Trim instability issue with PB Competition SRS

  • Driven by my PowerBox Competition SRS (V.25), the servos (MKS-HBL380) consistently power up with minor, but noticeable changes of trim. For example, through multiple power cycles, the rudder’s master servo comes up, seemingly at random, in one of two different, but very repeatable, positions. For one position, I need to give a +12 value of sub-trim in the Futaba T18MZ transmitter to bring the rudder to exactly neutral. The other position requires +18. However, if I power cycle the servo directly plugged into the PWM port of the R7008SB receiver, it consistently comes up at the same exact position, requiring a sub-trim of -35. The plane and its PowerBox are about 2 year old, but I noticed this issue only in the last few weeks of flying. I also noticed the servo movement is somewhat jerky when connected to the PowerBox, but smooth when connected directly to the receiver. Are these symptoms indicative of a problem with my PowerBox device?

  • Hello,

    if the trim moves it looks like a (temperature) problem with the Oscillator of the PowerBox. A little jerky movement is because of the different framerates between receiver/PowerBox/Servos. I´m pretty sure this is not part of the problem

  • Hello,

    if it were a temperature-related problem, wouldn't the trim move in the same direction, not back and forth with power cycles, as the internal temperature of the PowerBox increases, possibly due to heat generated by the voltage regulators?

    Regardless, it would seem there is a malfunction of my PowerBox. What do you recommend at this point?

  • I contacted PowerBox Americas and, while the old unit will be sent to Germany for repair, I was able to buy a brand new Competition SRS unit (V.25), which I installed today. The good news is the trim problem is no longer there and initially it seemed the new unit worked normally. However, the bad news is that I can no longer get the PowerBox information (Voltage, Capacities, RX info) via Telemetry (Futaba 18MZ and two R7003SB receivers). The other sensors' telemetry (receiver voltage, temperature and RPM) work fine. I tried to re-register the Teleconverter and also Reloaded the slot numbers of all sensors, directly connecting to the Transmitter port. No luck!

    I reconnected the old PowerBox unit to the same Teleconverter and receivers and its Telemetry worked fine.

    I am now out of troubleshooting ideas. What am I missing? Please help. Thanks

  • Problem solved! It was the "Telemetry" point in the "RX/TX Settings" menu, not documented in the Manual and now under the Teach Failsafe point. I believe it was not there in the V.24 version (but I am not positive). When I updated the old PowerBox to V.25, it must have defaulted to "Futaba", a reasonable choice since the "TX-System" menu point was set to S-Bus (which is only Futaba). Once I change "Telemetry" to Futaba, it all started working immediately. It does seem a bit redundant, given I had already chosen S-bus above.

    I'd recommend updating the Manual, if it has not already been done.

    In any case, I should not have missed it.

    Thank you very much for the quick response and your help....

  • As a follow-on question, I noticed that the trim value changes quite a bit going through the PowerBox, even with the new replacement, fresh from the factory. For example, I set rudder trim and sub-trim to zero in my transmitter. With tester connected directly to Ch 4 of the R7008SB receiver, I read 1522 microseconds. With the same receiver connected to the Competition SRS, via S-Bus and with Ch 4 mapped to Output C, I read on (C1) 1490 microseconds, a noticeable difference in the rudder deflection.

    Is this the normal operation of the PowerBox?