TT R 070 simultaneously servicing both a DC & DCC layout

[Captain_Francisco.]

I have a DC layout, and a DCC layout, in order to accommodate my locos of varying ages. The relative sizes of the layouts is slowly changing over time, and is favouring the DCC, however, DC will be with us for many a year yet. I also have an R 070 TT, which I prefer not to modify and thus lose it's ability to work off the DC layout. (If it ain't broke, don't fix it!) Would it be possible to use this item when being "fed" from either system. My thinking is:- an approach TT feed siding from the DC system, with the TT only wired up after passing an R618 double isolating track, with a second feed siding servicing a second "on-ramp" coming from the DCC system, also after passing an R618 double isolating switch. One could then isolate the DC, run a loco on from the DCC, isolate the DCC after the loco is in position, open the DC isolating switch, Turn the Loco, again isolate the DC, open the DCC, and run the loco off in the reverse direction. I appreciate that the "round house" ability would be restricted to the DC mode, but that is easily got around with the odd 'shed sidings' for the DCC locos. Is this feasable, and safe, for all the equipment involved. Thanking the 'Team' in advance.

[Captain_Francisco.]

I feel I should elucidate. When I said "feed" the TT I meant with a loco. The power only comes from the DC after the R618. The DCC just runs on, is turned, and runs off, no deviations to other "off-ramps." Apologies.

[Chrissaf]

First thing first.

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I also have an R070 TT, which I prefer not to modify and thus lose it's ability to work off the DC layout.

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Modifying the TT for compatibility for DCC working, does not in any way prevent the modified TT from being used on a DC Analogue powered layout......see drawing below for details. So in the scenario of a layout with separate DC and DCC running tracks. A TT that is modified for DCC is in effect the 'lowest common denominator'.

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The real issue is fundamentally how to maintain the DC & DCC separation if the TT becomes a common piece of track hardware to both previously separated parts of the layout.

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It is all well and good providing isolation tracks and switches to try an maintain that level of separation, but lapses of concentration do happen and the risk of running a DCC equipped loco off a TT switched to DC operation onto a DCC powered track is quite high. Or conversely running a DC loco off a DC powered TT bridge onto a DCC powered track. Or conversely running a DCC loco off a DCC powered TT bridge onto a DC powered track. Thus the risk of the DC voltage being passed through the loco wheels and pickups onto the DCC track and damaging the DCC controller is therefore also equally high with three possible invalid switching scenarios that increase that risk three fold.

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Personally, I don't think the risk is worth it.

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Just for info: This recently posted thread (earlier this afternoon) documents the recommended TT modification for DCC working.

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https://www.hornby.com/uk-en/forum/hornby-r070-turntable-problem/?p=1

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Postscript:

I recommend that any change-over switches that you do employ to switch from DC to DCC working should be a three way switch with a 'Centre Off' position. Some two way change-over switches are 'make before break' and would invalidate the separation required. A 'Centre off' switch provides 100% electrical separation with the 'Centre Off' position being a 'safe' position where nothing is connected to anything.

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For the elimination of doubt. The DCC modification that is documented on this forum recommends that the power feed to the bridge is removed from the inlet track and connected directly to the DCC track power connection. In your scenario, that feed would need to go to a 'three way change-over' switch mentioned above so that the bridge track and inlet track power could be switched between DC and DCC.

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With regard the image below. I am in no way condoning this as a solution for the reasons given above. I am just indicating one example of how it could be done with a DCC modified Hornby R070 TT using a three way 'Centre Off' switch.

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/media/tinymce_upload/cce19eb8e5a1411abac3f0400dc9261e.jpg

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With one centralised switch. The TT is either ALL DC or ALL DCC. But I must stress that the two dedicated input track pieces that connect to the DC & DCC parts of the layout are a potential risk area if the three way 'Centre Off' switch is in the wrong position for the inlet track being used.

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The downside to this is that if DC locos are parked on the TT sidings, then they are likely to shoot off when the switch is put into DCC operation mode. They will also move off when the DC Analogue controller is turned up and the TT is switched to DC mode. To overcome this. Choose which TT sidings will be potentially used for storing DC locos and put additional make / break isolation switches (not shown on the drawing) in their dropper power feeds, so that they can be electrically isolated when DC locos are parked on them.

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[Fishmanoz]

Oops, see also my reply on the topic in General.

 

It would make sense to consolidate the General and DCC forums topics into this one. A job for one of the Mods?

[Captain_Francisco.]

Thanks very much Chrissaf. As always an absolutely lucid, cogent, easily understood answer. Thanks for the better electrical alternative, although I will admit to  agreeing with you that the "human frailty" element, (at least until some form of "interlock" for the brain is perfected, and "water flows upstream,") the risk of (very costly) damage is too high for the idea to be viable. I was coming to that realization slowly myself, and your argument has settled my doubts. It was just a thought, allied both to 'space saving,' and a bit of 'aesthetic appeal.' I actually have an old black, hand operated, TT, (picked up in a "junk shop," will you believe,) which only has two leads for the cenre turntable track. I shall put my mind to replacing the "hand knob" with a motor, and set "Hornby Trackmaster" to solving the space constraint issue. Again, heartfeilt thanks.