Electronics > Repair
Fluke 289 not quite sorted.
Towlerar:
I have a Fluke 287 I acquired from eBay for cheap. At first I found that none of the power rails would come up and pressing the power button resulted in the LED coming on for a couple seconds, flashing briefly and emitting a quick chirp, then powering down with nothing on the LCD.
I found two shorted tantalum capacitors in the 20V section for the LCD bias (C97 and C98). These caps also appeared to have shorted the output driver in the 20V converter IC (U21). Removing all of these caused the meter to now stay on after turning on but the LED never goes out so it is still not booting correctly. There is no SPI communication between the MXS chip and the MSP430. The UART on the MXS does not output anything at all, nothing on the screen, no buttons respond. The crystal on the MSP is working fine but the crystal on the MXS is running at 100khz even after replacing.
I have come to the conclusion that the MXS chip is one sick puppy. It is getting slightly warm to the touch and indeed the thermal camera confirms this. I am powering the VBatt supply and the 20V supply from a lab supply. The 20V supply pulls about 10mA during powerup which seems normal. The 9V VBatt supply was drawing a whopping 258mA prior to finding and removing shorted components in the 20V section. Now it's at 50mA, which is still high. It seems normal is around 15-20mA on good meters according to earlier posts.
I think my next step is to obtain a JTAG programmer and upload the hex files from the existing application processor. It's unclear to me of there even is any programming in the MXS however. Can anyone confirm this? It seems maybe I could simply replace the MXS chip and it will upload the code it needs from the NOR flash? Either way, replacing the MXS chip seems to be my next obvious step.
Towlerar:
This evening I decided to try pushing on the PCB behind the MXS processor during booting and it got into the boot menu and complained about the date and time due to a bad supercapacitor which I removed. However, the meter never fully booted because no readings made it to the screen. I wasn't sure whether the issue was with a cracked ball on the BGA or a cracked MLCC so I started measuring across the various bypass caps and found C76 would intermittently short when I pressed on the board and released. The problem seemed to go away when I removed the cap but now I can't get the meter to boot anymore. Thinking maybe the problem is actually the BGA I tried reflowing it with the hot air rework station to no avail. So it seems the MXS chip is alive but perhaps there is a cracked solder ball that doesn't want to reflow. I may have no choice but to replace it anyway. Even still, I'm concerned the meter never fully booted, which might mean a missing or corrupted file in flash.
Towlerar:
This morning I replaced the suspected bad capacitor with a new one and I can consistently boot to the point that the power LED extinguishes and the buttons seem to mostly respond. Though, notably, the backlight button does not respond and I get nothing on the LCD anymore. This is probably no coincidence because both are tied to the MXS IC. The 9V Vbatt rail is pulling 18mA at idle now, which appears to be normal. I think the trick now is to check the UART debug communication and see if an error is being thrown out there.
Towlerar:
--- Quote from: kjk24 on January 03, 2020, 02:45:36 pm ---new fotos of the MC9328 & the Falsh & RAM
--- End quote ---
May I ask where you sourced your RAM from? I cant find it stocked anywhere.
Towlerar:
Today I attempted to get the debug data out of the UART port on the MXS IC. Try as I might I could not get the data out. My understanding is that the baud rate is 115200, 8, 1, N but I was not able to receive in TeraTerm or even decode with my Rigol DS2302, despite there clearly being data there. 3V TTL with nice, clean rise and fall. I was probably making some stupid mistake. Either way, the point is moot I guess because I made progress even without that data. I'll probably go back and figure out what I was doing wrong. I would expect even if my baud rate is wrong I would get gibberish.
Anyway, I took a break from the UART problem and decided to look at the weird issue where the LCD worked once and did not work again. I started by flowing contact cleaner into the flat flex connector and inserted/removed the flat flex several time even though both sides are gold plated and shouldn't oxidize. Then I heated up the solder joints on the connector pins, still no change to the display. Next, I pressed on the flat flex it multiple directions to see if it would come good and to my surprise I got a partial image. I then took the LCD out and inspected the flat flex under the microscope. The edge of the connector where the wipers contacted the gold pads looked fine. However, I noticed a micro strain hairline crack on the traces coming off the pads where the area of the flat flex with stiffener exited the connector body. It probably had small vibrations over long periods of time which ultimately strained the copper and cracked it. I'm guessing this meter had a hard life riding in a truck mounted toolbox based on the condition of the outside case. That can obviously set up sustained vibrations at certain vehicle speeds and eventually crack. Being that these LCDs sell for near $200USD on eBay, I was keen to try to fix it myself. I essentially tinned the gold plated fingers with solder and cleaned it with alcohol. I know it's not the best for longevity, but there weren't a lot of options. Also, this meter is now in a form of retirement and will have a cushy life on a bench in a temperature and humidity-controlled lab. The clearance became a bit tighter in the connector, but it worked! Again, I'm unsure what kind of longevity it will enjoy. I was still not getting the meter to fully boot, and given my UART problems I decided to try something different. I used some thin liquid flux and dropped it under the MXS applications processor and tried reflowing it at 315C with the hot air rework station. To my surprise this seems to gave worked! There was a definite improvement in how far the meter booted. However, at this point I was getting just a black line where the measurement should be regardless of what position the rotary switch was in. I verified that the notches between the switch and the housing lines up correctly. Then it struck me that if the switch was not making good contact with the PCB, measurements would not be able to come through. I cleaned the PCB and switch contacts with contact cleaner and rotated the switch back-and-forth several time and tested again. And... It's alive! It's almost a bit disappointing I didnt have to get into flashing any ICs. I think I'll still go back and figure out the UART issue that clearly is attributed to my setup a f not the meter itself. I'm also going to go forward with getting a JTAG programmer and will carefully probe around the MSP430 and MXS. It wouldn't hurt to have uploads of their hex files for the future. I'll probably also buy a spare MXS chip since those are EOL. I already cant find the RAM chips. The NOR flash and MSP are still available so I wont worry about that yet. I'll go ahead and order some replacement parts and get it put back together. This should just about wrap up troubleshooting for me. I just need to run some performance tests using my various calibrator to ensure everything meets spec. Who knows, perhaps there is still some calibration or even more troubleshooting in my future.
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