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Canon PowerShot A710 IS backlight

I'm trying to fix a Canon digital camera that got slightly wet in one corner. On disassembly, it looks as if only one corner of the rearmost PCB got wet, slight corrosion is evident there.

It appears fully functional apart from the backlight not being on. One can take pictures, view them via a TV etc, also see a slight image on the LCD if one illuminates it externally. So I think the backlight is the only problem to fix.

The LCD panel is marked Sony ACX336AKB-7 and 637A35F. The front half is the LCD glass, leaving the entire backlight "layer" only 1.2mm thick. This seems rather thin to me to incorporate a CCFL tube (as a laptop display would). Is this more likely to be some other light emitting technology? Or do they really make CCFL tubes that tiny?

There is a 2-wire flex from the backlight layer to the PCB. The wires are somewhat spaced out, suggesting reasonably high voltage? A meter shows infinite resistance between the two backlight wires. So presumably it's not LED based.

The driver circuit is somewhat hard to trace. (Anyone know of a source for a schematic... ;-) So far I've traced it back to a SOT23 sized 5-pin thing:

'large' _ _____ _ inductor drive?->--|_| |_|b-------@@@@@-----+------->

| XH |_ _|_ backlight | 6 |_|GND 'tiny' ___ ?-->

_| G4 |_ cap | a|_|_____|_|Vcc GND

I think "a" connects to "b" under the chip.

Where the second backlight terminal goes is a mystery, I can't manage to find continuity to anywhere. I wondered if it might have a series cap to GND (many CCFL invertors have a small series cap to the tube) possibly implemented as PCB copper layers? Or maybe it's just failed open. :-(

Has anyone any clues on this circuit? What is the SMT chip for example? I looked up XH and G4 on several lists, but none of the lists I checked mapped it even to a 5-pin device. Does this look like a typical way to drive any known electroluminescent panel? I presume the inductor and cap form a series-resonant circuit, thus giving a much higher voltage across them than the supply (which is probably 2.4V straight from the 2xAA NiCd batteries).

Any help would be most welcome!

Thanks, Mike

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One particular Sharp CD player from a few years ago comes to mind. That used an electroluminescent panel to backlight the LCD track / time display. That was a monochrome display, and the panel lit up blue as I recall, but I don't think that there's any reason that this material can't be made any colour, so probably white as well. A series of H&H PA amps used to use a long EL strip to illuminate the control surrounds, and this was a cool minty green colour. The EL panel on the Sharp was driven by a little inverter board, which supplied around 250v ac as I recall. Very tiny ferrite transformer on there, as negligible current draw.

Just as a thought, are you sure that the backlight has not been switched off in one of the menus ? I fixed a problem a while back on a Sony digital camera belonging to my daughter's boyfriend. It involved a very major Swiss watch-style stripdown to get at the zoom lens mech, and when I put it all back together, everything worked except the display. My immediate feeling was that I had left something off, but when he turned up, he took the thing off me, and 20 seconds later, the backlight was back on. Turned out that it had a 'theatre' mode, where the LCD backlight was doused, so as not to annoy the people sitting behind you, and framing was done through the optical viewfinder.

Arfa

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Arfa Daily

On 15 Jul 2007 23:07:24 GMT, u snipped-for-privacy@csi.com (Remove _ for valid address) put finger to keyboard and composed:

Could "a" connect to the junction of the cap and inductor, in which case the chip may be a self contained, fixed voltage, PWM regulator with integrated MOSFET, flywheel diode, oscillator, and error amp. "A" may be the feedback pin for the error amp, ie this pin may sense the regulator's output voltage. The "drive" may be an on/off pin.

Do you measure a diode drop between GND and "b" (anode to GND)? If so, then this could be an internal flywheel diode.

- Franc Zabkar

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Franc Zabkar

On Mon, 16 Jul 2007 17:08:03 +1000, Franc Zabkar put finger to keyboard and composed:

I believe that the backlight could be a white LED type. This would make the chip a down-converter which is consistent with the placement of the inductor and an [internal] flywheel diode. Or maybe it's some kind of switchmode constant current source ???

I haven't found any info on the A710, but FWIW the PowerShot A200/A100 both have LED backlights:

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- Franc Zabkar

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Franc Zabkar

Hi Franc,

"a" to "b" measures 0.01 ohm (with my Cap ESR/low ohms meter). By comparison, across the inductor measures 0.23 ohm. So I don't believe "a" goes to the other inductor/cap junction.

However I do suspect the chip contains what you describe. I found numerous 5-pin SOT23 led driver datasheets, though all seemed to have an external diode in a classic "boost" configuration and I don't seem to have one.

It does measure like a diode, anode to GND, just as you describe.

Good pointer. Thanks.

You are indeed correct. I tried some DC (with 10K series resistor) into the backlight but nothing happened. I then managed to peel the backlight unit apart (two very interesting plastic films in there with odd optical properties - diffraction gratings maybe) and got the back layer out of the casing. Under a silver sticky tape along the edge, I find two miniscule SMT white leds on the flex. The LEDs work fine. However I've discovered a hairline break in the flex cable wiring, near the connector, both the gold and the underlying copper have been 'unplated' from it. A fiddly job but I managed to bridge it with some fine wire.

I'll investigate the circuit a bit more before I reassemble, as I don't understand the apparently open 2nd backlight terminal on the PCB. It's a bit of a bastard to assemble and disassemble, since this means unsoldering wires, so I'd like to avoid reassembling it and then finding it still doesn't work...

Thanks for your help. Mike.

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