Home > CopterX Build Log, RC Helicopters > Mystery Gyro G401B (HK401B) modifications

Mystery Gyro G401B (HK401B) modifications

October 18th, 2009

The Mystery gyro G401B (sold by DealExtreme, HobbyKing, xmseller, etc) requires some modifications before using it. It’s a $20 gyro $14 gyro, so don’t expect it to have the same build quality of a $150 Futaba GY401 :P

Two major problems were reported in the G401B forum threads:

  • The gyro circuit board is not firmly attached to the plastic case, causing problems to the gyro operation.
  • The solder joints of the tiny switch for digital/analog/normal/reverse mode can break off the board when flipping the switches.

Items required:

  • Hot glue gun (I used a small 10W dual temp gun)
  • Small piece of soft foam – 2.5cm x 2.5cm, about 0.8cm thick (1″ x 1″, 1/3″ thick)

How to fix:

It seems like a lot of work but this is actually a 5-minute job:

  1. Unscrew the gyro case lid and carefully remove the circuit board from the plastic case.
  2. Add hot glue to the solder joints of the switch block (both sides) so it is secure on the board. If you add too much glue, it will be harder to put it back (I had to carefully trim the excess glue with an x-acto knife).
  3. Apply hot glue to the servo solders on the board, so the wires are held not only by the solders. Don’t let the hot glue gun touch the wires as they will melt.
  4. While you’re there, apply hot glue to the solder joints of the two vertical circuit boards.
  5. Apply hot glue to each corner of the case and insert the board again. Use something long and thin (like a small screw driver) to press the board against the glue so it gets fixed on the plastic case.
  6. Insert the square piece of foam when screwing the case lid back. It will help keep the board in a tight, fixed position.

NOTE: Steps 3 and 4 are optional and might be considered overkill by some. I did them anyway as I didn’t see any disadvantages.

Gyro circuit board, before applying hot glue:Mystery gyro circuit board

Board glued and gyro ready to be closed:Gyro circuit board with hot glue

Soft foam:Soft foam and case lid


UPDATE 02/Dec/2010: Trainboy64 has posted a nice video on YouTube showing basically the same thing:

  1. Litris
    February 13th, 2010 at 00:06 | #1


  2. Jorge
    February 14th, 2010 at 03:52 | #2

    Hi, thanks for valious comments about gyro. Does it have more problems? Do you recommend it? Thanks.

    • Sergio
      February 14th, 2010 at 03:58 | #3

      Hi Jorge,

      These are the only problems I know about. I’d totally recommend it, in fact, I’ve just received another one for my second heli! :)

      Also, read this post regarding the gyro mount (foam + metal plate + foam).


  3. March 2nd, 2010 at 08:03 | #4

    Sergio…. “you da man”! This is actually quite similar an issue to the (circa 2004) Walkera build quality issues. Remember the DF#35 and DF#4? Yeah, well that too had quality assembly problems. Serg, really NICE detail and good photos! Makes me want to take apart my little $7 Eflite HH special! Never did work in HH mode anyway, now you’ve got me thinking.

    • Sergio
      March 3rd, 2010 at 08:34 | #5

      Hello there! Thank you for the compliments :)

      I haven’t seen those Walkeras (heh, I’ve started with RC only in 2009!) and I bet they were not cheap at that time :P
      I’m curious to know if your E-flite gyro also has this problem! I mean, I bet you paid way more than $14 for it!


  4. glider-90
    March 30th, 2010 at 08:04 | #6

    Merci pour toutes ces infos!!! :-)



    • Sergio
      March 31st, 2010 at 02:13 | #7

      Avec plaisir! :)

  5. Hampf
    April 1st, 2010 at 17:57 | #8


    I setup my HK450 today including the HK401B and I have a bad tail wag.

    I tried everything already and read all the hints on the net but to no success..

    Tomorrow I ll try to fix the board inside the case by glueing or put some foam inside and see if it fixes the wag.

    If it doesnt Ill throw it into the bin.

    • Sergio
      April 4th, 2010 at 23:41 | #9

      Hallo Hampf! How did that go?

      It might be obvious, but did you turn the delay setting all the way to zero (assuming you’re using a digital servo)? The potentiometer goes a little bit past the “zero” mark until it stops, so that’s something to watch out for.


  6. glider-90
    April 2nd, 2010 at 02:59 | #10

    Oups!! …..

  7. Le
    April 13th, 2010 at 08:00 | #11

    hello friend, is very good that post
    I would like to know which servants you are using with that gyro
    I installed this gyro in my copterx with servant ino-lab digital HGD 202 Carbon to Frost and the tail is fanning a little.
    I will try to do this repair and to see in what gives…

    • Sergio
      April 13th, 2010 at 09:11 | #12

      Helllo there!

      The INO-LAB HGD 202 Carbon is a great servo, but it’s too slow to be used as a tail servo and doesn’t have enough torque. I’d definitely recommend the Turnigy (MKS) DS480. As an upgrade to the DS480, I’d get the INO-LAB 261HB which is faster and supports 6v (the DS480 does NOT support 6 volts). I’ve got the 261HB coming from HobbyKing and will use it with 3x 202MG on the cyclic, with everything running at 6v. It should work wonders with that setup! :)


      Opa, beleza? :)

      O INO-LAB HGD 202 Carbon e’ um servo bom, mas muito devagar e fraco para usar como tail servo. Eu recomendaria sem duvidas o Turnigy (MKS) DS480. Algo melhor ainda que o DS480 e’ o INO-LAB 261HB, que e’ mais rapido e suporta 6v (o DS480 NAO suporta 6 volts). To esperando o 261HB chegar da HobbyKing para usar com 3x 202MG no cyclic e tudo rodando em 6v. Devera’ funcionar super bem com essa configuracao! :)


      • lucas77
        November 6th, 2010 at 06:41 | #13

        hello sergio
        first congratulate you for this little blog … I hope you continue!
        I have to make a purchase, and I thought about buying 3 servos hg-202mg bb for the cyclic on my Trex 450 clone. plus 1 for the tail mod. dgh 261hb, gyro hk 401 b
        what do you think?
        and esc and ubecs …??? I hope this issue some day …!!!
        a greeting from Spain and thank you very much for everything.

        • Sergio
          December 2nd, 2010 at 00:57 | #14

          Hi Lucas!

          Using 3x 202MG for cyclic and the 261HB + HK401B for tail sounds like a great combo! This is exactly what I’m using on the new 450 I’m still building.

          If you’re using a switch-mode ESC like the Turnigy Sentry 40A, you should be fine with the built-in BEC. Note that the Plush 40A does not have a switch-mode BEC. If that’s your case, the Turnigy 3A UBEC should be more than enough.

          Cheers and thanks for the praises! :)
          I’ve got so many helis and planes to build but so little time :P


        • Sergio
          December 2nd, 2010 at 01:12 | #15

          Hello Patrick! It’s really an honour to have a veteran here, thank you!

          As there are many things to cover, I’d highly recommend you watch this MikeysRC series of videos on building the T-Rex 450 clone.

          He shows everything, step-by-step, including how to setup the servos and program the transmitter on the PC. You could start by watching these two videos below, as they’re specifically about the servos and radio programming:



          Let me know if this is helpful or if you have specific questions.


  8. SLS
    April 21st, 2010 at 07:05 | #16

    Thanks Man! Good job!!! :)
    You can also use Epoxy glue, instead Hot Glue. It´s easier to work.
    Sérgio S.

    • Sergio
      April 21st, 2010 at 20:15 | #17

      You’re welcome, Sergio! And yeah, epoxy would also work fine for this!

  9. Paul W
    April 21st, 2010 at 13:48 | #18

    Thanks for the advice, nice of you to share!


    Paul W

    • Sergio
      April 21st, 2010 at 20:21 | #19

      My pleasure! :)

  10. Framos
    April 25th, 2010 at 02:23 | #20

    Add an important detail, the problem occurred to me only when I changed to the “head lock”, I noticed a movement of the tail servo when touched in the control of the delay GYRO, I glued the circuit board on the plastic walls of the box with epoxy glue and the gyro has functioned perfectly. Before I had changed the tail servo because believed the problem was it.

  11. Ivan I.
    April 29th, 2010 at 18:44 | #21

    Hey, thanks for the great tips! My gyro was twitching and all over the place before I did your mods. This was my first gyro so I had no clue what I was doing, but your process was very simple to follow and worked wonders!

    My gyro is now happily situated in a RC hovercraft, and the difference is like night and day. No more drifting and the turns are super accurate! Thanks for posting the modifications!

    • Sergio
      May 13th, 2010 at 08:30 | #22

      You’re welcome, Ivan. Now, you *have* to show me a video of your RC hovercraft! Is it scratch built or a kit? Videos videos videos! :)

  12. bothal3
    May 11th, 2010 at 14:55 | #23

    Hallo There. Did anyone experience the problem that when switching on, the gyro rotates the servo to it’s maximum in one direction. (not kreep) All controls and trims are centered.

    • Sergio
      May 13th, 2010 at 08:27 | #24

      Hello! I’ve never seen that problem, but I’d try a few things:

      – connect the servo directly to the receiver to confirm it works fine
      – connect a different servo to the gyro to check if the same problem happens
      – make sure trim and subtrim is zero for your rudder channel (ok, you’ve confirmed that already)
      – make sure the gyro is sitting still when turning it on (because of the gyro initialization)
      – switch the gyro to normal mode instead of digital
      – change your gyro limit dial to around 100 just for troubleshooting it now
      – assuming you’ve plugged the gyro gain to another channel on your receiver, reduce the gain on your radio (maybe it’s too high?)
      – inspect the gyro board and look for loose soldering/connections

      Let me know if you find a solution!

      • bothal3
        May 26th, 2010 at 04:09 | #25

        Thanks for the reply.
        The problem was a factory faulty gyro. After setup the gyro did not respond to any movement.
        It looks as if some soldering/track came loose.
        I replaced the gyro and everything seems fine.

  13. M@
    May 29th, 2010 at 18:27 | #26

    For this fix, do I add the hot glue to the corners of the plastic case or to the board, then slide it back into place?
    There is not much work time with hot glue.

    • Sergio
      May 29th, 2010 at 23:02 | #27

      You should add the hot glue to the corners inside the plastic case. Indeed, you gotta be quick :)

  14. eduardo garces
    June 7th, 2010 at 00:12 | #28

    Puafffhhh!!!! I dont see anything with your hands.Can you put the camera in another place next time?

    • Sergio
      June 7th, 2010 at 00:23 | #29

      That video is not mine, and yes, it’s hard to see it sometimes because of the camera position.

      Basically, here is what you have to do: get your hot glue gun on the higher temperature and have the gyro board ready to be inserted (check the proper position with the plastic case for the switches). Then quickly add hot glue to the 4 corners inside the case and immediately insert the board.


  15. thomas d.
    June 16th, 2010 at 22:06 | #30

    Hi Sergio,

    thank you, for this easy understanding modification manual!
    I will do this tomorrow, before installing the gyro.

    By, and many happy landings,

    Tom ..

  16. rvc
    August 5th, 2010 at 01:37 | #31

    hello sergio, thanks for the tips. Actually i m still thinking to buy this gyro. is it worthed? how bout the performance? is it good as they said before? most likely futaba 401. anyone can give a comment for this gyro?

    thanks anyway.

    • Sergio
      August 5th, 2010 at 08:42 | #32

      Hello there!
      It works fine and many people consider it as good as the Futaba GY401. I can’t compare because I’ve never used the Futaba and don’t do 3D yet.

      My thoughts are: if you’re putting this on a budget heli (i.e., using cheap but good enouch components like in my CopterX build), then sure, go with the HK401. On my Align T-Rex 500 it wouldn’t make sense to use a budget gyro when everything else in the heli was top quality/brand name (original Align kit with Align servos, etc).


  17. rvc
    August 6th, 2010 at 05:45 | #33

    thanks sergio, i will order it and before install to my trex, will do your tips first.

  18. Daryl Wong
    August 22nd, 2010 at 16:53 | #34


    I bought the HK401 but it came with the switch on the DS and Rev missing. Cannot get any satisfaction on a replacement from Hobbyking. It appears to work. Although I am not sure as this is my first heli and gyro. No problem with the reverse as this is in the correct direction. Question is what is the setting for the digital / analog switch if the switch itself is missing? Is there a way to set it without the switch? I am using a digital servo on the tail. Any help / advice appreciated.

    • Sergio
      September 3rd, 2010 at 01:52 | #35

      Hello Daryl! Sorry for the delay, I’ve been quite busy dealing with non-RC stuff :(

      That’s a strange problem… Are you sure both switches are missing? It could just be a tiny yellow plastic layer covering the actual switch. Use something sharp to remove that plastic cover and you should see the switch. If the switches are really missing, I would simply buy another HK401 from HK instead of trying to fix it :)


  19. me
    August 31st, 2010 at 08:10 | #36

    nice share.!!
    thank’s.,i have this gyro.,and my heli’s always drifting left and right when it fly..
    after do this modd.,tail is ROCK-SOLID…
    thanks thanks thanks

    • Sergio
      September 3rd, 2010 at 01:43 | #37

      You’re welcome! :)

  20. Dave
    September 12th, 2010 at 15:37 | #38

    Thanks for the information. I watched the video on youtube, basically useless without your pictures and explanation of what to glue. The video shows fingers and aglue gun in the way for everything he does. He should re-shoot the video at a different camera angle so you can see what he is talking about and doing.

  21. Tim
    September 23rd, 2010 at 00:40 | #39

    I hope to use two of these HK401’s to stabilize a camera with a two axes mount on a fixed wing airplane. Would these be good for that? Can I mount them vertical and horizontal for yaw and tilt? If not these what would you recommend? Thank you for your time/help.

    • Sergio
      September 24th, 2010 at 01:09 | #40

      Hello Tim!

      I think they would work great for a 2-axis camera mount and the fact you can adjust the limit/travel on the gyro seems a good feature to me. You should have no problem mounting them horizontally or vertically.

      I considered the TG-380 (same as MKS 380), but they’re rate mode only and there is no travel adjustment, so I’d go for the HK401’s for your application! I’ve used the TG-380’s to stabilize the airplane itself (aileron/rudder).

      I’d love to see some pictures of your build and of course some videos afterwards :)


  22. Ciaran
    October 11th, 2010 at 13:19 | #41


    I have ordered two and recieved these, as I am a beginner,and I fly a belt cp v2. Wud you reconmend switching the belt cp v2 standard gyro to the hk one? Is the HK gyro better?

    • Sergio
      October 13th, 2010 at 19:27 | #42

      Hi Ciaran, I don’t have experience with the Belt CP gyro, so can’t say. If that one is not a heading-hold gyro, I’d certainly go with the HK.

  23. Milt
    December 7th, 2010 at 16:53 | #43

    Sergio -thanks much for the HK401 gyro mods. I have made them on my gyros, but still have twitchy and drifting tail on HK 450 pro with either Corona DS-919 or Futaba 9650 in both digital and analog modes. Delay set on 0, mechanical setup done in rate mode, no sub trim. I have tried two different HK 401’s. No slop in linkage, and linkage slides very freely. Played with gain in xmtr [Dx6i] from 54 to 70% – more stable around 64%. Rate mode also has tail drift, currently set on 35%. Only thing I did different is use Goop instead of hot glue gun. Would that give me a problem? [Also tried an HM4000 from an AXE 400 – but didn’t help].

    • Sergio
      December 13th, 2010 at 23:02 | #44

      Hello there!

      Looks like you’ve tried a bunch of things already, so I’ll step back and ask other basic things :)

      What about the headspeed? If it’s too low, it will cause your tail not to have much authority. I would also confirm the tail blades are not installed backwards, the belt is not twisted and the belt tension is right. Also confirm the tail rotor is turning to the right direction. It could also be a strong vibration from the main rotor (unbalanced main blades?) or unbalanced tail blades.

      I would stick to that Corona DS-919 as the Futaba 9650 seems too slow to me for a tail servo. It’s rated at 0.14s/60, while this DS919 and the MKS DS480 are 0.07s.

      Let me know how it goes!


  24. Milt
    December 14th, 2010 at 09:05 | #45

    Sergio – thanks for the tips. I was successful by making one change: I put in another HK 401b and everything settled down. I put the original HK 401b in my AXE 400 and it worked just fine, so who knows for sure. Always some manufacturing and electrical part tolerances. Head speed didn’t seem to make a difference, as it did same thing in idle up. My HK 450 Pro is the smoothest heli I have ever owned – very little vibration. [Just picked up TRex 500 and it also is very, very smooth] The 450 pro is torque tube and not belt. Blades are on correctly and balanced, also turning in correct direction. I am running the mod suggested in HeliFreak by Finless Bob for rate mode by having the tail grips following the blades instead of leading edge control. Out of four DS-919 servos, one started making my other servos jitter, and one had no output bearing. The other two are working fine. Thanks again!

  25. Ramón RamoNET@
    March 13th, 2012 at 06:07 | #46

    Hi Sergio. Thanks for your work.
    You said you’ve got the 261HB coming from HobbyKing; Are you going to switch the HK401 Gyro on DS (digital) or Normal to use with?

    • Sergio
      March 16th, 2012 at 14:34 | #47

      Hi Ramón, the Inolabs 261HB is a digital servo, so you should certainly use the HK401 gyro on digital mode. I haven’t used this combination yet but it should work fine. I’ve been playing with flybarless controllers lately :)

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