Okay, good news the LED backlight kit arrived (PCB model number GYD-09).
I was hoping to modify it to only need +6V. The board uses a DF6113 LED driver IC and I managed to find a
schematic online. The datasheet reference design is a fairly simple boost converter, however this board has repurposed the IC to make an inverter circuit. I don't think they are even using the feedback pin (FB) as it's being pulled down to GND. The current sense pin (CS) is used instead to regulate the current through the LEDs.
From testing it drops out and stops switching the MOSFET at about 8V. I thought I could lower this my changing the inductor value, but that just changed the switching frequency, it still drops out around 8V. I'm not sure what determines the voltage it drops out at?
Anyway, I decided to give up on that and power it from 10V (using the +6V and -4V rails) which worked. I modified the board a little bit, changing the 220uF input capacitor to a low ESR type, I also added a 10uF electrolytic across the LED strip. I'm not sure if this is really a good idea, but older board revisions do have a footprint for one there.
I also changed the current sense resistance (R4 & R7) to 2.5 Ohms (four 10 Ohm 1/4W in parallel, looks dodgy but does the job
). This limits the maximum output current as by default it can draw well over 1A, which is too much for the power supply in the TDS. I also added a trimpot across the ENA (3.3V), DIM and GND pins, which allows the brightness to be reduced, but I didn't end up using this and left DIM at 0V (max brightness). With this setup the display is bright and I'm measuring only 90mA across the sense resistors which seems pretty reasonable.
I cut the LED strip down to the correct size then just used double sided tape to attach it. I then stuck some sticky tape and foil over it to reflect any escaping light back to the screen. A little bit of a hack but works well.
I still need to go back and replace the capacitor aray which failed, I suspect it isn't critical though, I'll put four 0402 capacitors in its place. I'm also going to mount the LED driver board properly (it's only attached with double sided tape atm).
Also interesting, checking the error log there's heaps or errors there:
7 DI ADG421 Diag Failed, 1 SL SL_stopAcq: Proc B, 2 SL SL_stopAcq: A!=B. Only error "7 DI" is listed in the manaual, with the cause listed as ADG421 diagnostics failed / main board or power supply failure.
I'm glad I can call this one a success because I had my doubts there for awhile.
edit: I forgot to mention, I'll probably use plastic hardware to mount the PCB and put some kapton tape under it as the board GND is now at -4V. I'll go back and replace the sticky tape covering both sides of the foil with kapton as well as it should last a lot longer. I also measured the power supply rails to make sure they weren't being loaded down (-4.3V, +3.4V, +6.4V, +31V).
edit 2: I also forgot to mention that it successfully passed the self cal without error.
edit 3: Just occurred to me the input voltage required for startup is probably set by the forward voltage drop of the LEDs, maybe some large resistor in parallel would allow it to start switching at a lower input voltage?