Megger BM222 Please help with Fault diagnostics

[Dazz]

Hi
I own a Megger BM222 meter with a fault. It works on the 500V range, but there is no output on the 1000V range.
I am trying to figure out what the fault is so I can fix it.

On the 500V range, I can hear the internal relay clicking when the button is pushed.
When I push the button on the 1000V range, no clicking can be heard.

If you have a BM222, can you please tell me if you can hear the relay click on the 1000V range?
Does anyone have a service manual???

 

[westward10]

Not used one of those for a while so I can't remember. First thing I would look at is the contacts on the switch selector.

 

[Dazz]

OK I will give that a try.
It's an oldy but a goody. It is old enough that I can probably still get components for it.

 

[Moley]

I had a similar problem with a BM 230, worked on everything except 1000v .There's an IC ( possibly a mosfet, can't remember) near the bottom right of the board (near a small transformer) that had gone t1ts up. I've got a non working 222 somewhere. It's about time I took a look at it.

 

[Dazz]

I can see there is a switch mode power supply in that area and around the relay. 1000Vdc is a lot for an electronic component to handle, so it makes sense a mosfet or similar is toast.

 

[Dazz]

I think my BM222 has been crippled. The attached image marks where I think a diode should be. The diode has been cut out. It is wired as an OR gate with the adjacent diode.

I think it should be a 1N914 small signal diode.

Can anyone confirm the missing component is a diode? Part number? @Moley ?
Thanks.

 

[Moley]

Nothing missing. Mine is the same as yours.

 

[Dazz]

It looks exactly the same as mine. It also looks like a part has been cut out.

OK does/did your 1000V test work?

When I push the button, no relay click and the volt pointer on the LCD display is not shown.
I wonder if the 1000V test was factory disabled for all BM222. I don't have a manual or any data.

 

[Moley]

It did work but mine has a fault, different to yours. Mine tells me there's a DC voltage on the leads even with no leads attached. The 1000v wouldn't be factory disables as it's part of the spec of the 222.

 

[Moley]

I've also got a faulty 223 apart at the moment. That also has the same component cut out.

 

[Dazz]

OK so for my fault, I am working on the hypothesis that 1kV is being generated, but not switched to the outputs.
I have not probed the meter with an oscilloscope because 1kV will fry it. I have ordered an HV probe so I can poke around and see what is happening to the 1kV. The probe won't be here for a few weeks. I can probe with my voltmeter.

 

[Moley]

Well, an update on mine, well the 223. I've got the bm223 working. Turned out to be a dodgy switch. Looks like it's had something poured in it at sometime. Some isopropyl and 15 mins in an ultrasonic cleaner seems to have sorted it. Anyway, the relay should click in on the 1000v range BUT there is a 3 second delay before the 1000v comes in. You have to keep the test button pressed for at least 3 seconds. See the video (I hope attached). It does cut out with the low battery indicator.

 

[Dazz]

That is useful to know. I will try cleaning my switch first.

 

[Moley]

If you've got a fibre brush then give the contacts a good clean. The contacts/knob will only go back one way but getting the clip back on is a bit of a pain.

 

[Dazz]

I had a go cleaning the switch with IPA and without removing the clip. After letting it dry for a few hours and when I tried it on the 1kV setting, it worked. I rather suspect it has more to do with waiting the 3 seconds or so for the 1kV to start doing something. The relay clicks after 3 seconds or so.

It is entirely possible my meter never had a fault, but it has had a good clean.