voltages are found beyond this part.

[stj]

Re: Samsung HP-S4253 Power supply repair

what voltage between pin1 and the tab/pin2?
and what voltage between pin3 and the tab/pin2?

[cedward138]

Re: Samsung HP-S4253 Power supply repair

Pin 1 to pin 2 = 0 v
Pin 3 to pin 2 = 0 v