This PSU was installed in a number of Sharp products: PN-L702B; PN-E702; LCD-70X560A; LC-70LE632U; LC-70LE732U and LC-70LE735M to name a few.
I have an unusual backlight problem in this 70" Sharp PN-L702B touchscreen presentation display. When I turn the display on the backlight flashes for ~0.1 second and goes out. I can see the SHARP logo showing up under a flashlight so both the MB and the T-con appear to be working fine. I had to turn the lights completely off in the room in order to see the backlight flash - that's how quick it was. I even tried to record it with my video camera but at 30fps it was unable to catch the backlight flash. I am used to the classic 2-3 seconds to dark which would indicate a backlight failure, but this is definitely something else.
After I initially turn the display on:
The PCB names and TV modes are per the PN-E702 Service Manual (attached). For some mysterious reason the Service Manual has a schematic for the Main board but not for the PSU board.
The power supply (DPS-222BP-1 A) has all voltages present on PD connector:
This display uses a LK695D3GW35 panel. According to its datasheet (attached) each of the 14 sequential pairs of backlight LED strips is driven by +183v.
I tested voltages on the L1 connector on all 14 pins (LED1+ ... LED14+). The needle momentary jumps to +100v and falls back. The voltage jump occurred so quickly that I had to use my trusty 1958 vintage analogue voltmeter to catch it. My digital Wavetek 2030 multimeter was too slow to catch this spike although it did register about +45v in the MAX-HOLD mode. I presume that the voltage actually spikes to the full +183v but I don't have an oscilloscope to verify this. I read in a few places (for example here: https://www.badcaps.net/forum/showthread.php?t=54352) that a properly working PSU should be steadily providing this voltage on L1. The fact that I don't see this happening probably points to the defective PSU board in my display.
I even opened up the LCD panel and tested all 420 LEDs with a 100vdc power supply (15 LEDs on each strip; 14 strip pairs; 30 LEDs connected in series). They all lit up fine (after I burnt and replaced one LED https://www.badcaps.net/forum/showthread.php?t=56430) so the problem is definitely not with the LEDs.
I feel that the PSU tries to output +183v to the L1 connector but something interferes and interrupts the process.
All electrolytic capacitors in the PSU are name brand (mostly Rubicon); none of them are bloated. I did not not test any of the capacitors however.
I tested all of the diodes on board (including the D7951: https://www.badcaps.net/forum/showthread.php?t=51264 ).
I tested out of circuit all of the MOSFETS (Q7901, Q7700, Q7701, Q7801, Q7802 and Q7130) mounted on the top side of the board.
I tested in circuit all of the transformers for continuity.
I tested in circuit the three photocouplers on the board (PC7901, PC7701 and PC7902) on their LED pins only. Did not test their phototransistor pins - don't know how.
I did not test any of the IC's on the back side - don't know how.
I am out of ideas folks. Please help.
I have an unusual backlight problem in this 70" Sharp PN-L702B touchscreen presentation display. When I turn the display on the backlight flashes for ~0.1 second and goes out. I can see the SHARP logo showing up under a flashlight so both the MB and the T-con appear to be working fine. I had to turn the lights completely off in the room in order to see the backlight flash - that's how quick it was. I even tried to record it with my video camera but at 30fps it was unable to catch the backlight flash. I am used to the classic 2-3 seconds to dark which would indicate a backlight failure, but this is definitely something else.
After I initially turn the display on:
Code:
Time ACTION LED STATE MODE 00 seconds: Display turned ON LED is steady ORANGE PWR OFF (STBY) mode 09 seconds: Relay on "C RELAY PWB" clicks ON LED is steady GREEN PWR ON mode 16 seconds: Backlight lights up for ~0.1sec LED is steady GREEN PWR ON mode 32 seconds: Relay on "C RELAY PWB" clicks OFF LED blinks GREEN (~1 sec ON/~1 sec OFF) INPUT WAITING mode
The power supply (DPS-222BP-1 A) has all voltages present on PD connector:
Code:
PIN LABEL VOLTAGE 1 PNL 12v +12.04v 2 PNL 12v +12.04v 3 SGND 4 SGND 5 UR 13.2v +13.15v 6 UR 13.2v +13.15v 7 UR 13.2v +13.15v 8 UR 13.2v +13.15v 9 SGND 10 SGND 11 SGND 12 SGND 13 AC-DET +3.28v 14 PS ON +4.93v 15 BU 5v +4.97v 16 PNL-POW +4.96v 17 nc 18 nc 19 DIMMER +2.64v 20 nc 21 OFL +4.17v 22 ERROR +4.92v 23 nc 24 STB +4.96v
I tested voltages on the L1 connector on all 14 pins (LED1+ ... LED14+). The needle momentary jumps to +100v and falls back. The voltage jump occurred so quickly that I had to use my trusty 1958 vintage analogue voltmeter to catch it. My digital Wavetek 2030 multimeter was too slow to catch this spike although it did register about +45v in the MAX-HOLD mode. I presume that the voltage actually spikes to the full +183v but I don't have an oscilloscope to verify this. I read in a few places (for example here: https://www.badcaps.net/forum/showthread.php?t=54352) that a properly working PSU should be steadily providing this voltage on L1. The fact that I don't see this happening probably points to the defective PSU board in my display.
I even opened up the LCD panel and tested all 420 LEDs with a 100vdc power supply (15 LEDs on each strip; 14 strip pairs; 30 LEDs connected in series). They all lit up fine (after I burnt and replaced one LED https://www.badcaps.net/forum/showthread.php?t=56430) so the problem is definitely not with the LEDs.
I feel that the PSU tries to output +183v to the L1 connector but something interferes and interrupts the process.
All electrolytic capacitors in the PSU are name brand (mostly Rubicon); none of them are bloated. I did not not test any of the capacitors however.
I tested all of the diodes on board (including the D7951: https://www.badcaps.net/forum/showthread.php?t=51264 ).
I tested out of circuit all of the MOSFETS (Q7901, Q7700, Q7701, Q7801, Q7802 and Q7130) mounted on the top side of the board.
I tested in circuit all of the transformers for continuity.
I tested in circuit the three photocouplers on the board (PC7901, PC7701 and PC7902) on their LED pins only. Did not test their phototransistor pins - don't know how.
I did not test any of the IC's on the back side - don't know how.
I am out of ideas folks. Please help.
Attached Files
Comment