Bricked Fluke 177

[andyB2022]

How can I check that? I don't have any datasheet on the pins of the ASIC... I've probed something yesterday around some vias for the ASIC but found nothing....

[AVGresponding]

Unfortunately the only way is to trace it out the hard way... as you've found, with it being a multi-layer board it's a difficult proposition.

The 8x series uses the same ADC, its connections to the main ASIC are:

Pin 11 CS (ADCS*) --> ASIC ADCS*

Pin 12 SDO (D-OUT) --> 49k9 resistor --> ASIC D-IN

Pin 13 SCK (ADSCK) --> ASIC ADSCK

Obviously CS should be over rather than under-scored, but that's not so easy to do here...
The resistor in the data line will make it harder but not impossible to identify the pin on the ASIC.

Now, on the 8x series, the main ASIC is the main signal processor and also the display driver; I don't know if this is the case with the 17x series. If they are separate, the symptoms could indicate a dead MSP ASIC, with the display driver outputting those dashes as a kind of default in the absence of instructions.

Before any of that though, can we recap on some earlier measurements? If the ADC is meant to be getting 3.3V and it's getting 5V that's not good. In the 8x series the ADC is supplied with +/- 2.5V, the REF and VIN are connected to GND. It would be useful to know which is correct for your case.

[AVGresponding]

Here are the DSO captures for :LTC2435

As far as I know about SPI, SCK signal doesn't look good...(pin 13)

It's hard to say whether any of the overshoot might be artifacts of the measuring setup. Here's the excerpt from the 8x service manual on the ADC operation:

Analog to Digital Converter
U3 is a 20-bit Σ∆ analog-to-digital converter (ADC). C21 & C22 are power-supply
bypass capacitors. The DC signal at FE_O (U1 pin 19) provides the signal input for U3.
The reference for conversion is supplied by U8 as described in 1.3 to the REF and VIN*
(compliment of VIN) inputs. Since REF* (compliment of REF) is tied to -2.5 V and both
pairs of inputs to U3 are handled differentially, the dynamic range of the VIN is ±1.25 V
around COMMOM. The microprocessor U2 uses three digital lines to communicate with
U3. U3 signals the microprocessor U2 that a conversion is completed by pulling the SDO
signal line to logic low (-2.5 V), which is coupled through current limiting resistor R31
to DOUT. When U2 is ready for the reading, it pulls ADCS* of U3 to a logic low and
clocks the data serially out of SDO with the signal ADSCK applied to SCK. Connecting
F0 of U3 to -2.5 V sets the internal clock so that the normal mode rejection ratio
(NMRR) of the digital filter will provide adequate rejection of both 50 Hz and 60 Hz.

I've left the Fluke typos intact...

[andyB2022]

Before any of that though, can we recap on some earlier measurements? If the ADC is meant to be getting 3.3V and it's getting 5V that's not good. In the 8x series the ADC is supplied with +/- 2.5V, the REF and VIN are connected to GND. It would be useful to know which is correct for your case.

REF- is the only pin tied to GND.
VCC pin has 5V on it (datasheet says between -0.3V to 7V)
REF+  2.5V
IN+  2.5V
IN-  2.5V

All display related traces come exclusively from the microcontroller, most of them are on the top layer. So the ASIC has nothing to do with dispaly (I guess).

[andyB2022]

I've found a great offer for around 16$ shipped to my country (Romania, Europe) for the ADC and ASIC chips.
The question is, may it work with those things new? (I know meter will need recalibration).

[AVGresponding]

Before any of that though, can we recap on some earlier measurements? If the ADC is meant to be getting 3.3V and it's getting 5V that's not good. In the 8x series the ADC is supplied with +/- 2.5V, the REF and VIN are connected to GND. It would be useful to know which is correct for your case.

REF- is the only pin tied to GND.
VCC pin has 5V on it (datasheet says between -0.3V to 7V)
REF+  2.5V
IN+  2.5V
IN-  2.5V

All display related traces come exclusively from the microcontroller, most of them are on the top layer. So the ASIC has nothing to do with dispaly (I guess).

I'll go out on a limb and suggest that those voltages are correct.

[Fungus]

I've found a great offer for around 16$ shipped to my country (Romania, Europe) for the ADC and ASIC chips.
The question is, may it work with those things new? (I know meter will need recalibration).

It's worth a shot at that price if you have the equipment to replace them.

[AVGresponding]

I'd be reluctant to condemn the ADC without further testing, but if you can easily afford the parts, and if as Fungus mentions you have the tools to replace them I can see no harm.

A couple of things would be useful to know.

1. Could you sketch a quick drawing to show how exactly you connected it up when you tried to measure the current draw.

2. Is there a Fluke ASIC for switching the front-end ranges, like there is in the 8x series? Given your reported input impedance of 2-ish megohms, when it should be 11, I'd want to look more closely at this first.

If the MSP is getting bad/no data from this ASIC it might explain why it appears functional using some of the buttons but doesn't respond to the rotary switch properly, as the front end ASIC and not the MSP is the one directly connected to the rotary switch and the range function buttons.
Granted, I'm guessing a bit here, based on the 8x series design, but I think it's reasonable to suppose they'd use a generally similar system architecture, as it would save time and therefore money during the design stage.

[andyB2022]

Here is the way I did the thing, of course first time I saw nothing on the display and decided to swap the testing leads in between them... it was too late anyway as first time I connected the GND of the battery to COM lead...

As I've read here on the forum, the COM lead is very low impedance (like DSO GND) and you've gotta be really careful where you place it. Probing just with the V+ lead would be harmless (impossible to measure current without COM of course, as my intention was...)

I have a hot air soldering gun, flux, solder wick... I've soldered even smaller packages, so no issues here.
The only idea would be not to damage even more things while replacing the ADC and ASIC. Even if the ADC seems fine, it's worth ordering that thing too, just in case.

But, I have a stupid question here, the ASIC comes already programmed, doesn't it? I know these things can do only one task... from Bitcoin mining 

p.s. The idea of the whole topic is to try to fix those things and help other people... I could just go and buy a new meter and that's it, learn my expensive lesson, but that won't make the forum interesting, wouldn't it?

[Fungus]

But, I have a stupid question here, the ASIC comes already programmed, doesn't it? I know these things can do only one task... from Bitcoin mining 

ASICs aren't programmable, their functions are hard wired. It's like building a fixed-function computer out of 74-series logic chips instead of a CPU+MEMORY. They can also incorporate some analog elements in there as well.

Disclaimer: I'm sure there's some ASICs out there that have a programmable section of the chip and also some FPGAs with built-in opamps and comparators.

[andyB2022]

Thank you for that intel. I'll order the ASIC and the ADC. Keep you guys posted with this.

Meanwhile is it a good idea to just desolder the current ASIC and power on the meter without it? See if the behavior changes in any way?

[AVGresponding]

Thank you for that intel. I'll order the ASIC and the ADC. Keep you guys posted with this.

Meanwhile is it a good idea to just desolder the current ASIC and power on the meter without it? See if the behavior changes in any way?

Are you talking about the MSP? That is to say, the Linear Technologies Fluke chip N23297? I suspect you won't cause any extra damage by doing this, though I am leaning very much in the direction of a front-end issue rather than the MSP. I also suspect the behaviour will not change much if at all, as without this chip in circuit it won't be able to send any signals to the display driver, which will probably default to the same dashes.

The MSP ASIC may contain a small eeprom for the calibration data.

[andyB2022]

Could you please define "front-end"?

Recalibrating the meter won't be an issue as I have near me a specialized center that can perform this. As I've talked already in the idea of service with the guys out there, they've said that if the meter is too away from a calibration (having too big "errors") they won't be able to perform it as the meter will refuse storing new values (this explanation from them seems very odd to me)...

[AVGresponding]

By front-end I mean the analogue circuitry and the range dividers and switches. On the 8x series, the range switching though partly controlled by the rotary switch, also is largely controlled by an ASIC, which in terms of the signal path, sits between the input jacks and protection circuits, and the ADC.

So the complete signal flow (simplified) would be, input jacks --> protection circuitry --> rotary switch --> range + signal conditioning ASIC --> ADC --> Microprocessor.

I don't know if the 17x series does it the same or a similar way. A good look at both sides of the main board might help us figure that out.

[Per Hansson]

2. Is there a Fluke ASIC for switching the front-end ranges, like there is in the 8x series? Given your reported input impedance of 2-ish megohms, when it should be 11, I'd want to look more closely at this first.

andyB2022: I would also look into what AVGresponding asked above, as Alonso tested you should have 11M input impedance but it is 2.5M on your unit.
The unit itself can switch from 11M down to 10M when you press the range switch (if the processor is working that is).
In the attached schematic of a Fluke 77-III I traced the VDC input path in red for you (I take no responsibility for any mistakes I've made)

The classic failure is one of RT1, R1 or RV1, RV2, RV3 fails in a input overload scenario, and then the input impedance will change.
We know in your case this is not what happened, so we can rule out those components, along with the high precision Z1 reference ceramic network.
So basically the path I show we can assume is ok, but if you see you can also trace other paths from that into the front end, to some comparators.
If those are bad then you could be reading a lower input impedance, this is just an example to give you some ideas what to look for.

P.S: Please understand that the attached schematic is for the older model Fluke 77-III but some parts should still be similar!
Source: https://dam-assets.fluke.com/s3fs-public/77iii___smeng0100.pdf

[andyB2022]

I owe you guys some apologies, the 2.5M resistance was measured with a very dodgy meter, I've got this UNI-T 216C from a friend today to test again. Looks like the resistance between COM and V is infinite (above 60M at least, the maximum this clamp meter can measure)...
Second photo is to proof that the clamp meter measures correctly (tested it on a few 10k resistor too to make sure...)

So basically we are dealing now with an open circuit...

[Per Hansson]

The meter must be switched to the DCV mode to measure the input impedance.
Here an example of two meters:

Code: [Select]

UTE61E: AN8009:
OFF: 25.60M 36.4M
1V: 11.15M 10.982M
10V: 10.13M 10.085M
100V: 10.04M 9.995M
1000V: 10.03M 9.985M


[andyB2022]

Please watch below video with sound to hear to buzzer. I pressed on the case to show you that the meter it's alive.

https://www.youtube.com/shorts/pdBuUkCJIhM

[AVGresponding]

That's quite odd behaviour; normally when you can affect things by for example pressing on the case, it's a bad connection issue. The microprocessor seems to be cycling ranges; the impedance readings on V are what you might see with a constant range cycling. Have you inspected the leaf contacts on the rotary switch?

You need to set your clamp meter to read voltage when you check the resistance/continuity/diode functions.

[andyB2022]

The screws are not in the case that's why I've had to press on it, as I open it and put it back very oftenly nowdays (so the LCD makes contact and shows you guys something).

The leaf contacts are in pristine condition, so are the gold plated traces under them (just so you guys understand how much the meter was used it doesn't even have scratches caused by the contacts from many "switches" between different measurements).

[andyB2022]

Parts came in. Replaced ASIC and ADC, no change in it's behavior...

That means that those parts were ok?

[AVGresponding]

Most likely, yes, unfortunately.

[andyB2022]

I guess there is something wrong in the front end as you said previously.

[AVGresponding]

Tracing it and drawing it out will be an exercise in patience and concentration...

[andyB2022]

Yeah, it's a bit out of my capability... An X-ray machine will do the job in seconds... Anyone here with something like that? Or maybe someone has a photo with the PCB already?