Electronics > Repair
Fluke 289 not quite sorted.
MihaiV:
Hello,
I had to delay the repair for a while, but I'm on it again. I started troubleshooting the weird selector switch operation.
First, I checked all contacts, on PCB and the moving parts, to ensure there is no oxidation or faulty contact. No problems here.
In a previous post (nr 185) and with the help of Towlerar I focused a little on the voltage divider used for range selection, so I checked what voltage is required for what function. Also useful to see if the software does what it should.
I took a multi-turn pot (1K, the panel mount type), tied its ends to GND and the 2.5V reference and the wiper to R19, going to pin 95 of MSP.
All ranges are detected, and the results are in the table below. Measurements were made with a meter from work, set on Hi-Z. Putting 10Meg in parallel with 1Meg of R19 introduces a bit of error.
Fit the moving parts back (by the way, there is no way to assemble it wrong, maybe with the hammer, but there were no signs of brute force) and measure the output on each range (attached in the second table).
I noticed that all were a bit higher than it was needed. First, I thought that there was some flux residue under the first few resistors, so i cleaned them again, but there was no change. Using the 10Meg input also didn't change a thing. I wanted to fit a higher resistance above the first resistor in the divider, on the 2.5V side, an then it struck me to check R19. Out of the circuit, it was open!
I imagine I must have broken it myself when I desoldered it first time. Replace it with a new one and it works !!!
All functions are on their proper positions, none missing! Put the thing back together, to check the enclosure effect (meaning that things work when open and not when in their enclosure) and it works, all of it! Wow, I thought I wouldn't make it!
Big thanks to everyone that contributed to this forum and tipped me with great advice!
Here is the list of faults in my unit:
- Slightly corroded back-up capacitor. Remove and replace (don't know with what yet, but I saw a topic on it in the forum).
- Faulty solder joint under the MXS: showed the splash-screen only when pressing on it or warping the board. Reflowed the thing with liquid flux and hot air.
- Faulty MXS crystal, 32.768 KHz. Not oscillating, maybe cracked (?). Replaced with something else, and it runs. Make sure you use a low capacitance probe on it.
- Corrupted/wrong application. Fixed using the binary from AnJu, only the "2.fs 0x10040000.bin" file, I kept my own bootloader. Make sure you short JP4 at power-up. This removes the lock-bit problem when using the Jlink to write/delete the flash memory.
- Faulty resistor at the rotary switch, showing wrong voltages to the MSP430. Most likely because of me, not a fault from the beginning.
Glad to save it from the landfill!
I wanted this meter for my little brother, he got in the Polytechnic University last year, at some kind of light version of the electronics faculty, and I want him to start working with actual parts and tools, not only with paper and simulations. He saw it, but he doesn't know it's for him (he calls it "the yellow dumpster"). I can't wait to see his face when I'll give it to him! :-DMM
Thank you everyone!
lern01:
--- Quote from: nixxon on October 21, 2018, 02:08:15 pm ---
--- Quote from: ElecSeb on October 21, 2018, 07:12:07 am ---Expected non-working LCD to Working DMM.
This way when it works you know for sure the LCD works.
Other way around when good LCD attached to defect DMM, you know screen from new unit is good, but if it does not work, you know the Mainboard of the defect DMM is broken. But till that moment you still do not know if the suspected LCD is broken.
My 2 cents
--- End quote ---
I did as you suggested. The LCD from the 289 woke up fine when connected to my good DMM (287).
It seems that the LCD driver circuit on the 289 is no good. I noticed that one of the crystals had a low amplitude (200 mVpp) the way I measured it. According to the Fluke 27 Service Manual a similar 32.768 kHz crystal should measure 600 mVpp or more.:
* 4. Connect either an oscilloscope or frequency counter, using a low-capacitance probe,
to pin 54 of U1 or to the junction of C17 and Y1 (the crystal). A 32.768 kHz sine
wave with an amplitude of approximately 600 mV peak-to-peak should be present at
the junction of C17 and Y1 (3 V peak-to-peak at U1 pin 54). Note that U2 and the
display will not operate if the clock signal is not present. If the clock signal is not
present, the most likely causes are U1, Y1 or C17.
*
Can a missing (or low) clock signal be a part of the LCD problem when the rest of the meter seems to be working ok?
--- End quote ---
U1, C17, and Y1 are not in a functional circuit, and they seem to have nothing to do with the LCD circuit? Please advise, thank you!
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