Brymen 235 vs 869s vs 121GW smd schottkey diode massive issue

[4p0ksy]

Howdy,

Im repairing a board full of smd schottkeys -- possibly zeners. The schematic shows schottkeys however a search of the mfg marking (WG 2 subscript 0) implies zeners. from (smd.yooneed.one)

I've used three meters: a Uni-t, a fluke and a cheap astroai. Completely inconsistent readings. I just received the EEVBlog 235 today and it's given the most consistent so far. Most read OL, with a small section reading in a pattern of 0, 2.9v and then OL.
Only about a 1/4 of them read at all and I'm wondering if it's the meter. It shouldn't be but i've now tried 4 meters! They cant all be defective!

Is it the range? Can the 235 not read these? I haven't worked out exactly what diodes they are but they really fit an image of zener.

Do I need the 15v diode function of the 121GW? (the inaccuracies from reviews have put me off of it somewhat but it was the first choice for the diode test and data logging. As well, I noticed Dave writing that he gets the vibe the manufacturer doesn't want to continue making them. \

Is there a reasonable approach to getting correct or even ANY readings from these diodes?

I am considering a Brymen 869s however reading the range it shows only 3v for diodes. Im leaning toward these being 10v zeners but still, why would only a few give readings other than OL?

 
Yes I'm fairly new to doing this level of testing/repair.

Thanks for reading.

[joeqsmith]

In circuit?

[ArdWar]

What, and how, are you measuring?

Measuring things in circuit often results in "wrong" or non intuitive value. You really need to figure out the surrounding circuitry and deduce what the expected measurement and measurement method fit best.

Anyway if you're trying to measure zener *reverse* breakdown voltage, I don't think diode tester are best fit for that, save for the lower voltage zeners. Few handheld multimeter able to source more than 5V test voltage. Most are only 3V or whatever voltage their battery happen to be at.

[4p0ksy]

In circuit. On the board.

It's an analog computer and measuring with the probes strictly to check for blown components at first go.

The board is defective, giving wrong values for a function, so it works but I was told the problem is not firmware, it's to do with component. There's no clear visual signs of distress or bad joints so I checked all resistors to be thorough and figured it could be a shorted diode or IC.

Shouldn't the diodes at least give readings if they're not defective?

For sure @ardwar I noticed the range for handhelds is super low so that's likely the OL on so many.

[2N3055]

Shouldn't the diodes at least give readings if they're not defective?

In circuit they can show anything... Depends on the circuit.

[Kleinstein]

The typical defect of diodes is a short (e.g. < 10 ohm of resistance). One can often identify this in circuit.

Testing the zener voltage in circuit rately works. There are often other paths that can conduct.
The forward votlage should usually be visible. Usually relative high (e.g. 700 mV) for a zener diode and relative low (e.g. 300 mV) for a schotky diode. In circuit one may read lower, but should not get much higher readings.

For an analog computer one  should normally have some kind of schematics or description of the circuit - that is kind of needed to use it. Without it would be hard to do any repairs.

[4p0ksy]

Thanks for all the input.
This task is filling in big gaps I have in knowledge.

Onward!