Z usually denotes a zener diode. I don't know what the function of that component is in the circuit though. I don't know if 15M ohm is good or bad.
It might also be a gas discharge tube? GDT is another form of input protection. GDT is used in some Agilent and Hioki meters.
If it is a GDT, I *think* it should measure 0L ohms. I don't have an Agilent or Hioki meter to verify. If it is a GDT used for input protection and it measures 15M ohm, then I would desolder it to see if Auto ohms without leads shows 0L.
Yes, I too would normally read Z as standing for zener diode.
However, this is not the case here, as there are actual zener diodes on the board and they are marked with ZD (e.g. ZD1, ZD2, etc.).
I also know of GDT's being used in some meters for a similar purpose as MOV's and, while I can't claim I know much of GDT's, I personally very much doubt the item we're discussing here is one such component.
Two reasons I think this:
- so far, all GDT's I've seen have a specific physical construction which doesn't match with what we're looking at here (i.e. normally a ceramic body, much thicker and much shorter than our component).
- inside a GDT, I expect there to be physical separation between the two ends/leads of the component, with that gap being filled with a gas which probably has a specific breakdown voltage. OTOH, if you look at our component, you will see there is something looking like a resistor connecting the two leads. So, not much of a "discharge" that can happen there, IMHO (as in "sudden discharge").
Instead, I am guessing our Z component might be a high value resistor (100's of MOhm).
And the 15 MOhm in-circuit measurement is probably the total resistance of the circuit the resistor is connected to.
But all this is guesswork at this stage and, in the absence of any schematics or other data, there is only one way to find out.
I just need to muster some courage (for some reason, I'm always afraid of being too invasive with my troubleshooting)...