Westinghouse 37" LVM37w3 troubleshooting.

[Mad_Professor]

Brand Westinghouse
Model: LVM37W3


SO I just bought this for $80, they said it needs a new power supply so I said ok.

I brought it home, plugged it in. The light in westinghouse logo came on but when you press the power button nothing happens. Took it apart didn't see any bad caps but they were teapo.

While I was testing the voltages on the inverter board, some of my sweat from my forehead fell on the power board specifiably a rectifier, saw a spark from the short and now when you plug it in, the light in westinghouse logo doesn't come on.

I assume this is BAD.

Question is can I fix the power board? or has short doom it?

[Scenic]

Re: Westinghouse 37" LVM37w3 troubleshooting.

try not to use the preview function when uploading photos.
seems to be kinda buggy for some reason..

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

try not to use the preview function when uploading photos.
seems to be kinda buggy for some reason..

photos UP! seems to have trouble with picutres exceeding 4000x3000, does this forum come with automatic resizing?

[alexanna]

Re: Westinghouse 37" LVM37w3 troubleshooting.

photos UP! seems to have trouble with picutres exceeding 4000x3000, does this forum come with automatic resizing?

I use windows live photo gallery its free.Any photo program should work.You just have to resize to 2000x2000 max size

[jetadm123]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Can you point out which diode you "sweated" on? Have you tried checking the fuses?

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Can you point out which diode you "sweated" on? Have you tried checking the fuses?

Yes I check the fuse.


04n70bf
10031

It's this one

[jetadm123]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Rectifiers or diodes usually start with a "D". You're pointing to Q4, where "Q" stands for a transistor. 04n70bf is possibly a AP04N70BF mosfet.

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Rectifiers or diodes usually start with a "D". You're pointing to Q4, where "Q" stands for a transistor. 04n70bf is possibly a AP04N70BF mosfet.

So is there any chance of repairing the damage?

[retiredcaps]

Re: Westinghouse 37" LVM37w3 troubleshooting.

So is there any chance of repairing the damage?

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

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any ohm reading is less than 30 ohm you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

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

a) put black probe on pin 1 and red probe on pin 2 - read/record ohm
b) put black probe on pin 1 and red probe on pin 3 - read/record ohm
c) put black probe on pin 2 and red probe on pin 3 - read/record ohm

If any ohm reading is less than 30 ohm you might have shorted transistor. Remove the transistor and repeat the tests out of circuit to verify.

Just to clarify pin 1 being left next to print of Q4?

I got to go to work in few minutes, so I will have to do it when I get back.

[retiredcaps]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Just to clarify pin 1 being left next to print of Q4?

It doesn't matter. There are only 3 possible combinations -> 1-2 1-3, 2-3.

[kaboom]

Re: Westinghouse 37" LVM37w3 troubleshooting.

If that FET is shorted, R67 will probably be open. This confuses a lot of people. After they replace shorted semi's, bad caps, etc, those source/emitter resistors come out to play. .39ohm, 5% 1W metal oxide fusible.

That resistor is used as a current sense shunt to tell the current mode PWM what the primary current is. One end of something like a 1k resistor should go to the junction of the top R67 and Q4's source. The other end of that resistor will go to the current sense input or ancillary components by the PWM IC. Make sure those parts are OK, as the top of R67 will be at B+ when the FET shorts and R67 opens.

-Paul

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

It doesn't matter. There are only 3 possible combinations -> 1-2 1-3, 2-3.

I'm going to remove the transistor tomorrow.

pin 1-2 = I get quick reading before going OL
pin 1-3 = 01.0
pin 2-3 = quick reading before OL

If that FET is shorted, R67 will probably be open. This confuses a lot of people. After they replace shorted semi's, bad caps, etc, those source/emitter resistors come out to play. .39ohm, 5% 1W metal oxide fusible.

That resistor is used as a current sense shunt to tell the current mode PWM what the primary current is. One end of something like a 1k resistor should go to the junction of the top R67 and Q4's source. The other end of that resistor will go to the current sense input or ancillary components by the PWM IC. Make sure those parts are OK, as the top of R67 will be at B+ when the FET shorts and R67 opens.

-Paul

That went over my head except for the resistor, as you can tell I'm very inexperience when it comes to electronics like this. But if I understand you correctly... There is a resistor at R67 *orange white silver gold green bands* indicating as you said. Testing this I get 0.10 on the 200ohms setting of my DMM. One end grounds out at the main filter cap, the other goes to well I don't know, it goes to many things but the trace does terminate at Q4 or B4 *see below.*

I've included close up of the area in question, the ones circle in red are R67 solder joints.

Also there is this black thing on the third leg of Q4, the print next to it says B4*picture below what is that?

Btw got a new camera.

[retiredcaps]

Re: Westinghouse 37" LVM37w3 troubleshooting.

pin 1-3 = 01.0

1-3 suggests a shorted component. Remove and retest. If it still shows the same reading, you will have to replace it.

[kaboom]

Re: Westinghouse 37" LVM37w3 troubleshooting.

If reading across both anodes of a dual rectifier, and it's still in-circuit, you could be reading the DC resistance of the secondary. Pull it and retest if in doubt.

Looks good for R67. Remove one end of it and test again- you may or may not get a different reading. At this point, all you want to know is that it's not open. Depending on your meter, it may not be able to resolve values less than an ohm accurately.

You have rectified line voltage across the big cap?

-Paul

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Sorry guys, been busy doing double shifts at work and working on my car. Just haven't had the time to work on the board. Gonna try this weekend.

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

I've removed and retest the resistor and it checks out fine, and then removed the transistor and I get no continuity on any pin.
So whats next?

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

I've removed and retest the resistor and it checks out fine, and then removed the transistor and I get no continuity on any pin.
So whats next?

I was reading another thread that plainbill was participating in and they were talking about zener diodes being shorted out. From that I decided to check my board again and I have on the back side, ZD3 and ZD1, I assume these stand for zener? I test each one on the 200 setting of my DMM and I get 0.10 *closed?* on each, and this is with the transistor removed. Yet I test ZD4 and ZDS3 with the same band markings and get no reading *meaning open?* Then I tested D11 and get 90 ohms. I also remember messing around with my DMM few weeks back with the Q4 Transistor in and after the incident and it read the same. I'm wondering if I fried these zeners and if the Q4 Transistor is ok?

Band marking is
1.red
2. brown or orange *hard to tell
3. light green or green if there is no such color.

[Mad_Professor]

Re: Westinghouse 37" LVM37w3 troubleshooting.

I was reading another thread that plainbill was participating in and they were talking about zener diodes being shorted out. From that I decided to check my board again and I have on the back side, ZD3 and ZD1, I assume these stand for zener? I test each one on the 200 setting of my DMM and I get 0.10 *closed?* on each, and this is with the transistor removed. Yet I test ZD4 and ZDS3 with the same band markings and get no reading *meaning open?* Then I tested D11 and get 90 ohms. I also remember messing around with my DMM few weeks back with the Q4 Transistor in and after the incident and it read the same. I'm wondering if I fried these zeners and if the Q4 Transistor is ok?

Band marking is
1.red
2. brown or orange *hard to tell
3. light green or green if there is no such color.

Correction: Got a magnifying glass and found out ZDS3 has different band colors. I also tried again on 2k settings and tested ZD4 again which has matching band color of ZD1/ZD3 and I get 0.38, I test ZD1/ZD3 again and get 0.0.

[PlainBill]

Re: Westinghouse 37" LVM37w3 troubleshooting.

Correction: Got a magnifying glass and found out ZDS3 has different band colors. I also tried again on 2k settings and tested ZD4 again which has matching band color of ZD1/ZD3 and I get 0.38, I test ZD1/ZD3 again and get 0.0.

That certainly sounds like ZD1/ZD3 are shorted. The reading you got on ZD4 (380 ohms) is reasonable.

PlainBill