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Electronics => Repair => Topic started by: AndreyS on July 18, 2019, 07:05:03 am

Title: THOMSON DPL914VD home cinema PSU won't start (solved)
Post by: AndreyS on July 18, 2019, 07:05:03 am

My flatmate has left the flat and left this home cinema behind.
It had bulged 400V cap, shorted transistor, FAN7554 IC, and open fuse.
I've replace all these components and it just won't start.
The schematic is attached to this post.
431, optocoupler are not shorted. I didn't find any other shorted component.

But I've noticed difference in FAN7554's pins in schematic vs datasheet I found on the internet; pins 1-3 doesn't match. Are there at least 2 versions of 7554?
I've found IC that has the same pin layout as in schematic: FAN7601 https://lib.chipdip.ru/061/DOC001061746.pdf (https://lib.chipdip.ru/061/DOC001061746.pdf) But it's operating frequency is 300kHz (on contrary 7554 is 500kHz).

What is next step in diagnostics? What should I do? Are those ICs really have different pin layout? Can I just rewire pins 1-3?

Title: Re: THOMSON DPL914VD PSU won't start
Post by: AndreyS on July 18, 2019, 07:08:45 am

I forgot to mention that voltage on C805 which is connected to pin 2 (cs/fb) of 7554 is constantly jumping 0-1.3V. Measured with ordinary multimeter. Therefore real Vpp may differ.

Title: Re: THOMSON DPL914VD home cinema PSU won't start
Post by: AndreyS on July 18, 2019, 05:04:53 pm

I desoldered all solder jumpers (U26,U27,U29,U31), thus cutting out 12V,5V,3.3V from other components. The problem is still persisting, showing that the problem is indeed in power supply circuitry.
I've went through datasheet, and there is a principal diagram where the IC is connected exactly as in service manual of  the home cinema.
I might look for other failed components

Title: Re: THOMSON DPL914VD home cinema PSU won't start
Post by: fzabkar on July 18, 2019, 11:07:03 pm

ISTM that the FAN7554 IC matches the circuit on a functional basis according to the surrounding components, but the pin descriptions are in error. I would not substitute a FAN7601.

https://www.mouser.com/ds/2/149/FAN7554-1006916.pdf (https://www.mouser.com/ds/2/149/FAN7554-1006916.pdf)

Common failures in these types of circuits are C803 (dries out and goes high ESR) and R805 (startup resistor goes O/C).

Title: Re: THOMSON DPL914VD home cinema PSU won't start
Post by: AndreyS on July 20, 2019, 12:24:00 pm

the board had R809 1k and R811 0.47 burnt as well. So I replaced these and now it's back to life :)
R811 looked allright, and R809 is SMD resistor, I didn't noticed those.

Title: Re: THOMSON DPL914VD home cinema PSU won't start (solved)
Post by: AndreyS on July 20, 2019, 12:30:14 pm

By the way what happened to the circuitry? I mean what is the sequence of failure of components?
My guess is:
1) 400V cap dried out
2) it caused huge ripples and bad filtering
3) at this point I kind of lost. Now it is whether switching MOSFET or IC 7554

I attached the screenshot of the affected part of schematic