Mode Zero

On the subject of obscure British proportional sets, has anyone ever come across a Johnson RC set? (Not sure if I've spelt it correctly!)

Ed Johnson was quite a well known competition flyer from the Bristol area back in the 60s. Whilst visiting one of our club members who ran a "model shop" from a shed in his back garden (knowledgeable model shops were thin on the ground, out in the sticks!), I spotted a very smart looking transmitter awaiting collection by a customer.

He explained that it was a Johnson set, produced by the aforesaid Ed Johnson. The Tx had an all black folded aluminium case (vinyl clad, I think) and Bonner sticks. It was the only one I ever saw, and certainly never saw them advertised.

There's a challenge for you, Mike!

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Pete

Hi Pete

I've seen the adverts and scanned the "commercial developments" introduction for the Johnson proportional, but I've never seen one in the flesh or heard of anybody who's had one, let alone got one. A definite rarity!

Mike

Think we need a certain Mr Binks to share his memories of Multi Aerobatics in that era of which Ed was quite an expert and a big name before he moved on to become a sound recordist with the BBC?

Was it an analogue system to be able to have the throttle failsafe back then?

No, it was digital. Quite a lot of early digitals had fail-safe functions. It was actually a feature of the Bonner servos, which came in two varieties - center and end failsafe.

It worked by the receiver having an extra "loss of signal" wire going to the servos. When the servo amp detected this - er, "loss of signal" signal - the servo motor drive would switch from the output transistors to PCB wipers, similar to those found in "reed" servos.

It was pretty effective! The only time my Digimite went into failsafe was when a FlightLink transmitter got between me and my model! (Hence my dislike of FlightLink electronics! ) It recovered as soon as it emerged from the "shadow" of the FlightLink!

However, failsafe was quickly ditched on the grounds of cost and complexity. It wasn't "inherent", as was the case in many analogue systems.

I've never been aware of an early digital system that had a failsafe unless it was using Bonner servos (or maybe the related F&M Magnavacs?). However, I note that that write up of the Johnson system says it uses own design servo amps, so maybe the throttle servo had a pulse omission detector built in?

If you look at my "British Nats" video https://youtu.be/JaLFu8h_F9Y around 58 seconds in, you'll see a low wing model with black squares down the side of the fuselage. I'm pretty sure that was Ed Johnson's back when he was flying a Digimite radio. I think there's a brief flying shot of it in there somewhere, too, but I couldn't see it at a quick scan.

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Pete

Thanks Pete!, co-incidentally, I have just been taking some pics of the Bonner Set and I noticed that they had end or centre failsafe exactly as you describe!
That is that answered comprehensively & as we have learnt to expect from "our" resident Vintage Radio Tech Expert!

So do you think he copied the Bonner electronics to use in the Orbit PS2 servos mentioned in the article?

stuart mackay wrote: 12 Feb 2021, 09:27 So do you think he copied the Bonner electronics to use in the Orbit PS2 servos mentioned in the article?

I don't think so. I don't know if you've opened up a Digimite servo or not - be VERY careful if you do! Little ball bearings tend to fly everywhere! The Digimite-4 was quite different electronically from the original 8. I've never had one apart, but I think the amplifiers were at least partially in the receiver. The output rack had a couple of wipers pressed against a PC board at right angle to the main board:
DSCF2141 by Peter Christy, on Flickr

(Seen here above and beside the red gears)

One wiper ran along a linear wire-wound pot track soldered into the board. The other ran along the "fail-safe" tracks. One terminal of the motor connected to the centre-tap of the battery. The + and - ends of the servo supply went to 1) the amp, and 2) the failsafe tracks. In the event of a fail-safe, the motor would drive until the wiper left the tracks (gap in the middle for center failsafe, or at one end for end failsafe). As far as I'm aware, Orbit made no mechanical provision for this.

The receiver pack on the 8 was a 7-cell, with multiple tappings. For the servo drives, the important ones were -, +2.4 (centre-tap) and + 4.8.

I suspect that the Johnson throttle amp had its own electronic method. It may even have been a "digital" to analogue converter, so as to provide an automatic failsafe in the absence of a signal. That would have been simpler than having its own built in pulse generator to take over on loss of signal.

But in short, I don't know! The only one I ever saw was sitting on a shelf awaiting collection. I don't even know who bought it!

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Pete