Digimate Monitor Inverter Part ID needed

[Clyde]

Hi. Just got a Digimate 17 inch monitor with no display. Found a bad shottky diode on the inverter PCB so I replaced that and the fuse, now I have a screen, however after a few minutes it starts to flicker badly, on the inverter the transformer on the side I replaced the diode is getting hot. After a bit of tracing I have found a small SMD (8 pin, looks like an IC) which is shorted to all pins. I am unable to read the tiny writing on top and can find no information on what the part is. Does anyone have any idea, or possibly a schematic. Judging by the way it is connected (there is an identical one on the other side of the inverter, dual lamp) I think it may actually be a special kind of zener diode.

Anyway some numbers, the monitor is a Digimate L-1715 ( I believe the inverter is used in a variety of other brands ). The inverter PCB is part number IV185030HX. The part I need is identified as Q3 (maybe a transistor?)

Can anyone help?

 

[(*steve*)]

While you have it open, check all the caps in the power supply. You'll probably find some in the low voltage rails in the PSU that need replacing.

There's quite a few mosfets that come in 8 pin so-8 packages. Have a close look at the board and see if there are groups of 3 or 4 pins tied together. If it's a single mosfet, you'd expect to see 1 pin for the gate, 3 for the source and 4 for the drain, or something very similar.

If you can read a part number with a magnifying glass or similar, it will help a great deal.

 

[Clyde]

While you have it open, check all the caps in the power supply. You'll probably find some in the low voltage rails in the PSU that need replacing.

There's quite a few mosfets that come in 8 pin so-8 packages. Have a close look at the board and see if there are groups of 3 or 4 pins tied together. If it's a single mosfet, you'd expect to see 1 pin for the gate, 3 for the source and 4 for the drain, or something very similar.

If you can read a part number with a magnifying glass or similar, it will help a great deal.

Hi, thanks for that. Have already found a bad capacitor that had blown it's top, so replaced that as well. I think you are right about the MOSFET as it does have tied pins. Will try to get some means of reading the writing (it's very tiny)

 

[(*steve*)]

Hi, thanks for that. Have already found a bad capacitor that had blown it's top, so replaced that as well.

If you've found one there will almost certainly be more.

Will try to get some means of reading the writing (it's very tiny)

I feel your pain. I use glasses *and* a magnifying headset these days.

 

[Clyde]

Well I can only see two electrolytics near the power input, one of which was gone. The other checks ok on an esr meter. There is also one on the inverter board which also tests ok. I may replace the lot anyway for reliabilty's sake.

The mosfet is a pain, I gather it's a N channel chipfet, but still no further on the part number (The writing is almost the same colour as the package itself) I'm assuming it's a low power device, but would need some kind of datasheet to find out the ratings so I can find an equivalent. With it removed the lower lamp lights for a second and the screen illuminates before going out. Cycling the power produces the same result. With the shorted fet in circuit when I switched to standby the display went off but the backlights remained lit intensely. This has gone since pulling out the shorted fet. So far I've only spent a few pence on parts.

Will try to get hold of a magnifying glass and maybe read the markings on the good device since these seem a little clearer.

 

[(*steve*)]

OK, if you have an ESR meter then you can check them properly

The small SO-8 mosfets can be rated for anything up to 10A drain current! Typically they can dissipate only a small amount of power and are used as switches (with very low Rds and quite high gate capacitance)

A replacement shouldn't be too hard to find as soon as we know what the device is

The question though, is whether the mosfet failed first, or as a consequence of something else...

 

[Clyde]

To be honest, I have never had a lot to do with MOSFET's apart from in switching power supplies. So any help on this is great, at least I now know what I'm dealing with.

I personally think the capacitor was the first failure, followed by the FET, then the shottky diode. I also am not using the original power supply (it runs on an external 12v 4.16A "brick") so there is a chance that the original supply was faulty, which may have cause the capacitor failure, although electrolytics are not the most reliable things.

Still trying to get hold of some form of magnifier, as soon as I can read the markings will post here.

Thanks for your help so far.