4x 2SC4297 bpt in projector - reading e-c leak?? - diyAudio
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Old 30th May 2008, 10:33 AM   #1
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Default 4x 2SC4297 bpt in projector - reading e-c leak??

Hi,

I was given a Sharp XV-310P projector, and as always nothing is really free as this projector powers up for a second then dies. (by this I mean the fan spins and the 3 LEDs light up quickly).

When I disconnect the HV ballast board for the lamp the unit powers up as it should, so I download the service manual and start testing the hard worked components on the ballast board.

This unit has 4x 2SC4297 NPN triple diffused planar transistors on this board for switching, the problem is they measure ok until I do a reverse E-C leakage test.

When I hook my test bulb which is a 30v PSU is series with a light bulb and do a reverse E-C test the lamp lights, but very dim compared to when I do a forward C-E and apply a 6R resistor to the base to vcc.

Will this matter? The datasheets I found do NOT have anything written about a reverse diode internally and as an actual fact the schematic shows an EXTERNAL diode on the PCB across C-E.

Ok so I am stuck, do these transistors have an internal reverse diode? Are ALL FOUR faulty and leaking? Even though they are leaking reverse E-C will this really matter as there is an external diode to do this? Or do you think the transistor may break down forward C-E if I apply 100v or more?

I appreciate any help on this matter from someone who has delt with beasts like these before.
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Old 30th May 2008, 10:58 AM   #2
Mooly is online now Mooly  United Kingdom
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Hi,
Never worked on the Sharp but transistors do read C-E when reverse biased. Even an AVO 8 on high ohms will do this with any transistor, the 15 volt battery is high enough to break the junction down. Everyone is used to digital meters these days which will not in general do this.
If you pass a high enough current this way you risk damaging the devices. If the device had an internal diode it would read as such and your bulb would light at full brightness. A power transistor will hopefully survive what you are doing to it, small signal ones won't.
Regards Karl
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Old 30th May 2008, 11:27 AM   #3
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Thank you for the quick reply - Sometimes we forget these simple things! its been a long Friday!

This lamp ballast board only has two two pin connectors going back to the SMPS power board.

One set is coming straight after the bridge rectifier from the 240v mains so shares the SMPS boards filter and bridge, Logic suggests this is not causing the shutdown as no fuses are blown and nothing is monitoring this line and its not drooping.

The other two pins come from an optocouplers phototransistor labeled "lamp off signal" on the service manual, I am not going to try power the ballast board without this optocoupler connection as it is there for protection but I may try bring up a dc voltage from 30 - 200 VDC on a variac to the ballast input with a bridge and filter cap and do some tracing.

The bulb may well be on its way, its a metal halide 80v/120w and will cost AU$700 to replace so I am testing everything else first, I will NOT buy another lamp so worst case is this ends up as parts, I just want to be certain the bulb isn't blown.

Does anyone have any suggestions on how to test such a bulb?? I am taking every step slowly as I repair amplifiers for a living (including SMPS based DVD combo rubbish units) and have had training on safety and logical methods of deduction, but projectors are a new game for me!

Can I just get something close to 80VDC and test the bulb? I am not familiar with metal halide lamps.

Once again thankyou for any suggestions.
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Old 30th May 2008, 11:37 AM   #4
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I have quickly uploaded the relevant schematic if anyone has time to look (I know I know.....)


I am now waiting to see if anyone can give me a hint on testing metal halide bulbs or if anyone has any tips - In the mean time I will desolder some components and do cold testing.


Click the image to open in full size.

The lamp connects to the bottom right, the rectified mains goes to the top right and feedback is top left.
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Old 30th May 2008, 11:50 AM   #5
Mooly is online now Mooly  United Kingdom
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Hi,
Sorry -- no experience of projectors -- I have worked on thousands of SMPSU's over the years, and something like this, you need the full circuit and manual actually in front of you. Sorry
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Old 30th May 2008, 11:52 AM   #6
Mooly is online now Mooly  United Kingdom
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I know it's not much help but look up metal halide lamps on Wikipedia, a lot of info there.
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Old 30th May 2008, 11:57 AM   #7
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Quick update.

Reading the users manual (sometimes I get so involved with service manuals I forgot that a little bit of research of how the unit operates doesn't hurt!) I found that the bulb is NOT blown.

The user manual states a red error light would come on in this case, not a complete shutdown of the system!

The unit works without this ballast board connected, so this "lamp off signal" is causing this...

The lamp off signal goes back to the main SMPS board, The main SMPS board is taking this signal correctly as when the optocouplers transistor turns on it shuts down, otherwise when I disconnect the board the unit starts up.
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Old 30th May 2008, 12:15 PM   #8
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Thank you Mooly you have been more than helpful already .

I am now looking into a few flow charts and I found a schematic with voltages, which doesn't help yet as the PSU shuts down when the ballast board is connected.

Im looking more into the area where the feedback is sent at the moment.

But you have already been helpful so I thank you for that.
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Old 30th May 2008, 12:16 PM   #9
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That's something then. So what you are saying is that IC 1701, the opto is "incorrectly" for whatever reason telling the main board to shutdown.
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Old 30th May 2008, 12:20 PM   #10
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Is that opto used for the feedback signal for regulating the PSU on the main board. As I say you need the full manual in front of you for anything like this.
Good luck with it
Regards Karl
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