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Bricked Fluke 177

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AVGresponding:

--- Quote from: tom66 on July 15, 2022, 09:53:51 am ---If you look at the diagram for a full bridge rectifier it will be apparent why it tests open on the 'AC' side.  There is no path for DC,  it is like a bunch of valves that reverses the connection on each mains cycle.

I think the problem is that COM on a meter is not the same as battery ground,  it is going to be floating at some midpoint reference.  Maybe 3V for instance.  Then the ADC can measure voltages above and below COM potential.

If what you did is to either short COM to battery ground, or to +9V, then it's very likely that the reference will have been damaged.

Unfortunately on many meters this reference does in fact come from the main ASIC.

--- End quote ---

Not sure about the 177, but on the 87, COM is 2.5V above battery -ve, as the -2.5V supply is at battery -ve. I would not be surprised if the 177 is very similar. I'm not aware of a service manual for the 177 but the one for the 87 found in those other threads might give you some pointers as to where to look.

One thing I would say about measuring your supply voltages, is don't just measure them at the regulator, measure them at the IC's they are powering too.

Wrenches of Death:

--- Quote from: Fungus on July 15, 2022, 12:58:38 am ---I'll point people to this thread when they ask why anybody needs more than one multimeter.

--- End quote ---

Yep!

Sadly, this isn't the first post that I've read over the years of someone frying their meter trying to read the voltage of the internal battery.

It might be worth posting a warning about attempting to do this as a permanent sticky.

WoD


AVGresponding:

--- Quote from: Wrenches of Death on July 15, 2022, 01:53:45 pm ---
--- Quote from: Fungus on July 15, 2022, 12:58:38 am ---I'll point people to this thread when they ask why anybody needs more than one multimeter.

--- End quote ---

Yep!

Sadly, this isn't the first post that I've read over the years of someone frying their meter trying to read the voltage of the internal battery.

It might be worth posting a warning about attempting to do this as a permanent sticky.

WoD

--- End quote ---

This had me wondering if there was anything specific to this in the manual; there isn't, at least in the 80-series one. The closest to it is this:

"
• Make sure the battery door is closed and
latched before operating the Meter.
• Replace the battery as soon as the
battery indicator (M) appears.
• Remove test leads from the Meter before
opening the battery door."

But this is under the general warning about avoiding shock risks etc, which I guess might lead people to think if they aren't measuring any dangerous voltages it's ok to ignore the warning.

robert.rozee:
see the photo from this thread:
https://www.eevblog.com/forum/testgear/fluke-177-failed-and-need-support-to-repair/?all




check voltages on the pins of the 16-pin chip labelled "24351". this is the analog-to-digital converter, an LTC2435-1:
https://www.analog.com/media/en/technical-documentation/data-sheets/24351fc.pdf

with the F177 set to DC VOLTS, input leads shorted, and using pins 14, 15, 16 as your ground point, you should see:
3.3v on pin 2 (supply for the ADC)
2.5v on pin 3 (reference voltage)
0.0v between pins 5 and 6 (in+ and in-)

report back the voltages returned, as well as what is on the F177's LCD.


cheers,
rob   :-)

andyB2022:
You mean shorting COM and Voltage plugs on the DMM and then doing those measurements? Meter set to V DC or mV DC?  Will try the measurements and come back with some updates.

In my opinion, if it was a problem with the ADC, the meter would work normally just it won't be able to measure anything correctly...but it doesn't even want to "cycle" to different measurement options when I spin the dial.

Also if you guys have an ideea of what IC I can remove from the PCB and then test the meter without it, I'll try it... nothing to lose right now...

Regardless of the screen, it's still the same as in the video of the first post. Wherever the dial is set it will stay in continuity mode and will beep sometimes randomly like it would detect a dead short on a PCB / wire etc.

If anyone can tell me the input impedance of their meter between V and COM? I measure 2.5M ohms with a crappy UNI-t...

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