Nixie DMM updateI replaced both trannies in the trigger circuit. Looked in my stock for trannies with a similar round metallic package. First one I found was a 2N2905A.
Looked up the datasheet. PNP general purpose. current, voltage, hFE, speed... all better than the Germanium trannies.
Checked them with the tester before soldering them on the board, just in case... at the same time checked pinout. Same as the Germanium ones so can't go wrong.
Powered up the thing... no change. Still won't generate a trigger signal. Trigger circuitry still shows zero sign of life.
So...some other component in there must be bad... ![Confused :-//](https://www.eevblog.com/forum/Smileys/default/confused0024.gif)
OK, progress !
After some more playing, I felt there was some indication of a capacitive effect : assume the counter is free running. I then short the comparator output to stop it from free running. Now I power off the meter. Then power it back up immediately. Meter wakes up NOT free running. It survived the 2 seconds during which it was powered off. I did that 20 times in a row, never misses a beat. But, if I power off the meter and wait for a short while, just a few seconds, like 10/15, then upon re-applying power, the counter wakes up free running ! Works every time. So there was a pattern there, it seemed.
So I replaced a couple 100nF film caps on the board, see below. One is in the trigger circuit, on the right hand side input, the one solicited by the push-button when using Manual trigger. I assume it must be used to debounce the push-button. So not a very critical application, but well, replaced it anyway.
Second cap, identical one, is the one that does the RC differentiation to generate / derive the Reset pulse, from the output of the trigger circuit. That one I had more hopes about.
Well that fixed it ! Trigger circuit now works as it should, both Manual and Auto modes, and the ramp generator is now back to life as a consequence... which now allows the meter to take measurements !
In short : Vince 1, Meter 0
![Fighting :box:](https://www.eevblog.com/forum/Smileys/default/fighting0030.gif)
However, once I had pulled the caps, I tested them on the Chinese meter, and they were both fine. I also did a leak test with my lab power supply, recreating the sort of voltages that the caps are subjected to on the board... and they don't leak anything. Under 30Volts, one leaks zero, and the other like 50nA....
So not sure what's going on here... maybe the caps as such were not bad... but their solder joints needed some freshening up, and that's what I did when I desoldered them ?
I am tempted to put the old caps back on, and chances are it would still work !
![Face Palm :palm:](https://www.eevblog.com/forum/Smileys/default/facepalm.gif)
I now can't get the counter to free run anymore. Might come back to haunt me soon I bet, but for now, it looks like I was right. The problem was most likely a dodgy reset due to a failed trigger circuit.
We shall see how long it stays working...
So... that means that replacing these two Germanium 2N1305 trannystors with basically the first random silicon PNP I picked in my drawers, a 2N2905A... works just fine !
![Laughing :-DD](https://www.eevblog.com/forum/Smileys/default/smiley_laughing.gif)
A bloody 2N2905A.... there you go !
![Evil >:D](https://www.eevblog.com/forum/Smileys/default/evil.gif)