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 restesting 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, 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.