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Servos - a ghost in the machine?

Richard Mawer

Entry posted by Richard Mawer

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As mentioned in the last entry, over Christmas I made my second MERG Servo4 electronic module. You will recall I put too high a voltage through the first one (and one donated by a friend) due to H&M Duettes delivering far more voltage than they say when they are not under load - BEWARE!

 

This third module tested ok and looked right. I plugged it into a new (checked) 12vdc supply and still all looked and tested ok. I plugged 4 servos in and they worked. I wired it up the power for common return, wired it to the panel switches and added piano wire bent double back into the cranks of the servo and offered them up under the points through holes in the baseboard under the tie bars. I originally hadn't centred the servos up first, so some bent the piano wire and were working at odd angles as a result. I redid this with the laptop and now have much better positioned servo armatures. The points then changed via the panel switches in the desired manner and all was well. I am pleased to say the double sided sticky pads are still working well. Not the cheap polystyrene ones: they pull off. These are very strong. One did not feel overly secure and then I realised I had stuck it on the label side of the servo and of course the label was coming off - well, the servo was coming off and the label was well secured to the baseboard.

 

I was then able to play trains for a while and enjoy the fruits of my labours over the last year. This is as far as the last blog entry got to. I got a few more locos out and was enjoying life when all of a sudden a train ran off the continuous run and into the loops - but I hadn't changed the points!!! Was there a ghost in the machine?? After a bit of experimentation I found that one of my Dean Goods and particularly my 61xx caused the points for the storage loops to change over. This happened with the suspect locos running, but with them at random places around the circuit. Both locos are in need of a service, but why did only 2 of the servos change and only in one direction???

 

I have to say that the MERG modules are very easy to make and you do not need to know anything about electronics. Most of the MERG membership know a lot and they are very helpful and I received loads of advice when I put my situation up on the forum. We tried lots of things to find the cause and a solution. One of the MERG members in my club (High Wycombe and District Model Railway Society) had not only very kindly offered to mend my first two modules, but also had some ideas about my phantom point changes. We swapped out the offending module for one of the repaired ones and hey presto the problem had gone. Well almost, I still had some "twitching" of the servos at times, but no complete change over.

 

It turns out the problem was twofold.

1) although I thought I had made the module well and it looked and tested ok, it turned out there were some dry joints. Mark had found some on the other boards too.

2) The suspect locos are producing RF from poor running and the module was picking it up. The dry joints were making the module interpret the RF in an odd way.

 

The "twitching" is quite common, but can be dealt with. The solutions are to improve the running of the locos, which I need to do anyway, and to keep the track clean. Separating the Servo wiring from the track wiring as much as possible will also help. There are also some configuration changes I can make to the module to reduce it. These are all standard.

 

Overall, I am really pleased with the look of the slow changing points and also with how easy the modules are to build. I also like how cheap the combination of servos and Servo4 Modules are. The members of MERG are brilliant and pleased to help. So a big thank you to Mark, Chris, Keith, Bob and all the others in MERG.

 

I have learnt that dry joints are hard to spot, but cause lots of problems. Just because a module works doesn't mean it works fully and faultlessly. I hope my future soldering is better and I shall certainly be checking very closely indeed to find any possible dry joints before I install the modules.

 

I shall certainly be using servos powered by MERG Servo4 modules for all the other points and signals.

 

Now, this time consuming interlude has been concluded I can get on with the next baseboard construction - at last!

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Hello Richard, I also experimented with servos as can be seen from my blogs. Never did manage to resolve the twitching problem until I did away with the electronics and modified the servo. Interesting blog, thankyou.

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Donw
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Interesting problem this.

Some years ago working on telephone cables we were upgrading a Shrewsbury Wolverhampton coax cable. This had been running fine on the frequency multiplex but was switching to digital. We found quite a number of dry joints which had caused no problems with the frequency system but on digital the impeadance change at the dry joints would cause echos of the signals and hence misinformation.

It was easy to test on cables send down a pulse and measure the time intervals of any echos these gave the distance to the problems. I cannot see how you would test on acircuit board.

Don

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