Help Repairing East Tester ET5410 Electronic Load - "COMM FAIL!" Error - Page 1
Forum section: electronics / repair

author: tantonsen
author_rank: Newbie
timestamp: August 11, 2025, 10:16:14 am
content: Hi everyone,I hope you're all doing well. I'm reaching out to this wonderful community because I'm a bit stuck trying to repair my East Tester ET5410 electronic load, and I could really use some friendly advice from folks who might have experience with similar gear. I've been tinkering with electronics for a while, but this issue has me stumped, and I haven't been able to find any identical problems online (though I did spot a somewhat related thread on DIY Solar Forum about an ET5420 showing "CH2 COMM FAIL," which was fixed by checking the voltage selector switch—mine seems correctly set for 230V here in Denmark, but worth mentioning).The problem: When I power it on, everything seems to start up normally at first—the display lights up, and I can see communication happening between the display and the processor on the main board. The relay on the board clicks as the last step before the error appears, and then it just shows "COMM FAIL!" on the display. No buttons respond after that, and it won't function.Here's what I've done so far to troubleshoot:- I've carefully disassembled the unit to inspect the boards.- Measured the power rails on the back of the PCB: Getting stable +15V and -15V, so the power supply seems okay.- Disconnected the sub-board to simplify troubleshooting—no change.- Confirmed the processor on the main board is getting power.- Regarding the relay: It only activates if the 5-pin ribbon cable (connecting the display/front panel to the main board) is plugged in, which suggests to me that there is at least some communication happening between the display and the chip on the main board. If I disconnect that ribbon cable, the relay doesn't click at all, but the "COMM FAIL!" error still shows up.Beyond this, I'm not sure what else to try or where to measure next. Maybe check specific pins on the processor or look for faulty components? Or could it be a firmware issue? I'd be incredibly grateful for any suggestions, schematics if available, or pointers on common failure points with these units.Thank you so much in advance for your time and help—EEVblog has been such a great resource, and I appreciate any insights you can share!Best regards,Thomas Antonsen

author: Kean
author_rank: Unknown
timestamp: August 12, 2025, 05:19:22 pm
content: From your description I am guessing you are not using the USB or RS232 interfaces when this occurs?If so, then that likely points to a communication issue between the front panel STM32F103 processor and something else probably on the main power/analog board. That other thing could be UART comms, maybe through opto-isolation. Or with the general "COMM FAIL!" message it could also possibly be I2C or SPI device.As it doesn't happen immediately after power on, I would first suspect either mechanical or thermal fault. But maybe time it to see if it is consistent - maybe it is a hard failure and the result of a firmware timeout.For mechanical issues, check the cables and connectors for any sign or breaks or contamination.For thermal issues, you can try see if the fault appears quicker if you start it from warm or cold. Maybe use a hair dryer or similar to raise the internal temperature above 60C.

author: MathWizard
author_rank: Super Contributor
timestamp: August 12, 2025, 11:51:23 pm
content: Do you have an oscilloscope ? I don't see any error codes or troubleshooting in table of contents, of the 2 manuals I just checked.Really you want a Logic Analyzer, and some fine test leads to solder onto the chip or nearby. The 8-channel $20 ones on ebay/amazon should be fast enough to look at something like this.Maybe it's trying to talk to an external chip on the PCB, like an EEPROM, but has a comm problem ? Or something silly like a USB chip has a problem(if there's USB), and the MCU program decides halting everything, is the best course of action.1 bad scenario, is the MCU is screwed. If that happens, I would still try and make some basic Vin circuit, to drive the output section. It shouldn't be too hard to make some simple adjustable circuit for constant DC current at least, just have some built in limits in case it goes out of bounds.