# PothosSDR-2021.05.01 - Tx Calibration: MCU error 5

**URL:** <https://discourse.myriadrf.org/t/pothossdr-2021-05-01-tx-calibration-mcu-error-5/7274>\
**Category:** LimeSDR\
**Created:** [24 July 2021 12:24 UTC](https://discourse.myriadrf.org/t/pothossdr-2021-05-01-tx-calibration-mcu-error-5/7274 "2021-07-24T12:24:18Z")\
**Posts on this page:** 1\
**Page:** 1

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**Author:** ![exodus](https://avatars.discourse-cdn.com/v4/letter/e/958977/32.png) [@exodus](https://discourse.myriadrf.org/u/exodus)\
**Post date:** [24 July 2021 12:24 UTC](https://discourse.myriadrf.org/t/pothossdr-2021-05-01-tx-calibration-mcu-error-5/7274/1 "2021-07-24T12:24:19Z")

</div>

Hello

My board is LimeSDR-USB rev 1.4  
I’ve installed PothosSDR-2021.05.01 and updated FW and GW to the latest versions.  
As you know, GNURadio performs automatic calibration before start.  
So, channel A calibration is ok, but channel B always fails.

I’ve tested calibration via LimeSuite, channel A calibration is ok, but channel B always prints message **“ERROR: Tx Calibration: MCU error 5 (Loopback signal weak: not connected/insufficient gain?)”**. But with enabled MIMO calibration is performed successfully.  
I did read old topics and the wiki, nothing could help me.

Is it a hardware bug?  
How can I fix it?