First Tube Showing All Segments

[troj]

My thermometer has been working well for a while when I noticed today that the first tube has all the segments (including the decimal point) lit. This is happening regardless of the mode the thermometer is in.

I suspect something has failed....I just don't have a clue as to what it is.

-Kevin

[Tony]

Is it lit even when it souldn't be in a particular display mode?. If so is the LED on or off - if it's off and the tube is still lit then it'll be one of the driver transistors (the MPSA42 or MPSA92).

[troj]

Is it lit even when it souldn't be in a particular display mode?. If so is the LED on or off - if it's off and the tube is still lit then it'll be one of the driver transistors (the MPSA42 or MPSA92).

It's always lit, in all modes -- when the display blanks, it blanks. But as soon as any tube is on, it's on with all segments lit. The LED is only lit when that tube is supposed to be lit.

Looking at the two transistors behind that tube, I see one marked 2N 6520-C27 and another marked KSP 42-C11

This leads me to these two parts:

https://www.fairchildsemi.com/datasheets/2N/2N6520.pdf
https://www.fairchildsemi.com/datasheets/KS/KSP42.pdf

If these are the right parts, I can easily order some and swap them out.

-Kevin

[troj]

Replaced both transistors, and all is well with the world.

-Kevin

[Tony]

Sorry about the late reply, for some reason I thought I'd already replied....

Yes they're fine - as long as they're high voltage NPN and PNP TO92 parts almost anything will work.

Tony.

[troj]

Sorry about the late reply, for some reason I thought I'd already replied....

Yes they're fine - as long as they're high voltage NPN and PNP TO92 parts almost anything will work.

No worries; I just figured you had something come up.

It's working, life is good. Just waiting for ThinkGeek to get more in stock so I can buy a second one!

-Kevin

[Tony]

I haven't had a request for more kits as yet but I'll let you know if they do order any more.

Tony.