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Vintage Futaba servo FD17M - safe voltage?

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JulianJ17/01/2020 14:05:36
53 forum posts
11 photos


Could anyone tell me a safe voltage for a vintage FD17M Futaba Ripmax servo? I am bringing a vintage glider back to life & would like to power the receiver & servo's directly from a 2s 7.4v 1.6 MaH liPo.

The voltage is not a problem for the modern Spectrum receiver but I can't find any spec on the servos.

Can anyone help?

Kind regards


Alan Gorham_17/01/2020 14:29:56
1158 forum posts
133 photos

Back in the day when that servo was current, 4.8V NiCd packs or 6V dry battery packs were common. I'd say you would be pushing your luck trying to use 7.4V. If you want to use the LiPo, why not fit a uBec to regulate the LiPo voltage down to 5V?

Don Fry17/01/2020 14:59:13
4413 forum posts
52 photos

Old, agree, 7.2 would push your luck, but they were designed to be used with a 6 volt dry pack. We use a lot of 6 v designed servos on a 2s LiFe battery, 6.6 nominal, without problems. I am restoring an aircraft at the moment, 1980 build, and I will be sticking to the installed original Multiplex servos using a LiFe pack, and they don't object.

Brian Hammond17/01/2020 15:13:57
336 forum posts

I would not like to try that,s fully charged 2 cell Lipo is more like 8.4 volts!

Jonathan W17/01/2020 15:14:43
108 forum posts
11 photos

I think you'd be pushing your luck going anything over 6 volts. These servos date back to late 70's. In fact, I'm looking at them now in the 1979 Ripmax catalogue!! When using 4 x dry cells, in those days these would quickly sag to below 6v as soon as you put a load on them. But most often it would be a 4 cell NiCad battery pack, nudging 6v fully charged but again sagging to 5.something once under load.

I would stick to 4 cell NiMh, or a UBEC set to 5.0 or 5.5v.

By the way, the FD17M was large "heavy duty" waterproof servo most commonly used in boats and RC cars of the day. They are quite a bit larger than a "standard" servo, so I'm surprised they were used in a glider.

Edit to say: by 6 volts max, I mean 6 actual volts, not a "nominal" 6 volts battery which might be 6.6 or whatever. If you blow these, no spares are available. They'll be slow anyway, at any voltage, compared to anything nowadays. Ripmax catalogue quotes 0.4 sec transit time.

Edited By Jonathan W on 17/01/2020 15:21:22

JulianJ17/01/2020 15:25:28
53 forum posts
11 photos

Thank you for all the replies, very informative. I have been given a Graupner Cirrus with the servo's already fitted. There isn't a lot of life left in the airframe but it's good enough to fly.

Enclosed pic for those that are interested.

Kind regards


1 img_4590.jpg

Jonathan W17/01/2020 15:56:12
108 forum posts
11 photos

Indeed they are the big old 17M's!!

Also note, the old M series radios had servo neutral position at 1.3ms pulse width, whereas modern gear is at 1.5, so the neutral will be shifted. You might be able to correct for this with sub-trim. The servo output shafts are square, not splined, so you won't be able to simply move the arms around. Some of the arms had square sockets, giving only quadrant alternatives. Other arms had 8 point star sockets to give 45 deg position adjustment steps.

The other alternative is to dismantle the servos and move the pots around a bit to reset the neutral.

In any case, you might run out of mechanical travel at extreme end point of 2ms pulse width. It might be necessary to limit end points in your transmitter programming.

J D 817/01/2020 17:43:11
1358 forum posts
78 photos

I have a couple of these and was trying them out the other day. They still work but slowembarrassed or what. Servos have come a long way and are a hell of a lot cheaper.

Former Member18/01/2020 22:36:33
3578 forum posts

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