Worked great until I fried something while checking TP2

[Jeff]

I completed the kit last night and it was working great. I heard the faint whine as the power supply started, and I was receiving the appropriate number of background clicks. I took it to my kitchen to test against some granite, where I expected more clicks. Not satisfied, I decided that I should test TP2. It read around 200V. My meter is an Extech EX330. It's a fairly nice meter and in my ignorance I assumed it would have a fairly high input impedance (TIL: it's actually 10 MegaOhms so 200V makes sense). As I puzzled over why I was seeing 200V instead of 400V, I cranked the HV dial to max, and then I decided that I must have not been grounding to the correct place and decided to continue my theme of ignorance and accidentally grounded it against what I assume was probably the battery voltage somewhere on one of the other components. A very long beep ensued, the readings on my meter went haywire, and then the geiger counter ceased functioning altogether. A comedy of errors, which could have been prevented had I simply checked the input impedance on my meter first. Lesson learned.

I've read through all of the other support requests and have tried to collect as many of the diagnostics that you have requested from others, which are as follows:

Battery voltage: 3.04 V

TP1: 3.04 V
TP2: 2.81 V

U1 voltages:
1: 0 V
2: 0 V
3: 0 V
4: 187 mV
5: 2.02 V
6: 0 V
7: 0 V
8: 3.04 V

Removed U1, measured voltages for pin 4 and TP2:

Pin 4: 187 mV
TP2: 2.81 V

Shorted U1 pin 3 and 8, measured voltages for pin 4, TP2, TP1, and left leg of R8:
Pin 4: 54 mV
TP2: 0.93 V
TP1: 2.88 V (and beeps when probed)
Left leg of R8: 200-300 mV then falls steadily to 0V (and without 3 & 8 shorted as well as when U1 is in place: 0.4 mV)

No beeps from shorting the tube clips with or without the tube in place.

Shorting TP1 to ground results in a beep.

I swapped U1 with an identical part which I had on hand and the results and readings were the same.

I also no longer hear the whine from the power supply starting.

Any ideas as to what kind of mess I've caused? Any help would be desperately appreciated!

[mightyohm]

I believe I know what the problem is. Your voltage measurements are extremely helpful! Thanks for taking the time to measure them!

Pin 4 is always close to 0V, even when the 555 output (pin 3) is low. I've seen this before, and I believe Q3 has failed and is shorted on. Try replacing Q3 with a readily available 2N3904.

That should fix the problem.

[Jeff]

That was it! I replaced Q3 and now everything is working great again. Thank you!

[mightyohm]

You're welcome! Enjoy your kit!