Westinghouse TW-80001-S024A Flashing red power led

[sceva]

This monitor was knocked over by the household kitty (another reason to have a dog). It caused the connection where the external power supply connects to be a little loose. The owner tried to fix it himself, and it worked for awhile, but eventually it must have shorted something out.

The power supply tests at 12.5v. I found bad caps at EC1 and C58, both 470uf, 16v, on the power board. I had replaced these and powered it up.

When turned on, the power board/inverter connection shows proper voltages, but the on/off wire is not on and the red power led flashes. I then found another bad cap at C5, 220uf, 25v, on the inverter.

The inverter also has a blown fuse at F1, not sure what size.

There are two 8 pin smd's on the inverter board labeled U1 and U2. When I test them with the ohm meter two legs across from each other on U1 show a closed circuit, which is different from U2. B3942, C8501, T0820 is what is printed on each of them, and appears to be a mosfet?

I am learning electronics and have only gotten to this problem a couple of times and am in over my head. Can someone help me with what steps to try next to figure out if this can be repaired within reason before trying to replace one or two parts at a time and hoping it will work? I realize something shorted to blow the fuse, but is it the mosfet, or does something else short that causes the mosfet to short also, could there be a dozen components that also got shorted?

Thanks.

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

Just for kicks I tested for shorts in the transformers at T1 and T2 on the inverter bd. Both showed .8 ohms with the meter at 2k, across the secondary.
T1 on the primary side showed the bottom 4 leads all closed (with each other), and the top 3 open.
T2 showed all of the primary leads closed with each other except the 3rd leg from the top.
Don't have a flyback tester...

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

Any suggestions on whether anyone thinks the transformers are OK or not? If they are OK, I think I will try to find the mosfets and fuse and see what happens. Thanks.

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

I am having trouble finding the replacement mosfets. The number is B3942, and I have attached some info from a datasheet (the complete pdf was too large to attach), but I don't know how to find a suitable alternative.

(I am beginning to think that kitty jokes are not in good taste in this forum?)

[smason]

Re: Westinghouse TW-80001-S024A Flashing red power led

This monitor was knocked over by the household kitty (another reason to have a dog).

I'm not so sure about that. We were watching our 50" plasma the other night and a commercial came on that had a large dog, full screen.
Our 1 year old Golden leapt off the couch barking at it. Luckily the 50" is above the fireplace. Had it been lower, I might be posting plasma boards for sale....

'course we have 2 goldens AND a cat (formerly 2 cats, that fought) and I recently finished fixing a DMM that got, umm christened by one of them.

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

Our 1 year old Golden leapt off the couch barking at it.

It's incredible how lifelike TV is these days.

Any suggestions on how I can find alternative mosfets or if it appears the transformer is bad? - btw, I tested the transformers on the board, and maybe the shorted component is in circuit with the transformer?

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

Something is shorted out on the inverter board. I don't have the best tools for working on a board like this. Does anyone have a spare they would like to sell? The part number on the back of the board is Honja, HI04AO-7A3. A pic is in the first post. Thanks.

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

I have given up on repairing the inverter, or finding an exact replacement, but I am now wondering if I can swap another inverter board. There is plenty of room in the case to modify it.

On the current inverter the pins (per attached datasheet) are:
pin 1 & 2 12v : 10.8 - 13.2V
pin 3 Von/off : On 2.8 - 5V, Off 0 - 1V
pin 4 Vb dimming control : 0V Max, 5V Min
pin 5 & 6 ground

I have another inverter out of a Viewsonic VP930B, part number IS159. I can't find a datasheet for it, but I was able to test it with its original Power supply and it showed these voltages when the backlights were on:
Pin 1 & 2 : 12V
Pin 3 & 5 : ground
Pin 4 : 3.6V
Pin 6 : 2.4V

I was not able to dim the Viewsonic to see if pin 4 or 6 was the dimming control.

Anyone know how I can figure out which pin is Von/off and which is dimming?

Thanks.

[Rtech]

Re: Westinghouse TW-80001-S024A Flashing red power led

Trial and Error is probably the quickest,Apply and external(bty)3volts to either pin 4 or 6 ,and the one that lights the lamps is the on/off.You cannot damage anything if you put it on the wrong one first
regards

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

Trial and Error is probably the quickest,Apply and external(bty)3volts to either pin 4 or 6 ,and the one that lights the lamps is the on/off.You cannot damage anything if you put it on the wrong one first
regards

It was pin 6. Thanks for the help!

[lethalsnakes]

Re: Westinghouse TW-80001-S024A Flashing red power led

Any luck in finding the B3942 I have a smoked chip as well. Everthing else checks out good. cant find a replacement. main board is good but inverter board cap was bad and shorted and burned up one of the chips label B3942.

[PlainBill]

Re: Westinghouse TW-80001-S024A Flashing red power led

Any luck in finding the B3942 I have a smoked chip as well. Everthing else checks out good. cant find a replacement. main board is good but inverter board cap was bad and shorted and burned up one of the chips label B3942.

Sceva only supplied a partial datasheet, and I haven't had any luck identifying the original part. The part doesn't seem to list the design voltage and current, and I'm not smart enough to figure out what they are from the data given.

What you are looking for is a dual N and P channel MOS FET an SOP-8 (or 8-SOP) package. A POSSIBLE replacement part is the Rohm SH8M24TB1

PlainBill

[sceva]

Re: Westinghouse TW-80001-S024A Flashing red power led

Any luck in finding the B3942 I have a smoked chip as well. Everthing else checks out good. cant find a replacement. main board is good but inverter board cap was bad and shorted and burned up one of the chips label B3942.

I don't have the datasheet any longer, but here is some info I found that might help in your search:
B3942
30V/6.9A,
RDS(ON)=25m&@VGS=10V (N-Ch)
30V/5.8A,
RDS(ON)=40m&@VGS=4.5V (N-Ch)
-30V/-6.1A,
RDS(ON)=35m&@VGS=-10V (P-Ch)
-30V/-5.1A,

RDS(ON)=58m&@VGS=-4.5V

Super High Density Cell Design for
Extremely Low RDS(ON)
Exceptional On-Resistance and
Maximum DC Current
SOP-8 Package

By the way, I kept this one and it is working well after several weeks. My only complaint is the resolution isn't as high as I prefer.

[retiredcaps]

Re: Westinghouse TW-80001-S024A Flashing red power led

Original datasheet at

page 41 at (long URL at google docs, hopefully it cut and pastes properly)

[bonney]

Re: Westinghouse TW-80001-S024A Flashing red power led

Will this help

[PlainBill]

Re: Westinghouse TW-80001-S024A Flashing red power led

Will this help

Yes, it does. We're looking for a dual N-channel P-Channel mosfet in a SOT-8 package rated at 30V and 6.9A (N-ch) and 6.1A (P-ch). The Rohm SH8M2TB1 might work, but the maximum current is rather low. Mouser doesn't seem to have anything at all that meets the description.

I suspect I'm not entering search parameters correctly; this is a common configuration on inverters.

PlainBill

[bonney]

Re: Westinghouse TW-80001-S024A Flashing red power led

Have a look at Fairchild Semiconductor FDS8958A

[PlainBill]

Re: Westinghouse TW-80001-S024A Flashing red power led

Have a look at Fairchild Semiconductor FDS8958A

Those have enough current capacity. I have two reservations about them. The package appears to be different 8-SOT vs 8-SOIC and this has logic level inputs. I'm not sure what kind of inputs the original parts have.

PlainBill

[dragonfly]

Re: Westinghouse TW-80001-S024A Flashing red power led

Hello to all, i am a new member here.
I use an fsd8958a on the place off b3942 and work fine to me.

[matyy]

Re: Westinghouse TW-80001-S024A Flashing red power led

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Hello to all, i am a new member here.
MY Westinghouse TW-80001-S024A AFTER I REPLACE THE INVERTER NO DISPLAY
BUT THE bakelite IS OK
CAN ENY BODY TEL MY WHAT TO DO ?
SORRY FOR MY BAD ENGLISH