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I have tried all the troubleshooting topics first

Checked connections reflowed solder
Checked component installation
TP1 to ground gives single click + red led flash
No click or red led flash if I short across the Geiger tube

I was getting a weird triple? click red led flash once a second if I shorted the negative terminal of the Geiger tube to the second pin to the right on the serial jumper

Any ideas?

Can I exchange for a working board?

Can you hear a whine from the speaker?

Have you checked the voltages on the 555 (U1)?
Examples here: http://www.mightyohm.com/forum/viewtopi ... t=10#p5489