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Bricked Fluke 177
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? :scared:
AVGresponding:
--- Quote from: andyB2022 on July 17, 2022, 07:11:17 pm ---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? :scared:
--- End quote ---
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:
--- Quote from: AVGresponding on July 17, 2022, 05:41:11 pm ---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.
--- End quote ---
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) :D
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
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