Hornby Turntable - R070

[RDS]

I have bought an R070 Turntable for my DCC layout.

 

The Hornby instructions for converting this item for DCC operation state that:

For operation of the Turntable, the contacts at the end of each rail must be carefully removed. If left in place

 

they will cause a short-circuit.

 

I fully understand the reason for the removal of the contacts if the turntable outlet spurs are connected individually elsewhere to the track, via say a DCC bus but I would like to ensure that an outlet spur is only

 

live when the turntable is in the correct position for that outlet.

 

I realise that my loco's (motor or lights) will not operate until the turntable is in the correct position but I prefer it like that.

 

I would like to achieve this by leaving

 

the contacts at the end of each rail in position.

 

Question: Can I leave the contacts at the end of the turntable rails in position as long as I do not connect any of the spurs to the main layout and rely on the turntable to provide the connection.

 

[St1ngr4y]

Hi RDS,

I, too, have just bought and installed a Hornby Turntable. I think you have made two conflicting statements in your post...

1) " I would like to ensure that an outlet spur is only live when the turntable is in the correct position for that

 

outlet." This implies that an outlet spur will take its power from the table rails via the brass contacts.

2) "I realise that my loco's (motor or lights) will not operate until the turntable is in the correct position but I prefer it like that." If you

 

mean when the loco is actually on the turntable, then this implies that the table rails will take their power from an outlet track via the brass contacts. If you mean when the loco is in an outlet track, then that's fair enough.

 

However, let's assume

 

that you only have one outlet track connected to the main layout and that is via the outlet which has two other brass contacts which touch the underside of the X918 lead-in track which fits into this slot. These brass contacts take power via wires under the

 

turntable to supply the rails on the table itself. There will come a time when the table turns that the brass contact under the left rail of the table will come into contact with the right rail of the lead-in track (or vice-versa) causing a short..

 

Ray

 

[RDS]

Hi Ray

Thank you for a very comprehensive and prompt reply.

 

I had overlooked that the LH rail of the turntable would at some stage contact the RH rail of the lead-in track (and vice versa). I had only considered the lead-out tracks, which because

 

I was leaving them unconnected, had assumed it would be OK.

 

I will have to revert to my original plan which was to remove the contacts as per Hornby instructions and then wire all the lead-out tracks (under the turntable) to the lead in track. Is this

 

OK?

 

It's a pity that Hornby do not market a Digital version of the turntable with these connections already installed with a set of Brass contacts for all X918 outlets.

 

Having said all that, why is the 'shorting' problem not also applicable to

 

DC?

 

 

[walkingthedog]

It's time they revamped the TT. I had one over 30 years ago.

[walkingthedog]

Is the problem because DCC uses AC. As the turntable track provides power to the outlet tracks it wouldn't matter if left was touching right with DC because it wouldn't be shorting anything as the outlet tracks are dead.

[Rog RJ]

walkingthedog said:

Is the problem because DCC uses AC. As the turntable track provides power to the outlet tracks it wouldn't matter if left was touching right with DC because it wouldn't be shorting anything as the outlet tracks are dead.

The

problem is that DCC controllers are much more sensitive to short circuits than dc controllers and shut the power off in a tiny fraction of a second. dc controllers are usually fitted with a very slow acting overload protection circuit

[Fishmanoz]

Some furry thinking here. Can I start by saying even the Hornby thinking behind DCC operation of this turntable is less then ideal and it is not recommended that the contacts at the end of the bridge tracks be removed. There are better ways.

 

Let

 

me get that posted for a start to save more TTs from being butchered and I'll then post some more.

[Rog RJ]

PS

 

There have been many topics about converting the turntable to DCC, which a forum search should bring up and explain ways of making the conversion without removal of the bridge rail contacts. One method is to insulate the contacts of the "inlet

 

track" (or not use it at all) which then allows all the approach tracks to be live without causing a short circuit every time the turntable tracks pass by. As the turntable lines up with an approach track it becomes live through the bridge rail contacts. The

 

unfortunate side affect of this is that the turntable rails are continually receiving power and the going dead as the turntable rotates.

[Fishmanoz]

The problem with DCC operation is not that DCC power is AC or not, and we won't get into that argument here, it is that the power is applied all the time. This causes a short every time the bridge rotates past a live track.

 

With DC operation, you

 

drive your loco onto the bridge at the inlet track, the one with the brass contacts under it, and so far so good. Once you get it there, you stop it and now the DC track voltage is zero. It stays that way until you want to drive it off after rotation. Further,

 

when you drive it on, inlet rail A is connected to bridge rail A, and B to B on the other side. As soon as you rotate it left at the inlet, bridge rail B contacts inlet rail A. If you go right, then bridge A contacts inlet B. This happens similarly as the

 

bridge rotates past any outlet track. So bridge rotation leads to multiple contacts between rails A and B. For DC, there is no problem as the power is always off during rotation. But for DCC it is always on so the shorts are real and immediately you rotate

 

the bridge.

 

The next potential problem for DCC is that half the outlet positions are at reversed polarity to the inlet. This is due to the way the connections are made to the bridge under the turntable and you can look up Brian Lambert for an explanation

 

in his coverage of TTs.

 

So what's wrong with the Hornby solution, apart from butchering the TT and voiding the warranty? After all, it removes all potential shorts? But it is also heavy handed and requires all used outlets to be bus wired back to the

 

DCC supply, although not a big problem for those running a bus.

 

The more elegant solution is to isolate the inlet track and only the outlets which go back to the layout by cutting the rails themselves and inserting a small piece of insulation to stop

 

them sliding back into contact. Nothing is needed for dead end sidings as the TT power goes to them when it is turned to them, so you can drive locos in and out at this time. On the live outlets, whatever power arrangements you have to provide layout power

 

to them continues to do so and the isolation continues to avoid shorts during rotation. All you need to be aware of is outlet polarity on the reversed polarity outlets and getting that correct with respect to layout polarity.

 

How do I know all of this.

 

There is extensive coverage of this on the forum going back well over 12 months since last discussed. One forum member even produced and sold the modified isolated outlet tracks. Do a search and you'll find the discussions over a number of threads. I'm going

 

to do one now as I'm not sure I've described the inlet track arrangement correctly. And my head is refusing to work it out logically just now.

[Fishmanoz]

Oh and RDS, no matter whether you use the isolated outlet track method, or the Hornby butchering method, the bridge is live all the time, whether you need it or not. Those with sound locos need it live to keep the sound running during rotation.

[Fishmanoz]

I see Rog has posted between my 2 with more of the story. Still not definitive on inlet though. Except if you fully isolate the input track, you should wire DCC directly to the bridge to stop it going on and off.

[Fishmanoz]

Now I remember. Have a look at https://www.hornby.com/forums/hornby-forums/hornby-digital/3140/ post by Blackbird who produced the modified inlet track. You have to gap the inlet, not the outlets and use IRJs on any outlet connected to the layout.

[2e0dtoeric]

RDS did say he prefers the bridge to be dead unless it is lined up with the feeder spur(s)

SO - simply do not use the first spur that connects to the central feed wires from the bridge! Use the spurs that are not hard-wired underneath to the centre

 

feed. Just connect the two from the motor to the controlling switch and power. That way, as soon as the table begins to move away from the live track, it will be isolated until it lines up again with another spur. (assuming you are not using spurs 180' (or

 

opposite sides!) apart.

This way, although lights and noises go off as the table rotates, you can be sure there are no shorts or reverse loops created.

No butchering involved!

[Fishmanoz]

Yes 2e0, that will work for getting onto the bridge, but now you can't get off unless you power every outlet because now it's the inlets and outlets powering the bridge. Not only that, if you have outlets (powered) at 180 degrees to each other, the bridge

 

will short them out when it aligns with both (remember these will have reversed polarity so when aligned, A will meet B and B will meet A).

 

So RDS, there is a downside to every way you set up this TT because it's not designed for DCC. Even so, you

 

have 2 choices: use the 2e0 method with dead bridge, not using the inlet track, powering all the outlets and don't use any at 180 degrees to any other; or gap the inlet between contacts and bridge, use IRJs on all outlets, and make sure any outlets connected

 

to the layout either have the correct polarity or use an RLM if not.

 

Simple? I think not but yes you can make it work as long as you are careful. Then again, there are other TTs out there designed for DCC operation and in stock last time I looked.

 

 

 

And I'm not sure why you wanted the bridge dead as I can't see a down side to having it live if you then isolate it.

[RDS]

Thank you all for your inputs.

Sorry for the delay in responding but I was keen to see as many alternatives as possible.

 

Fishmanoz, I don't think I said I wanted the bridge dead (assuming by the bridge, you mean the rotating turntable rails).

 

I would prefer the outlets to be dead though, apart from the one that the turntable is lined up with.

 

I will only have one of the spurs (the in-line) connected to the main layout. All others will be dead ends, used to park loco's.

 

The option

 

I favour so far is the one to insulate the brass connectors from the in-line X918 and then just ensure that it is not possible to turn the turntable through 180 degrees back to the in-line. From what I can gather, this will ensure that no short circuits can

 

occur as long as none of the outlets are connected to the main layout. This appears to be what I had thought originally, with the exception of insulating between the brass contacts and the in-line rail.

[St1ngr4y]

RDS said:

The option I favour so far is the one to insulate the brass connectors from the in-line X918 and then just ensure that it is not possible to turn the turntable through 180 degrees back to the in-line. From what I can gather, this

will ensure that no short circuits can occur as long as none of the outlets are connected to the main layout. This appears to be what I had thought originally, with the exception of insulating between the brass contacts and the in-line rail.

Hi

RDS,
I have discovered something about the turntable which you may not be aware of. There is a position that the turntable bridge arrives at when the polarity of the rails on the bridge flips the opposite way, so if you do rotate 180 degrees, the polarity

will have changed once so that a short won't occur as you feared. Where is this point I hear you ask... Ok if you view the turntable at the in-line end where the brass contacts are positioned, and numbering the in-line as position 1, and the others 2, 3, 4

etc in a clockwise direction. Then take a line starting between outlets 4 and 5 through the centre of the table exiting between outlets 12 and 13 then this is the "polarity switch line". In fact when the bridge nears this position, the rails actually are dead

for a second or two. You can tell this if you rotate with a sound and/or lights loco on the table.
So for those people who are doing it the Hornby way, they have to be aware which outlets need their DCC a-b and which ones b-a, if you see what I mean.
Ray

[Fishmanoz]

Ray, I covered this in detail in my posts above, including the link to Brian Lambert - the Hornby TT is a slip ring design.

 

RDS, would like to know why you want the bridge, yes the rotating rails, dead please? I can't think of any reason but happy

 

to learn something new.

 

But again, if you do this, you have to power all of your outlets to sidings or they won't work. The standard way is to have the bridge power them when aligned,but not if the bridge is dead.

 

That now means that all of

 

you inlets and outlets are live. Now if any two are at 180 degrees, not just the inlet and the outlet opposite, you will get a short when you turn to that pair. This first because any opposite pair have reversed polarity to each other and, even if they didn't,

 

there would be one anyway as A and B rails pass each other.

[RDS]

Fishmanoz said:

RDS, would like to know why you want the bridge, yes the rotating rails, dead please? I can't think of any reason but happy to learn something new.

I don't want the bridge dead, (see my post from 8th

Feb at 23:07) just the outlets that are not lined up.

(I have had my turntable apart today. Very clever the way it works.)

[RDS]

@St1ngr4y

Hi Ray,

Thank you but yes I did know about the swop, which Fishmanoz covered in his post earlier in this thread.

 

Incidentally, have you installed your turntable and which method have you used?

 

[St1ngr4y]

Sorry, Fishy, I'm afraid I skipped your posts in my hurry to publish my discovery. I've since read Blackbird's article on his modified X918 track. Having thought a little more about it, would this not work....

1) Connect the two contacts under the inlet

 

track to the DCC bus.

2) Place over these contacts a piece of plastic to prevent them contacting the rails on the X918.

3) Use insulating rail joiners on both rails of every X918 used (whether the track is joining the main layout or not)

4) On dead-end

 

tracks provide a DCC connection to the bus.

This will allow the contacts on the bridge to be left in place in order to provide power from the bridge to the X918's. It also means dead-end tracks will still have constant DCC for sound loco's.

 

Or have

 

I over-simplified it ??

 

Ray

[St1ngr4y]

@RDS

I have installed my turntable removing the contacts as advised by Hornby. However, what I have found is that these brass contacts also keep the rails located centrally on the bridge, and that they can easily move, not only fouling the inlet tracks

 

electrically but also mechanically. So tomorrow I will replace the contacts and try out my theory which I have described above.

Ray

[Fishmanoz]

RDS said:

I don't want the bridge dead, (see my post from 8th Feb at 23:07) just the outlets that are not lined up.

(I have had my turntable apart today. Very clever the way it works.)

RDS, in that case use the Blackbird

method as I have also described. Gap the rails on you inlet track between the brass connectors and the TT and don't use IRJs on any of your outlets, except any connected to the layout rather then being sidings. Now the live bridge powers the sidings when aligned.



I don't think you need use Ray's method above as he is trying to make the bridge dead, and this is not needed.

[St1ngr4y]

Fishy, I'm not trying to make the bridge dead at all. By doing step 1, connecting the DCC bus to the brass contacts under the inlet track, this gives the bridge its constant power. By doing step 2, placing a piece of plastic on top of these contacts, and

 

putting a pair of insulating joiners onto the inlet X918, this makes the inlet X918 dead except when aligned with the bridge.

Ray

[Fishmanoz]

Ray, I suggest you do as above rather than yours. Just cut the inlet tracks and install insulation in the gap to stop them sliding back together. Although insulating the contacts and powering all the outlets with correct polarity will certainly work except

 

sound locos will lose sound when rotating except for a blip as they align with outlets as they go around.

 

Just a thought too, and another reason for not using your method. That blip of power on the unpowered bridge every time it passes a powered outlet

 

may cause decoder resets, faults or attempts to do a DC runaway. Much better to GDP the inlet and leave the bridge continuously live.

[St1ngr4y]

Fishy, I'm sorry to insist on this but you have got my suggestion completely the wrong way around. With my way...

1) the bridge will have constant DCC power except for the 2 seconds or so when the bridge passes the polarity switch point.

2) The only

 

pieces of track which will be electrically dead are the X918 pieces, and even then only when they are NOT in contact with the bridge via the contacts which Hornby suggest removing.

Ray