Do you remember what jacks were in use when measuring the DC/DC?
Were you using the meter for measurement of current, power or voltage?
The energy to do that couldn't have come via the V jack, there is at least a PTC + 1K in series with every path. Even if the contacts shorted the VA range contacts and the ohms contacts, worst case path is V jack to GND via a PTC + 1K. So, it's weird
So I can only presume the path is via the low impedance A jack somehow...
Maybe one of the range switch contacts wasn't installed properly and was floating around? I presume you hadn't taken off the range switch before?, as that's required for the shim replacement.
Kean, can you post a photo of the range switch contacts please.
Edit: you can probably work out the orientation of the range switch based on the contamination shown on the plastic underside. You can see the carbon deposit is not near any of the contacts points.
Also, note that a while back another member had posted some good pictures of their switch showing a fair amount of contamination in this same area. I am not sure why it would accumulate here.
Do you remember what jacks were in use when measuring the DC/DC?
Were you using the meter for measurement of current, power or voltage?The probes were definitely in the V and COM jacks for voltage measurement, and meter in auto ranging DC V.
Sure, see attached photo. They look squeaky clean to me.
So you weren't measuring Power (VA) at all?
I'm now at a loss as to how this happened. Need to think some more...
IMO, the weak link of the meter's front end is that 15V diode check mode. I marked the switch to make it easy to follow on the schematic.
Also, note that a while back another member had posted some good pictures of their switch showing a fair amount of contamination in this same area. I am not sure why it would accumulate here.
A bit of damage for a low energy source. Is there more to this story??
From Joe's annotation if you measure what is labeled 37 and 15 you get an open circuit, on the schematic these should be a short.
Whereas if you measure between 12 and 36 it is a closed circuit indicating these rows are swapped.
Attached are what I believe the connections to be.
I'm not so sure about the internal trace now, but you can see there is one that goes directly under that crater.
Can someone explain how to display the burden voltage when measuring current?
I can follow the instructions by pressing the SETUP button and changing the setting to "bd.on", but after that I cannot see how to get a voltage shown on the display? (I assume the voltage should appear in the secondary display above the current reading?)
Any thoughts?
The energy to do that couldn't have come via the V jack, there is at least a PTC + 1K in series with every path. Even if the contacts shorted the VA range contacts and the ohms contacts, worst case path is V jack to GND via a PTC + 1K. So, it's weird
So I can only presume the path is via the low impedance A jack somehow...
Maybe one of the range switch contacts wasn't installed properly and was floating around? I presume you hadn't taken off the range switch before?, as that's required for the shim replacement.
Any thoughts?
The energy to do that couldn't have come via the V jack, there is at least a PTC + 1K in series with every path. Even if the contacts shorted the VA range contacts and the ohms contacts, worst case path is V jack to GND via a PTC + 1K. So, it's weird
So I can only presume the path is via the low impedance A jack somehow...
Maybe one of the range switch contacts wasn't installed properly and was floating around? I presume you hadn't taken off the range switch before?, as that's required for the shim replacement.
At 600V 2.5Kohm would still be 250mA. Maybe enough to cook the PCB. Seems possible.
It looks like your DC supply is good for about 10mA. You could try to turn it up to 600V with the 121GW set to the DCV mode, then turn it to ohms. You may not have enough current to damage it, if your goal was to replicate the damage.
My goal was only to show that the contacts do not arc over at these voltages.
Please take a few pictures of R82 and U9 under the microscope. Maybe even ohm out R82. What ever went though these two traces had to go through more parts. There should be some pretty good damage to the top side as well.
Please take a few pictures of R82 and U9 under the microscope. Maybe even ohm out R82. What ever went though these two traces had to go through more parts. There should be some pretty good damage to the top side as well.R82 measures fine (100.3ohm), and no visible damage in the U9/R82 area. In fact no visible damage anywhere else on the PCB, although I haven't looked under the LCD assembly. The fuses are intact.
I also checked with my colleague, and he hasn't use this meter at all. I will reassemble it and run some functional tests shortly, but don't want to do too much before I meet up with Dave.
My goal was only to show that the contacts do not arc over at these voltages.
Yes, but the important factor is turning the selection knob while the voltage is connected. Under these conditions the contact springs can partially bridge the gap between traces on the circuit board and increase the chance of an arc occurring. That's exactly the test Joe was doing in his videos.
Is it allowed that you turn the knob when voltage is connected? But this could be a problem in combination with the lose knob without the shim.