Trouble with Hitec HS85 Servos

[Andy Stephenson]

 My winter projects have been rescuing and servicing my collection one by one from the loft. After 4 years up there, my Bullet areobat has developed servo issues. 3 out of 5 of the HS85s have just stopped working and just sit there buzzing and getting hot. Though not all at once but after running a soak test. I now won't trust any of them. I took the bottom of a couple of them to find they worked if I pressed on the feedback pot but as soon as I release the pressure they revert to form. I'm wondering if heat cycling has got to them, as it gets quite hot up there in the summer and cold in the winter. The servos were new and I'd only flown it 3 times before going into storage. None of my other installations with different servos have suffered the same fate. Has anyone else had similar problems?

[Piers Bowlan]

Presumably all servos purchased at the same time from the same source? Duff batch?

Now for one of my sillier ideas… have the control linkages seized up or corroded in some way restricting their movement? I said it was a silly idea!

I have a lot of HS85 servos in models stored for years in an unsuitable location (a barn) but never experienced this kind of problem. Were the models stored with batteries connected in them? Black wire corrosion?

[Andy Stephenson]

Piers,

I don't know about the duff batch but it does appear so and yes they were all bought at the same time. Linkages were all fine, I always like linkages as free as possible, even to the point that they should drop under their own weight. The battery was removed and stored separately definitely no black-wire corrosion. It looks as if there were a duff batch of feedback pots in these servos.

[Gunnar Borseth]

I’ve had the same thing happend to one HS85 with only one flight on it. After winter storage it did not move. A spray of WD-40 or contact cleaner in the pot had it going again. It was one of four aileron servos on an Ultimate so It would not have been a disaster if it had failed in flight. Even at full deflection. But I would not have used it as a single servo on any control surface. The easy answer is don’t use them if you don’t trust them. 

[Andy Stephenson]

I've consigned all 5 to the spares bin and ordered replacements of a different make, though I won't be surprised if they look like a badged run-of-the-mill Chinese generic type.

[Paul De Tourtoulon]

Are you running them on an analogical receiver or Digital HV ?.

 

try them on a servo tester.

[Nigel R]

13 hours ago, Andy Stephenson said:

I took the bottom of a couple of them to find they worked if I pressed on the feedback pot but as soon as I release the pressure they revert to form. I'm wondering if heat cycling has got to them, as it gets quite hot up there in the summer and cold in the winter.

 

How hot and how cold are we talking?

 

If they've been sat doing nothing, the only other things that could really happen are related to moisture and rust.

[Andy Stephenson]

Some of the 5 servos were OK and were running on the receiver as set for analogue servos and also on a tester but as I got to running them some more on soak-test more failed. The plane was wrapped in bubble wrap to prevent dust ingress but moisture, possibly?

I've read about other types of Hitec servos suffering from reliability servos so I will probably avoid that brand in future.