No 5V on inverter board of Dell 2007FPb

[tommy_b]

Hello everyone,

First time posting here, and I know I am not following protocol at the moment by not providing pics, but I was wondering if anyone is familiar with the circuits of the inverter board for a Dell 2007FPb monitor. I am not able to get the backlights to light up (although, on occasion, I have seen the Dell logo for a second or two before it fades to black). Normally the display is just black (no flashes or anything), and the two green LEDs are lit.

I've already recapped with Panasonic FM and FC series caps, to no avail. I also replaced the power supply board with an external laptop power supply (19.5V), as I wasn't seeing 19V out of the power supply board. After poking around on the USB board, I do see that there are 5V, 3.3V, 2.5V, and 1.8V being produced. However, I do not see 5V on the inverter board to run the 20-pin PWM chip (OZ964GN PWM Controller) (i.e., the 5V supply pin, pin 5 - Vdda, is measuring 0.9V. That would explain why no backlights). It seems like the inverter board must be designed to produce the 5V locally, but am not sure. The Vdda pin is connected directly to a diode (perhaps a 5V zener?), but other than that I haven't attempted yet to trace out the circuit.

Does anyone happen to know how the 5V for the PWM Controller is generated? Am I perhaps mistaken and 5V should be coming in externally from one of the other boards?

I will try to get some pics of the boards posted later.

[budm]

Re: No 5V on inverter board of Dell 2007FPb

The PICO fuse on the inverter board is OK? What is the DCV feeding the Inverter board?

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

budm, many thanks for the quick response. I believe the PICO fuse is OK, as it measures 0.3 ohms across. (The PICO fuse is FP801, yellow resistor-looking part near the edge of board, yes?). The primary DCV is ~19.5V (I think it's supposed to be 19V nominally, by design).

I'm beginning to suspect that the 5V is supposed to be supplied from the USB board to the inverter board, but its not making it out of the USB board for some reason. (Makes sense logically). I've identified three signals (TP716, TP714, TP713) that originate at the USB board and terminate at the inverter board (they're simply routed via visible copper traces from the USB connector to the inverter connector on the original Power Supply board. Since I'm not using the original Power Supply board, I've temporarily hand-wired these Test Point connections between the USB board and inverter board. I'm 99.9% sure the connections are wired correctly).

I just went back and measured each of the above Test Points and noticed that there is 3.3V on one of the pins, and the other two are at 0V. Maybe those other two pins are supposed to be 5V and ground? I will have to see if I can find a path from the 5V rail on the USB board to one of those connector pins. Maybe there's an open resistor or something in the path. Having a schematic sure would help. :-) Do you know anything about the USB board circuit that may be of help? Sure would be appreciated.

[budm]

Re: No 5V on inverter board of Dell 2007FPb

Can you tell me what DCV you have on all the pins of the connector of the power supply board to goes to the inverter board?
Those pins with small traces are for BL-ON, DIM, the third one I do not know what that pin is, are those the pins you test?
I do not believe the 5V is fed to the inverter board, it probably has its own drop down voltage regulator to convert 19VDC to lower Voltage to run the IC, that is how most the inverter boards I work on use.

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

The pins on the inverter connector measure:
1,2,3 = 19V
4,5,6 = Ground
7 = 0V
8 = 3.3V
9 = 0V
I started to trace pins 7,8,9 on the inverter board and you seem to be correct about the 5V being generated on the inverter board. I only have a partial schematic sketched out (and it ain't pretty yet), but its starting to look like the 5V is generated off of the 19V input, as you said. I almost have enough traced out to determine how the 5V is generated, so I will post that portion of the schematic when I get it cleaned up. Might be helpful to others.
Thanks for letting me know about the BL-ON and DIM signals.

[budm]

Re: No 5V on inverter board of Dell 2007FPb

Pin 8 is probably the BL-ON pin.
You may be able to trace out VDDA pin of the IC to see if it goes to pin 7 or pin 9.
I can look at my board (need to find it first) to to see what is going on.

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

I've got a good part of the circuit traced out, but not sure how accurate it is. However, now I've got a new problem. My external 19V power supply is shutting down, I presume due to overcurrent. I measured ohms between the 19V and Ground pins on the inverter board connector and am getting 3.5 ohms (~5.5 Amps). My external power supply can only go to about 4.25 Amps at 19/20 volts.
Don't know what changed, as I was mostly just poking around with the DMM with power off. Before this, the external 19V power supply was staying on and I was measuring 19.5V. I wonder if I poked a bad solder joint with the DMM probe and made it reconnect again, such that it's now drawing a normal level of currrent (5.5A) from the 19V supply, whereas before it was drawing less. Arrrrggh! If it does normally draw 5.5A, then I'll need to find a bigger 19V power supply (or maybe I could drop the voltage a bit to get more current? ... my external supply is adjustable from 15V/16V - 19/20V; or maybe I could try fixing the original 19V supply). I'm giving up for tonight, but thanks for your help. Maybe I'll get back to it tomorrow night after work.

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

Just a reminder to myself to try measuring the ohms between 19V and Ground on each board separately, to see how each board (USB or Inverter) is loading down the supply.
Yeah, that's the ticket! :-)

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

Well, I found a new problem on the inverter board. One of the MOSFET chips (8-pin, dual MOSFETs) now has a short across one of the MOSFET's (gate to drain?). There may also be a short across a 4.7V zener and/or a 5.1K resistor in parallel with the zener (haven't pulled them off to check each one individually yet) that feeds the MOSFET gate pin. The zener and resistor are connected to 19V coming in on the connector, so the short was causing my external power supply to shutdown.

Anyone know what the part number is for the dual MOSFET chips? None of the parts on my board have markings. I guess I'll try replacing the bad components and give it another try.

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

Found the MOSFET part number in another post:

p/n: SP8M3 (Rohm)
http://www.alldatasheet.net/datashee...HM/SP8M3.html?

[tommy_b]

Re: No 5V on inverter board of Dell 2007FPb

I've given up on this monitor (bought another working 2007FPb last night, so my dual monitor setup will soon be complete). Too many problems to debug on this one and no schematic made it too much work. However, I am thinking about converting this one to LED backlighting, since I already have an external power supply that might come in handy for that. Haven't ever disassembled a LCD panel, so that should be fun. :-)

Anyway, just wanted to say thanks to Budm for helping me try to debug this unit.