When powering on HD70, power light goes on and the fan cycles a couple of times, but no light and then the Bulb light flashes and power goes off. Replaced bulb, and still will not light. Is this the ballast? Can I test the ballast before buying a new one?
I'm not going to pile on here and say call Optoma. What I WILL say is that you have done all you can to test, and that is install a known, good lamp.
That failed, therefore you have a bad ballast. There is no way you can test the ballast yourself except with a lamp. Lamp ballasts strike the lamp with a very high voltage, and once the lamp has "ignited", settle down to a running voltage. The circuit needs the lamp to operate so there is no way you can meter the output because it will not work.
It needs to go in for repair, I don't believe you will be successful in finding someone to sell you a replacement lamp ballast, and it has to be replaced. You can't repair them
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