Running a servo off HL 6.0 and 7.0 (and earlier MFUs) Tamiya too

[Ecam]

I have received a request for a how to use a servo with non 6.1 and 7.1 systems.

This is not full servo control, only to use/power a servo from the 6.0 and 7.0 MFUs.

I started pulling things apart and tinkering back in the 70s, sometimes they'd work when reassembled.

Old servos from the eighties and 90s.



One of the old ones along with a newer digital servo on the right (5 pack for $25 USD off Amazon).



The Heng Long MFU ports for the airsoft, barrel lift and turret turn are all on off. The Tamiya lift and spin are proportional. Simply remove the circuit board from the servo along with it's power lead. Run a HL or Tamiya lead from the appropriate port on the MFU and solder leads directly to the servo's motor.





Most servos have 180 degrees of travel, 90 either way from center. Which means a servo would reach full travel in no time and could be damaged by straining on it's stop. So to get past this I remove the stops in the servo gear train.





The bar on the nylon gear (at 3:15) and silver pin on the brass gear. Now the servos are miniature geared motors for use in turret spin, rotary drive, winches and with some creative gearing could work a lever for barrel lift and push pull (recoil). Still working on that last one for my TDs but they've been moved to the back burner for now.

The airsoft, turret turn and barrel lift ports are all strong enough to run the airsoft motors as I've done in my Priest and M32 builds. The winch on my M32 can lift/pull around 4.5 lbs or 2 KG. A modified airsoft motor running the spool (tested with the HL system and then switched to the Tamiya)



There are other ways to do this but are more complex, here is an example: viewtopic.php?t=31735

This should provide full servo control or switch to a 7.1 board.

[Ecam]

As a contrast, I wanted to run a HL smoker motor for the winch on my large DUKW running a six channel radio (Flysky FS-GT5). In this case I use the servo's electronics just not the servo motor. I steal the power leads for the servo motor and extend them to the HL motor. It gives a proportional control for motor speed.



Slight alteration is needed to the servo gears as the dash pot (signal feed - red wire) has to be able to be adjusted. Here is a the typical gear set on the top of a servo.



The center and right gear are removed (center and lower in picture below).



Within the brass bushing is the dash pot that is the servo signal wire. Now the servo will act like a speed control for the external motor. The output gear will be used to center the servo. Like some speed controls forward may be 70% and reverse 30% moving the arm will find that center.



On the DUKW I found the center kept moving so I added a turn buckle for adjustment and holding center (no problem since). The Flysky unit I use is a pistol style with 4 channels for full servo control (1-2 & 5-6). 5 and 6 do not have a center detent so I elected to use a 3 position switch (ch.4 - but no trim adjustment) to control the winch for out/off/in. My propeller channel is 6 so I could have used 5 but finding center or worse bumping channel five could damage the winch while driving the DUKW.



I first did this mod in the late 70s when I was in high school. I had a Blue Devil destroyer and I wanted to motorize it. I used this servo conversion to power the props (back then we used wiper arm resistor speed controls). Granted full speed was limited but most will argue that scale speed is quite slow. The really cool thing was at minimal (slow) speed the props would just barely turn (120-180 rpm). Run time was enormous compared to using larger high speed motors. Back then we typically had a separate power supply (4 AA battries) for the radio and soldered together ni-cad packs for main drive power. I used a 4 cell ni-cad pack to run the radio and the receiver powered the props.

[Herr Dr. Professor]

Eric, if I can reduce your explanation here to terms accessible to those to whom "pot" is something in which one cooks or that upon which one sits, apparently one rig "borrow" the electronic functions of a servo to operate a motor of greater power. Yours is the kind of ingenuity I admire. Then, too, that "Blue Devil" kit appealed to me since it was first sold. I never bought one, and I am, what, somewhat "over-kitted" now. But the appeal endures. Do you have an photos of yours?

[Ecam]

Goodness no, didn't think to photograph back then. The first one succumbed to some homemade pyrotechnics and a rock quarry pond around 1980. I don't remember the fate of the second one but I do remember it never made it in the water. Had the Lindberg PT109 a couple times too, pretty much the same story on those as well.

Motor of slightly larger size, little more power. I didn't get silly to see what the receiver and servo electronics could handle (I didn't let the smoke out). I did use two servos on a home made "Y" harness to run the two motors/props and the Cox Sanwa radio held up.