So I bought this cheap(er) Mastech gas sniffer a couple of years ago to help find a leak. It did work at the time. It has been sitting in the garage for a couple years. A use for it popped up and I go to test it and...it doesn't work anymore. With nothing to lose I crack it open. The behaviour is that it alarms when you turn it on. Before it would not alarm until you turned up the sensitivity dial.
I checked the dial pot. When it's off, the pot seems to work great (B503 label - from ~2 ohms up to ~.50k ohms). When the unit is turned on...the pot is open and turning the dial doesn't do anything. I start to get out of my depth at this point.
The pot has 3 legs. Two of them are connected to this voltage reference diode (D4 on the board) LM385 Z-2.5 (
http://www.ti.com/lit/ds/symlink/lm385-2.5-n.pdf). Pins 1 and 2 are connected to the pot. The 3rd pin is open.
I don't fully understand what this voltage reference diode is, how to confirm if it's working or why the pot is open when the unit is turned on. My thought was that something was shorted to ground. I tried to check all the 3 legged transistors for shorts and I re-soldered all the legs as a shot in the dark.
It would be cool to fix this and learn something. I do feel it might be a bit beyond my knowledge though and I thought to reach out to this community and maybe someone can help me. I would appreciate it if so. I don't care much about the unit. It would just be nice to figure out what was broken and learn something in the process. Thanks for any guidance in what to do next.
![](https://i.ibb.co/vYMmZGD/side1-Copy.jpg)
![](https://i.ibb.co/XsYc5pz/side2-Copy.jpg)