Fujitsu SCENICVIEW P15-1A power board repair

[sermulis]

Hi,

Fujitsu SCENICVIEW P15-1A monitor was not showing any led activity.
After opening it - one cap was bulked C922, which was replaced. After than i Was able to switch monitor on two times between not able to switch it on about 20 times.

Checking the power board i came to conclusion, that i'm not receiving power in secondary circuit - (caps C922-C925 0V DC).

I am receiving variable DC readings on C906 and IC901 (1 to 7 pin) from 11 to 15 Volts. Also, I am hearing ticking component in primary circuit (guessing that IC901 is not able to start)

What you suggest me to check? Should the voltages be so unstable?
Thanks in advance

[Rtech]

Re: Fujitsu SCENICVIEW P15-1A power board repair

Have you checked the two SChottky diodes on the Heatsink ?? Also the most common problem on these are the 4 transistors Q 209, Q210 etc.these are common faults and used on many Boards,usually at least one pair go short cct.

[sermulis]

Re: Fujitsu SCENICVIEW P15-1A power board repair

hi,

what readings should I get for good diodes? should I test them out of circuit?

transistors can be checked in circuit for shorts in source to drain pins?

thanks!

[Th3_uN1Qu3]

Re: Fujitsu SCENICVIEW P15-1A power board repair

No voltage on secondary means the PSU doesn't start. Replace the startup cap of the PWM controller, it's the one next to it. C906 in your photo. There can't be anything shorted since the monitor does power up sometimes.

[sermulis]

Re: Fujitsu SCENICVIEW P15-1A power board repair

hmm. but why than I am receiving variable voltage on C906? I will replace this cap and post here results

[japlytic]

Re: Fujitsu SCENICVIEW P15-1A power board repair

I see this power supply has Hermei capacitors - a known bad brand.

[sermulis]

Re: Fujitsu SCENICVIEW P15-1A power board repair

after replacing C906 monitor came back to life

[Th3_uN1Qu3]

Re: Fujitsu SCENICVIEW P15-1A power board repair

Congrats. Want me to explain the whole "startup-cap-bad" effect? I'm drunk enough to do so, but i feel like tomorrow i'll be wide awake and unwilling to, so it's your choice.

Ok here goes. The power supply controller (most likely a UC384x variant) has two sources of power. One is a resistor from the primary HV side to its VCC pin, which supplies just enough current for it to start oscillating. The second is an auxiliary winding on the SMPS transformer, which sustains a 15-30v level for the controller to operate on, once the power supply started working and the monitor is ON.

There are two things that can trigger erratic operation. One is a legitimate short on the secondary, on which the supply will enter what's called "hiccup mode". Which is what you've been seeing on C906. If there really was a short, replacing that capacitor would have yielded the same behavior. That means you have to check the secondary diodes, on which you most likely you would have found a short.

The second is that the startup cap itself goes bad. This is manifested by the supply trying to start, then dropping out of regulation because the startup capacitor is dry and unable to supply enough power to the PWM chip for it to keep running, so when it starts drawing current to keep the thing running, it falls back flat on its face. This is what you have been experiencing here, and it's a common issue with primary side controllers.

Long story short: Any time you see a PSU with a primary side controller ticking, replace the startup (Vcc) cap first. If that does not fix the issue, THEN look for short circuits in the secondary. Have fun and enjoy your stay on Badcaps.net ! Th3_uN1Qu3 out.