JVC Model: JLC37BC3002 - Backlight not working

[kyrinth021883]

Hello! I have dabbled a little with fixing a few TVs here and there as kind of a side hobby when I'm not busy. I am still totally a beginner though. I work as an avionics technician and so have a basic understanding of electronics principles.
I was recently given a JVC flatscreen from a friend at work who said the picture went out. I brought it home and did a quick check. TV does power on after plugged in, standby light good, remote functions good, TV powers on from standby good, and checked the LCD with flashlight and verified the LCD is working and seems to have a picture, but the back light does not seem to be coming on at all.
I opened up the back to check for anything obvious (bad caps, burnt marks, loose connection, etc). After not seeing anything obvious, I put a meter on it and tested each pin at the connector (the one connecting the power supply board to the main board). The following are the results (going from top to bottom of connector):

With TV in standby:

VBR_B - 0 VDC
STATUS - 0 VDC
ON/OFF - 0 VDC
ON/OFF - 0 VDC
12V - 0 VDC
12V - 0 VDC
12V - 0 VDC
GND - 0 VDC (GND ohm check good)
GND - 0 VDC (GND ohm check good)
GND - 0 VDC (GND ohm check good)
GND - 0 VDC (GND ohm check good)
5.1VSB - 5.1 VDC
5.1VSB - 5.1 VDC
5.1VSB - 5.1 VDC


With TV in ON:

VBR_B - 0.53 VDC
STATUS - 3.06 VDC
ON/OFF - 3.26 VDC
ON/OFF - 3.28 VDC
12V - 11.98 VDC
12V - 11.98 VDC
12V - 11.98 VDC
GND - 0 VDC
GND - 0 VDC
GND - 0 VDC
GND - 0 VDC
5.1VSB - 5.21 VDC
5.1VSB - 5.21 VDC
5.1VSB - 5.21 VDC

I'm not 100% sure, but I think these voltages all look correct?

When first turned on, i think I see some faint blue-ish light from the top left corner (looking at the back) through some of the holes in the metal backplate. But this only lasts a second or two. Can't tell if this is the backlighting trying to fire off, or something else.

After not getting anywhere, I decided to inspect the boards more closely. The only thing I could find that might be a possible cause of fault is what looks like a small dead bug (ant maybe?) laying across two legs of a mosfet.
Looks like "Q602" on the power supply board (see pictures). Not experienced enough to know what 2 legs of the mosfet the dead bug are laying across. My question would be could this possibly be the cause of the fault and how could I check it? Also, what else could I check to further trouble shoot this problem. I feel like I am on the right track with it being a back lighting issue. Thank you in advance for any assistance.

[budm]

Re: JVC Model: JLC37BC3002 - Backlight not working

https://www.badcaps.net/forum/attach...6&d=1507079126
That is the is the NOSFET for the PFC Voltage booster circuit. is he 3.15A fuse close to it good?
You need to verify that you do have 3800~400V DC between the two legs of the main filter cap, the backlights inverter circuit needs that to operate.

[dick_barton]

Re: JVC Model: JLC37BC3002 - Backlight not working

There's a second fuse on the board. Have you checked it. Also occasionally the two transformers need to be resoldered.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

I have checked both fuses on the board and both are good.

https://www.badcaps.net/forum/attach...6&d=1507079126
That is the is the NOSFET for the PFC Voltage booster circuit. is he 3.15A fuse close to it good?
You need to verify that you do have 3800~400V DC between the two legs of the main filter cap, the backlights inverter circuit needs that to operate.

I checked the voltage between the two legs of the main filter cap (assuming this is the largest one next to the NOSFET picture I posted?) The voltage was about 385 VDC when the TV was turned on.

[dick_barton]

Re: JVC Model: JLC37BC3002 - Backlight not working

It seems as though your backlight control voltage may be to low.
In another thread similar t your set the VBR_B voltage is much higher and they do have the backlights working. See post #4

https://www.badcaps.net/forum/showth...ighlight=VBR_B

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

I see that! Thx for linking that post. If that is the problem, where would I check next?

I searched the forum some more and found a thread with similar backlight issue but the resolution was not clear other than they used a repair kit on the power supply board from ShopJimmy.

i searched ShopJimmy and found a repair kit for a Vizio/JVC 0500-0412-1300 board (link here)
But my board is a 0500-0412-1310. (Mine is 37", that repair kit is for a 32") will they be the same kits since its just for the power supply board? (Seems all parts and boards for my model TV are out of stock everywhere) I would think that the power supply board would be similar, but the main boards would be different between different sized TVs of the similar model ranges. Not sure how that all works though. Would just hate to purchase a repair kit for a wrong board if it wont work for me.

Also, without knowing fully yet what component is bad, I am hesitant to buy anything.

[dick_barton]

Re: JVC Model: JLC37BC3002 - Backlight not working

I don't think you have a power supply board problem.
If you disconnect you power supply from the mainboard and on the power supply, attach a 1K resistor between 5.1VSTB and ON/OFF and a second 1K resistor between 5.1VSTB and VBR_B and check that your backlights come on and stay on.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

I don't think you have a power supply board problem.
If you disconnect you power supply from the mainboard and on the power supply, attach a 1K resistor between 5.1VSTB and ON/OFF and a second 1K resistor between 5.1VSTB and VBR_B and check that your backlights come on and stay on.

Ok, I will try that. I have to get some resistors first though. Don't have any laying around at the moment. Thx. Will post my findings afterward.

[Dumah Brazorf]

Re: JVC Model: JLC37BC3002 - Backlight not working

Post a pic of the back of the power board.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

Post a pic of the back of the power board.

I have taken some, please let me know if you want me to try to get better quality or closer up. I can try.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

I don't think you have a power supply board problem.
If you disconnect you power supply from the mainboard and on the power supply, attach a 1K resistor between 5.1VSTB and ON/OFF and a second 1K resistor between 5.1VSTB and VBR_B and check that your backlights come on and stay on.

Ok, got some 1K ohm resistors. I did the test you mentioned. Disconnected the main board from the power supply. Put 1K resistor between one of the 5.1VSTB and ON/OFF (INV ON/OFF, had to look at back of board to know which on/off it was) and also put one between a 5.1VSTB and VBR_B.

The results: Plugged the TV in..... and nothing. No backlight. Getting good 5.1VSTB's. 4.95VDC on the INV_ON/OFF pin (power supply side of resistor). I get 0.6VDC on VBR_B pin (power supply board side of resistor). And no 12VDC's at all. (Guessing that is normal since connector is disconnected?)
Is the backlight supposed to turn on and stay on with just this set up using the resistors? Still trying to figure out if the power supply/Inverter board or the main board is bad.

Reconnected main board connector. Decided to take voltage readings again and watch while toggle the ON/OFF button. All voltages are as stated above in previous post, with the one new thing I noticed. When TV is in standby (just plugged in and red LED light on front) VBR_B voltage reads 0VDC. When I turn the TV on (w/ power button) VBR_B goes to 1.2VDC. After about 3-5 seconds, what I thought looked like the backlight in the top left corner (through some holes), turns off, (I turned the lights down to see it better) and the VBR_B voltage drops to 0.4VDC. The VBR_B voltage stays at 0.4VDC until I cycle the power switch again. Then I get the same result each time. Turns out that the blue light is actually a blue LED on the bottom side of the power supply/inverter board and apparently a blue LED in the top right corner under the metal someplace. Just LEDs not the backlights lol.

[dick_barton]

Re: JVC Model: JLC37BC3002 - Backlight not working

Can you post a picture of the underside of the board showing what the connector pin names are.

[dick_barton]

Re: JVC Model: JLC37BC3002 - Backlight not working

Having the correct pinouts as seen from underneath the board it looks like you have to jumper 5.1VSTB to Remote ON/OFF and 5.1VSTB to Inv ON/OFF using the resistors and then check that your backlights come on and stay on.
Explains why your 12V did not come on in the previous test.

Photo's courtesy of Shopjimmy

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

Can you post a picture of the underside of the board showing what the connector pin names are.

Hi, sorry it took so long to post a pic. Couldn't take one until I got home from work. Here is the pic you requested.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

Ok, repeated same test, this time with both the 5.1VSB + INV_ON and 5.1VSB + REMOTE_ON jumpered with 1K ohm resistors respectively.

Same thing happens as when I push the power button in normal configuration. The aforementioned blue LED lights turn on for a few seconds then fade out. Still no backlight. This time i did get the 12v.
Read the voltages on the connector:

VBR_B - 0 VDC
STATUS - 3.31 VDC
INV_ON - 5.01 VDC
REMOTE_ON - 5.22 VDC
12V - 11.84 VDC
12V - 11.84 VDC
12V - 11.84 VDC
GND - 0
GND - 0
GND - 0
GND - 0
5.1STB - 5.1 VDC
5.1STB - 5.1 VDC
5.1STB - 5.1 VDC

[budm]

Re: JVC Model: JLC37BC3002 - Backlight not working

When using force on, you must remove the signal control wires that go back to the main board first.
VBR_B (dimmin control) - 0 VDC = no backlights, it has to have 1V or more typ.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

I'm sorry, I'm not following. I disconnected the power supply board connector from the main board for that test. I do not see any other connections to disconnect.

[budm]

Re: JVC Model: JLC37BC3002 - Backlight not working

OK, I think I understand what you describe now after reading this statement again "Same thing happens as when I push the power button in normal configuration. "
So it looks like you need a third resistor (1K) between VBR_B and the 5.1STB also to get the backlights to come on for this power board testing.

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

Ok, I will try using 3 once I get home

[kyrinth021883]

Re: JVC Model: JLC37BC3002 - Backlight not working

Ok, I was able to test using 3 resistors. Using a 1K resistor for each one, I jumpered 5.1V to INV_ON, 5.1 to REMOTE_ON, and 5.1 to VBR_B. I get the same as before. Once power is applied, all I get is the blue LED lights come on a few seconds then fade out. No backlight. I read the voltages, all the same as listed the most recent time above, onl difference is the VBR_B voltage being about 0.68V.

Since you mentioned it should be above 1V, I used a lower ohm resistor (470 ohm) and tried again. This time VBR_B voltage was 1.2V. Same result. No backlight. Blue LEDs behaving the same. Starting to think its just a back PS/Inverter board. Curious to try to find what component(s) are bad on it though.