Instek GDM-8251A -- dead current input

[kcbrown]

Like so many others here, I went for one of the Instek GDM-8251A meters that was for sale on eBay.   At the price they wanted, it was really hard to pass up.

It seems to function well except for one thing: the current input is dead.  Shows almost, but not quite, zero amps.

Further testing shows that the issue is with the 1A and 10A ranges.  When it switches between those and 100mA and 10mA ranges, you can hear a relay click.  There's a fuse on the low-current common input jack, and that works just fine.  I don't know if there's a similar thing for the high-current circuit.  If there is, it doesn't appear to be easily replaceable.

I may be facing a dead relay.  I suppose it's possible that the 10A current shunt resistor is dead, or perhaps one of the amplifiers.  I don't know anything about how this is architected.

Anyone else have similar issues with theirs?  I know that I was supposed to take it apart before I turned it on, but I couldn't resist.   

Anyway, could make for an interesting repair attempt...

[kcbrown]

Well, it *would* have made for an interesting repair attempt.  I opened the unit, checked the large current protection fuse (it's fine), checked the current shunt resistor (it's fine), and then realized that maybe, just maybe, I'd screwed up and instead of my standard providing 1A like I thought it was, it was providing 1mA.  So I double checked that and, sure enough, it was only providing 1mA, too small for the 1A/10A range to really show.

So I hooked it up to my current-limiting power supply, gave it 200mA, and it worked just fine.

So -- I'm an idiot.  But at least I had a good excuse to take it apart.   
 

[kcbrown]

This does raise the question of how the meter does current "autoranging".  I can hear it switching the relays when I measure current, but it always stays in the 10A range no matter what amount of current I actually have it measuring.  It appears that if you want to use the 100ma and 10ma ranges, you have to use the low-amperage common terminal, but the meter won't autosense there at all with the probe placed there -- you have to manually switch to get it to sense current through that common terminal.

Anyone have experience with this specific meter who can shed some light on this?  Is my meter acting the way everyone else's does in this respect?   Autoranging with voltage works just fine.

[kcbrown]

Aha!  Looks like you have to use the "LO" black port as the positive terminal in order to get low current autosensing.  That makes sense in retrospect.  I originally thought there were two different common terminals, with one having a small current fuse, but having the "LO" port be the positive terminal makes a lot more sense.

Sigh.

Apologies for the "stream of conciousness" ("unconciousness"?)...