An electrical disaster (A geek tragedy)

This is how I managed to destroy an Intellibox (IB) and two boosters at once. Well OK, it is still somewhat of a mystery, but here is the story...

I have 4 power districts on my layout, powered by my Intellibox and three boosters.

Everything shares a common ground, including other small devices around the layout that run on 16VAC or 5VDC or 12VDC. I run using mixed Motorola and DCC protocols.

One of those devices is a turntable which takes 16VAC and rectifies that to drive the motor using DC. There are six contacts that go to the turntable:

• Track center rail
• Track running rail #1
• Track running rail #2
• Motor positive
• Motor negative
• Solenoid

The track connections are digital, i.e. a square wave AC signal.

The turntable motor connections are all about 20V DC 

I was trying to resolve a contact problem and I was measuring the voltages on the contacts shown above and I may have measured the voltage between the turntable power contacts and one of the rail contacts. A voltmeter should not be able to conduct much current between its probes.

The next thing I noticed was that the IB screen was dark. It had blown the 3A fuse on its power supply line. I replaced the fuse. The Intellibox complained of a short and then presence of an external voltage. After that it would no longer power on. I attempted a repair of the Intellibox.

After placing another Intellibox (running version 1.55) in service, I found that the sensor bus was reporting nonsense, and the last booster in the chain of boosters was making trains in its section move, and lights flicker. I traced the problem to the first and second Märklin 6015 boosters! Once I removed them, trains in the last power district worked normally and there were no spurious sensor reports.

I obtained a used Märklin 6017 booster and tried that, and I got the same result. No matter which of the three Märklin boosters I connect, I get s88 interference and trains start running even when the IB is in STOP mode. It is as if they are putting analog current out.

I have an old Modellbahn Ott 100W booster and I connected that in the same place as the Märklin boosters and it does not exhibit any problems.

Update 2024-01-05

I obtained another Märklin booster - a 6015.

This one does not send bad output to the tracks, but when rail power is on, I get the junk s88 readings. This is now making me suspect that there is something wrong with the s88 bus of the Intellibox. Track output seems to be leaking into the s88bus, but only in the presence of a Märklin booster. I will be retesting to see if I can locate a defective s88 module, however I believe this is not the case because with the Ott and Power 3 boosters, they all work fine.

Special Option 901 = 3. This controls the asymmetry of the digital signal, and a value of 3 is recommended when using 6015 and 6017 boosters.

Here is a summary of the behavior of each of the 6 boosters I have:

A = Märklin 6015

B = Märklin 6015

C = Power 3

D = Modellbahn Ott 100W booster

E = Märklin 6017

F = Märklin 6015

IB 1.203   IB -> A -> B -> C. Everything was OK

IB 1.55   IB -> A -> B -> C. STOP: flickering and trains moving  GO: s88 garbage

IB 1.55   IB -> A.  STOP: flickering and trains moving  GO: s88 garbage

IB 1.55   IB -> B.  STOP: flickering and trains moving  GO: s88 garbage

IB 1.55   IB -> C.  All OK

IB 1.55   IB -> D.  All OK (See update 2024-01-11)

IB 1.55   IB -> D -> C.   All OK (See update 2024-01-11)

IB 1.55   IB -> E. STOP: flickering and trains moving  GO: s88 garbage

IB 1.55   IB -> F.  STOP: OK  GO: s88 garbage

Update 2024-01-06

Sometimes, when one of the Märklin boosters is connected, I can select a loco address if the track power is off. If the track power is on, then an attempt to select any loco is met with the message "Error" in the loco address area. 

(Note the irony in that the error message has German style hyphenation on an English word! I noticed this error is not in the IB control though.)

I wrote to Uhlenbrock support and described this problem and they replied saying this error occurs if the Loconet is corrupted and that the Intellibox is in need of repair. (See below on an update on this.)

Update 2024-01-11

We have found that if we disconnect the connection (red) to the rails, from a Märklin 6015 booster the corruption of the s88 bus stops, and returns as soon as it is connected again.

My son and I spent a day disconnecting everything from the red feed in one of the power districts (all trains and K83 / k84 modules) and it makes no difference. To me this now suggests that there is electromagnetic interference occurring between the bundles of s88 sensor wires and the track power lines. The s88 bundles are all shielded cables and have never given any problems in 20 years.

We also found that if we waited long enough, the Modellbahn Ott booster does indeed also precipitate false s88 changes, but way fewer than with the working 6015 booster we have.

If we connect the Power 3 Booster to any of the 4 power districts, no corruption of the s88 bus occurs.

I now believe that the IB is not defective in terms of a bad s88 bus port or booster output. I also think the corruption of the Loconet is because the IB gets overwhelmed by the number of s88 changes. This explains why the loconet goes haywire when track power is on, as that is when the false s88 messages come flooding in.

So despite the IB working perfectly with Märklin boosters for decades, a 6015 no longer works with this IB. The only difference I know of is that this IB is running a newer version of the firmware (1.55). If version 1.55 really did not work with Märklin boosters I am sure that this would be a well known problem though.

Uhlenbrock now maintain that the Märklin boosters do not work well with DCC signals. This is strange because they have worked fine in the past and there is a special option (901) which specifically adjusts the symmetry of the digital signal to support them. Uhlenbrock recommend using Power 3, Power 4 or Power 40 boosters instead.

The difference in behavior of my original 6015 boosters (A & B) plus the 6017 (E) I bought on eBay compared to the 6015 (F) I bought on eBay, make me believe that both my original boosters died and the 6017 I bought, also suffered a similar fate before. That booster (E) has now been returned.

I am now looking for Power 3, Power 4 or Power 40 boosters.

I am also going to test with sending only motorola packets to rule out DCC as being the issue, and I am also going to get an old computer going that will allow me to load an older firmware version to see if that resolves the problem.

Update 2024-01-14 

I got rid of all DCC signals by setting the following special options:

SO 6 = 1

SO 12 = 0

SO 22 = 6

SO 25 = 2

SO 901= 1

SO 904 = 28

SO 907 = 1

SO 909 = 198

SO 931 = 1

As expected, the Märklin booster still causes corruption on the s88 bus.

Update 2024-01-28

I managed to downgrade the firmware of the IB to 1.203 - which is what my original IB was running.

It behaves the same. So it is not the firmware version.

I have some second hand Power 3 boosters on their way from Germany. I hope they resolve the problem.

Resolution

After replacing the Märklin boosters with Uhlenbrock Power 3 boosters and adding a fourth booster instead of using the IB output, everything is working again.  The nature of the failure is still a mystery.

Upgrading the Märklin 7286 turntable

My Märklin 7286 turntable was not moving so it was long overdue for a service. While I did that, I also decided to do some upgrades to make it work better than before.

I describe here how I carried out the following upgrades:

• Improved rotation
• Improved electrical contacts
• Added lighting to the control shed
• Added control desk direction indicators
• Added an operator in the control shed

Improved rotation

The turntable was designed to work with analog layouts. Track power is provided to the running rails and the center rail from below, and there are contacts at both ends of the bridge to transfer this power to the current spoke track. This allows one to only power the spoke track one is using, allowing locomotives to be left on other tracks unpowered. On a digital layout, all the tracks can be powered all the time, making all these contacts superfluous. Not only do these contacts add resistance to the rotational movement, they also make clicking noises and they even cause short circuits between the center rail contact  and the running rails of the spokes. I had previous defeated the connectors around the edge of the turntable by jamming them in. I decided it was time to take it a step further..

The center rail on the turntable has two metal plates that lie between the running rails. I cut the ends off both with hacksaw, filed them smooth and painted the ends black.

On the underside of the bridge, at the end of each rail was a small contact. I cut all four off with some small side cutters:

This results in a much smoother and quieter rotation.

Electrical contacts

The turntable bridge pivots about on a central support ("Königsstuhl") which has some concentric contacts. These contacts needed cleaning so I rubbed them with a very fine sandpaper and also cleaned them with some contact spray applied with a cloth.

I also cleaned the finger contacts on the underside of the bridge that ride along on the contacts:

Despite good contact on the supporting side, I could not get one of the running rail contacts to work, despite bending the finger downwards to apply more pressure, so I gave up and soldered a wire between the two rails.

Control shed

It always annoyed me that there was no light in the control room, so it was time to fix that. The shed unclips from the bridge providing good access to the interior. The glass windows are made from a single translucent piece that has a small dimple in the ceiling, a perfect size for adding a surface mount LED.

First I looked closely at the LED with a magnifier to determine which wire was connected to the end with a small dot. This is the side that has to be connected to negative. I tied a small knot in the other end of that wire.

I taped the LED into the top and ran the fine wires to the the corner of the building .

I then inserted the glass into the shed, keeping the wires between the glass and the outer frame at the corner so they cannot be seen.

There is enough room in the hole that the building clips into, to pass the two wires through to the underside of the bridge without having to drill a hole.

I decided to power the light using the power from the track as I did not want it only going on when the turntable was moving. Since the track contains the digital signal, it is AC and thus needs a diode to only allow the current to flow in one direction, plus the usual protection resistor.

The rails provide one side of the power source for the light, so I needed to get power from the center rail. I soldered a wire onto the small contact that conducts the center rail current to the parts between the tracks:

I drilled a small hole next to where the contact sits:

and threaded the wire though and soldered it to a 1N4001 diode.

The other wire was soldered to a 2K Ohm resistor soldered to one of the running rail contacts.

A quick test showed the light worked well.

Control desk direction indicators

I thought it would be nice to put red and green LEDs into the control desk so that when the bridge moved one way, a green light would go on, and the red LED would indicate the other direction.

I drilled a very small hole in the middle of the control desk.

I then connected the two LEDs in antiparallel. (I did not have a green SMD LED so I used a red SMD and a much bigger green one)

This illuminates the red LED when the current flows in one direction, and the green LED when the polarity is reversed. These were then inserted into the hole on the underside and one side was soldered onto a 2K Ohm resistor.

The pair of LEDs then had to be connected to the motor terminals, so that red will indicate one direction and green the other. One motor terminal is easy to connect to, but the other required a contact to be temporarily removed to solder a wire on.

It works:

however, the control room light is too bright for them to show up well. I need to increase the size of the resistor on the room light.

Operator in the control shed

I found a person that looked suitable and glued him in front of the control desk.

How the Märklin turntable (7286) works and how to control it digitally

I needed to understand the internal working of my turntable so that I could optimize its digital operation. I have documented what I found below.

The Märklin turntable (7286) is a rebadged Fleischmann product and adapted for 3-rail running. It is a very nice prototypical looking turntable and has 48 possible track positions at 7.5° intervals.

It can be controlled manually on an analog layout and digital control is also possible, but not out of the box. (More on that later.)

Both ends of the turntable bridge have contacts that make connections with tracks around the edge. This allows transfer of power from the turntable to spoke tracks, ideal for analog layouts, but it can cause a problem with digital layouts due to contacts shorting to the always powered tracks as it turns.

To remove the bridge, unscrew the screw between the rails in the middle. Lift up the screw and a contact clip and the two metal plates that make up the center rail.

Two edge slots, three slots apart, have to be removed from around the perimeter in order to lift the small end of the bridge out. 

The bridge sits atop a set of six concentric contact rings. Below the turntable they are connected via a ribbon cable to a small interface board. Some of the connections are bridged with capacitors on the underside.

The contacts are as follows:

Contact ring	Label on Interface board	Function
1 (center)	B	Center rail of bridge
2	2	Motor - positive for clockwise movement
3	3	Motor - negative for clockwise movement
4	1	Relay - negative to turn it on
5	0 (near 1)	Rail adjacent to shed
6 (outer ring)	0 (near end)	Rail opposite the shed

On the underside of the bridge is a motor mechanism. The entire mechanism can slide a short distance along the length of the bridge. This allows a driving gear at the outer end of the motor mechanism to engage with, or be disengaged from, the teeth around the side of the turntable pit. A small hand lever can be pushed towards the center of the turntable to disengage the mechanism, allowing the turntable to be freely repositioned. A small spring reengages the motor mechanism when released.

The motor mechanism has a relay which is connected in such a way that when the relay is energized, it will close a switch in the motor circuit. In addition, the relay is mechanically connected to the main drive gear so that the switch is held closed except at 4 positions of the main drive gear, 90° apart. Thus, the motor circuit switch is closed and operable, if either the relay is energized or the drive gear is not at one of its 90° positions. 90° of rotation of the drive gear corresponds to 7.5° of rotation of the turntable bridge. There is also a third way the motor circuit switch can be closed, the hand lever that disengages the motor mechanism can also be pushed towards the outer edge of the turntable.

Rotation of the turntable bridge is achieved by energizing the relay and applying a voltage to the motor. To stop at the next turntable position, the coil is de-energized. The bridge continues to turn while the mechanical latch holds the switch closed. When the drive gear reaches one of its 90° positions, the latch is pushed into a notch on it by a spring, which opens the switch in the motor circuit, stopping the turntable. It is locked in place until the coil is energized and voltage applied to the motor.

To stop at a specific track beyond the track adjacent to the starting track, the coil must remain energized until it is within the last 90° of rotation of the drive gear. There is no feedback mechanism to know the speed of the motor so control systems using this setup have to rely on a time delay before de-energizing the relay.

Negative voltage on coil (4) will engage the relay if either motor line (2/3) is positive.

If the motor switch is closed, a positive voltage on 2 (and negative on 3) will drive the turntable clockwise and a positive voltage on 3 (and negative on 2) will drive the turntable counterclockwise.

The wiring schematic in my turntable is as follows:

Note that there are numerous versions of the turntable sold through various companies, and the wire colors and connection labels vary. The principle of operation is however, the same.

Inputs to control the turntable

Electrical input is direct current (12 - 16 Volts DC). If you want to use readily available 16VAC you should convert it to DC using a bridge rectifier (there is one you can use in the manual controller that comes with the turntable).

Controlling the turntable using a K84 type module.

One can use 3 addresses (A, B & C below) of a k84 module to control the turntable. Here is an example:

Connections

Turn clockwise

Port/ring	Voltage	K84s
3/3	Negative	A Green
2/2	Positive	B Green
1/4	Negative	C Green

Set port 1/contact 4 to positive (K84 C = Red) to stop at the next position

Turn counterclockwise

Port/ring	Voltage	K84s
3/3	Positive	A Red
2/2	Negative	B Red
1/4	Negative	C Red

Set port 1/contact 4 to negative (K84 C = Green) to stop at the next position

Immediate halt

Port/ring	Voltage	K84s
3/3	Positive	A Red
2/2	Positive	B Green
1/4	Positive	C Red

Multiple position movements

In order to move a specific number of positions (for example 24 positions to move 180°) automatically you will need to time how long your turntable takes to move one position, and multiply that by the number of positions you want to remove, and subtract one half period, to determine when to set port 3/contact 4 to negative. For example, say it takes 2s per position, to move 5 positions you wait for 2s x 5 - 2s/2 = 9s.

Note that turntable speed can vary with temperature and even direction of movement.

Order of k84 commands

If the K84 commands are sent in the order C, then A, then B, no unexpected movements will take place. Note however that with an Intellibox at least, there is no absolute guarantee in the order in which K84 commands are carried out.  This is because if the Intellibox turnout buffer happens to fill up, some commands are not accepted and the software has to resubmit them, and that may result in them being executed out of order. Other systems may be similar.

Other control methods

A number of third party companies have produced control systems. The most sophisticated I have read about is from the DSD2010 Digital-Bahn but you do have to modify the mechanism extensively. It also provides lighting for the control room and outputs for signals, and an option for a sound module.

Märklin used to sell a digital controller (item 7687) but early versions were very unreliable, and a number of people have opted for the k84 approach instead.

To get reliable turning to any specific track position, there has to be feedback from the turntable as relying on timings is fraught with sources of error. One approach I am considering is placing 48 reed contacts, connected to a single sensor, in a ring on the underside of the turntable and mounting a single magnet under the bridge. As the bridge moves, successive reed contacts will be closed and software can count the slots traversed and know how far the bridge has turned.

Bw

In my layout control software I can define turntables as follows:

And then it looks like this when running:

Red on a track indicates that it is occupied.

I can optionally display radio buttons to select any track directly, or even any slot that is 180° away from any track.

If the turntable is not motorized, the software makes a voice prompt to move the turntable to the required position.

Upgrades

I have done some upgrades to the turntable.