Hi Ron
I used your guidance to perform some Resistance measurements using a DMM
Since this is a packaged, 2x MOSFET device and has a common source (the gold color base), the gate1 (A) and gate 2 (B) and the top two terminals, the drain1 (C) and drain2 (D) are the bottom two terminals, here are the values i get -
R(source-drain1)=8 ohms [this doesn’t seem to be high value resistance]
R(source-drain2)=1 ohm [this doesn’t seem to be high value resistance]
R(gate-source1) =infinity [this is open circuit]
R(gate-source2)=568 ohms [doesn’t seem like open circuit]
R(gate-drain1) = 1326 ohms [doesn’t seem like open circuit]
R(gate-drain2)= 439 ohms [doesn’t seem like open circuit]
Seems like the FET has ended its life?
I have already pulled it out and awaiting a new FET arrival.
I will repeat the above measurement on this new device, before i install.
Using a multimeter is not infallible for FET testing but it’s still useful.
Drain to Source should be at least 1 Mohm. Test with Gate connected to Source to clamp its voltage at 0. This may not be necessary but ensures stray charges do not cause the FET to be partly turned on and is a good lab technique.
Gate to Source should also be at least 1 Mohm.
As FETs are sort of symmetrical I would expect similar results with the Drain and Source reversed. Some have protection diodes which can give different readings.
You can see why I invested in a smart semiconductor tester.
However it does look very much like a damaged FET.
No need to reply but just by way of confirmation. It’s taken me 3-days but I finally found my two new and unused spare MRF-186’s. (I found my spare MX-P50M and my MX-P50A hiding in the same place too).
The DMM test findings of 2017 are confirmed but there are different values for A and B forward, depending on the DMM used to measure them (as you’d probably expect).
The 2017 values of .57 Volts forward drop in my first post were for a quality Beckman DMM.
Today using two cheapo DT830D DMM from ebay, the values were .41V as measured by the yellow DMM and .77V by the black DMM. This proves that when it comes to DMM’s colour matters! (This would likely be the case with diodes too).
Just kidding about the colour of course but for for A and B forward on a new device you can probably believe (accept) anything from .41 to .77V +/- a bit. Both MRF-186’s measured the same as one another.
I must agree that this is certainly not the best method for checking these things but from what we know so far, it can distinguish a new MRF-186 from a blown one.
Two terminal measurements on resistances under 10 ohms are notoriously inaccurate even after zeroing the meter. You can get a lower reading by pressing a little harder on the probes.
But when it doesn’t matter if the reading is 0.1 or 0.5 ohms accuracy is less of an issue.
Radio amateurs only occasionally need four terminal measuring ohm meters.