> The UC3842 datasheet does show you can draw up to 20 mA from the Vref pin, > which should certainly power up a 555. But, this is a 5 V regulator, and may > not be working properly. it may be that driving several mA to the 555, E2 and > other components has cooked it over time. With the 555 removed, apply > resistors between pins 1 and 8 of the 555, and see how much load the UC3842 > can supply. I'm guessing that at 1 - 2 mA the Vref output is going to collapse, > indicating the UC3842 is "weak". If the Vref does droop, make sure the Vcc is > still well above 5 V. If so, it has to be the UC3842. If the Vcc is also drooping, > then follow the chain back to the power input to find the defective component. > It could be Q1, R27 R28, > R32 or for some reason the crowbar D19 is being fired.
I just tried this with a 1K resistor I had to hand, so 5ma not 1-2ma, and the Vref droops. Vcc does not droop though, so I think the UC3842 needs to be replaced. I will try that. Incidentally, the UC3842 datasheet says that Vcc must have a short-path bypass capacitor to ground. If I socket the UC3842 is that going to be an issue? Thanks Rob
