Review: Corona DS 538MG servo

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So who's doing this reviewing then?

Well I've been building and flying or driving radio controlled models for over 40 years and during that time I like to think I've built up a reasonable amount of knowledge.

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For the past nine years I've also been involved in the design and manufacture of some rather sophisticated engine technology and UAV flight control systems.

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Review: Corona DS 538MG servo

QUALITY CONTROL SINKS AN OTHERWISE GREAT PRODUCT

Dated: 21 Mar 2011

It's hard to find a good, cheap, reliable standard servo from a Chinese manufacturer -- and goodness knows, I've looked long and hard.

While I'm normally a big fan of Chinese products, I find myself reflecting on the fact that the good old HS425BB and S3001 servos from brand-name manufacturers such as Hitec and Futaba still seem to deliver levels of reliability that the Chinese just haven't been able to match.

Every time I think I've found a good "standard" servo (45-55g weight, 3.5Kg-5.5Kg torque) I end up being disappointed.

So, when I first popped the Corona DS 538MG on the test-bench I thought I'd found a servo that might not only match the 425s and 3001s of this world but even exceed them by a good margin.

Corona's offering is more than a standard servo - but at a standard-servo price.

I've seen these for sale from as little as US$10 from various online suppliers and the specs are pretty impressive. A huge 6.5KG/cm of torque, a digital amplifier, metal gears and not too slow either. Woohoo -- what a beauty!

And indeed, if Corona could get their act together in the area of quality control, they might have a winner on their hands -- but alas, I fear they have succumbed to the temptation to simply ship products without proper testing.

So here's what I found when I put these servos to the test...

Performance

As far as centering, precision and torque go, these servos are more than a match for any "standard" servo. Indeed, although they fell short of the advertised 6.5Kg/cm of torque, the samples I tested could deliver well over 5Kg/cm of torque before stalling and that is a very credible figure for a $10 servo.

Being digital, they also showed good holding torque, easily able to resist loads several Kg higher than the stall-torque.

When loaded, they made the typical digital-servo whining noise and delivered credible (if not outstanding) resolution and centering with no sign of overshoot or undershoot.

Durability

Now this is the hardest thing to test when reviewing a servo. I have been reviewing a lot of servos here at RCModelReviews but am only just getting around to publishing the final reviews because it's very clear that some servos work just fine "out of the box" but simply don't last.

Gears wear prematurely, amplifiers fail, feedback pots wear out and other problems often appear, only after a few months of regular use.

The reason I'm publishing the Corona review first is because it was the first servo to show signs of stress when actually put into use.

The first thing that became apparent is the softness of the gears.

When used on the ailerons of a 50cc gas plane, it took only a handful of flights before the gears were so worn that the backlash was unacceptably high. Okay, so you wouldn't put a "standard" servo in a gas plane, but these are touted as a hi-torque metal-geared servo so I would have expected them to last more than a couple of flights. Also, even when used on a nitro-powered model, the backlash quickly grew to unacceptable limits that encourage flutter and imprecise control.

But it gets worse...

Three of the four test servos have suffered catastrophic gear failures, all in exactly the same way...

There is a bell-shaped gear which is supposed to have the smaller part of the gear securely pressed into the larger bell-shaped section.

On three out of four servos tested, this interference fit has failed -- causing the gear to fall into two parts -- as in the pictures on this page.

This is totally unacceptable -- especially when one of the test servos exhibited this fault straight out of the box, with the motor spinning continously when powered up and no sign of any output shaft movement. On inspection it was obvious that the small section had never actually been pressed properly and punched (to expand it) into the larger gear section.

Sorry Corona -- this is a fail of epic proportion.

What a shame... if these servos had a better gearset they would simply make the good old plastic-geared analog servos redundant and become my preferred option for many models.

Unfortunately, all I can say is that if this is the standard of quality control being exercised by Corona, I could not honestly recommend anyone risk a model to these servos.

Perhaps it was just one bad batch -- but that is what Quality Control is there to catch so it clearly indicates a total lack of any real testing of this product or monitoring of quality levels out of the factory.

Maybe if/when Corona get their act together and deliver a DS 538MG with harder gears that are *properly* constructed and tested they can let me know and I'll take another look. In the meantime, I'll just keep looking for a great standard servo out of China.

Pro's: