Hi All, I need help from someone knowledgeable in electronics. I'm trying to fix my completely dead osziFOX pen oscilloscope.

I've narrowed the problem to the power supply circuitry. Power from the battery/transformer is fine before and after the diode, and up to the input of the chip in question, but zero after the chip. The chip tests open, and nothing is getting to the 78L05A voltage regulator, so no +5 volts to the rest of the circuitry.

I pinpointed the exact fault to the pale blue SMD chip immediately after the power supply diode. This chip is completely open. If I short the 2 terminals on the defective chip, the device works perfectly.

I'm assuming that the chip is a fuse that has blown, or less likely an open resistor, and the diode is for reverse polarity protection. I don't think a capacitor would make any sense

I'm not sure how I blew the fuse since the scope worked flawlessly last time I used it.

I'm not very familiar with SMD's. There are no markings on the chip, and it is a pale blue in color. It looks like either an 0603 (most likely), or an 0805 package. Can anyone help me to identify this chip? I've reverse engineered the circuit (since I can't find any schematics on line) and I think fuse makes the most sense.

The osziFOX is rated for a 9-15 volt dc power supply, typical current draw of 85mA. The 78L05A voltage regulator apparently puts out a maximum of 100 mA at 5 volts dc.

I could just solder a wire across the bad chip's terminals and it would probably work fine, but I would rather replace the fuse and have some protection in case some other component shorted the circuit and blew the fuse.

I was thinking the correct fuse value would be around 250 mA, or possibly 500 mA, probably fast blow. Does this make sense?

It's gonna be a real bitch doing the work without a microscope, but I'm game to try it. Any advice would be appreciated. I love using the scope when I'm working on my Gottlieb boards.

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