Has DCC power damaged the lighting on my Pullman coaches?

[Chrissaf]

Interesting. The published circuit diagram on that link is in essence the same as my derived one awaiting moderation. Didn't read the full back story to the post, but I saw that the lack of a Capacitor in-rush current limiting resistor was causing issues for some DCC users. I would suggest that 56-68 ohms would be about right for that task if it was felt worth adding for extra protection.

.

Now that we know the value of R1 & R2 is 470 ohms. Then even if the LEDs were short circuit, the resistors would not allow the high current to pass that Colin says he has got with these faulty coaches. Thus this revised component information would make the capacitors look even more suspect as they go directly across the DC side of the bridge.

[2e0dtoeric]

It may be an irrelevance, but in the pic showing the underside of the board with the cap on it, there appear to be two fine bridges across to adjacent tracks. It may well be the camera angle, of course, and the bridges are untrimmed 'tails' of the components.

Also I'm intrigued by that square--section red wire! I don't recall ever having seen any square wire!

In the earlier pic of the top of the boards, the red wire going into j3 connection has a lot of crumpled bare wire before the drilled hole. This may be a dry joint.

[ColinRE]

Chis & FB, 

Apogies for my poor AVO reading skills, I got the decimal place wrong on the meter and the yellow and violet stripes were enough to confirmed my error.

Interesting thread from 2011. Makes me feel less alone, although up until I had the fatal short circuit I have had all 16 coaches sitting on the track without circuit breaker issues at start up - hundreds of times. Also gives me some confidence that most of my pullmans will conform to the same electrical design, although 4 iof the problematic ones are very much older and inherited.

Agree given R1 & R2 at 470 ohms, the capacitors now look to be the most likely cause. I have got as far as ordering some from Radio Spares but experimenting with them will have to wait till I get back from hols in June.

Many thanks for your help; will let you know how I get on then.

Colin

[ColinRE]

Re 2eOdtoeric's comments

The red square wire is in fact a rubber band used to hold wires in place for photographs

The back of the PCBs isn't the best picture and yes what you can see is just tails rather than a fine bridge.

The joint isn't dry, electicity will flow through it OK.

Colin

[Chrissaf]

and the yellow and violet stripes were enough to confirm my error.

.

Yes Colin, that's what caught me out too.....as I said before enjoy your Holiday.

.

At least it is a very simple circuit and not reliant on some over priced non existent obscure spare part that can't be ordered. Even if you had to rebuild them all from scratch with new components using the existing PCB, it wouldn't be that costly an exercise.

.

Colin had a global event that took out 13 of his 16 coaches with the same fault at the same time. The boards in the photos are just one example of the 13 faulty coaches taken at random. Thus we have (maybe incorrectly) been taking little notice of anomalies in the photos that would only affect a single board.

 

[ColinRE]

Chris,

My lunch hour again. Thanks for all your hard work with the diagram and particualrly the efforts in producing Notes 1 and 2; - much appreciated and hugely helpful when it comes to swapping out components.

Colin

[2e0dtoeric]

'Square wire' = elastic band! AARGH!

Doesn't the 'flat' on the led indicate the cathode?

 

edit to change a word, a rubber is something different in America!

[Chrissaf]

Doesn't the 'flat' on the led indicate the cathode?

.

Yes absolutely, but not all LEDs are round. I have some square ones with long 'Anode' legs without a flat 'Cathode' side. Personally, I find the long legs easier to spot on a new unused LED. By suggesting testing for a +ve voltage pad, just trying to make it easier for Colin. The number of times I have unsoldered an LED, taken a mental note of what side was flat, then promptly forgot when I returned to the board to fit the new one. But that's my 'short term' memory loss for you. 😆 😉

[ColinRE]

Hi Chrissaf,

A long time since my last note so thought I’d better update you on the final status.

What I found in June was that removing the capacitor from each coach resolved the short circuit and increased the overall resistance of each coach from circa 110 ohms to 360 ohms (the same resistance as an intact coach). In this state, I found the coach lights worked OK, albeit flickering a little more that with a working capacitor. I then had a go at replacing the capacitors. I did this with 4 coaches and my soldering is fairly proficient, but what I found was that it didn’t act as a capacitor but rather reinstated the short circuit. I spent a long time trying to understand this without success.

Recently I concluded that I would live with the flickering LEDs and removed the capacitors from all 13 faulty coaches. The only problem I then found was when I tried to operate more than 10 flickering coaches in that they seemed to increase the propensity for short circuits when locos traverse points.

My final status is to run with the 3 ‘intact’ LED coaches, 9 flickering ones and 4 with disconnected circuits.

Thanks again for your help

Colin

[Chrissaf]

Interesting, I can't at present think of a theory that fits in with your observations, except maybe one**. Given that the circuit we deduced from your photos and reproduced on page 3 is so simple.

.

** The only thing that could be still affecting your circuit might be a diode or diodes in the bridge rectifier have gone faulty due to peak voltage spike damage. There may be an element of AC creeping onto your (DC) LED circuit part of the PCB (a scope trace would be needed to confirm or not). Reinstating the capacitor would potentially look like a short circuit to an AC waveform. The Electrolytic capacitors used as power reserves and smoothing components in this type of circuit, don't like AC voltages across them and can be damaged. Hence why they have a positive and negative side to them. You might want to try replacing the bridge rectifier on one of the worst affected coaches as a test and optionally reinstate the capacitor on it as well and see if the issue is then resolved.

[2e0dtoeric]

Or just as a trial, place one diode in series with the cap, so it only sees the positive-going 'side' to the power supply!

Admittedy that will not then smooth out the flickering LED's but it will point to the problem better, because if the apparent short has gone, you'll know where to look for the fault.

[bms]

A long time after the event but worth suggesting testing would be adding a resistor between the bridge rectifer (diodes) and the caapacitor to reduce the initial in-rush of current to the capacitor which will look quite lo resistance at switch on.

On my layout on switch on, 2 pullmans are sufficient to trip the overload and it wont reset until the cars are tipped over to remove contact. Everything else then burst into lofe including sound equipped locos then I can return the cars to full rail contact and all is well.

[96RAF]

Four years after the event, but someone may be watching.

[Fishmanoz]

I have no problem with 6 Pullmans at switch on or otherwise on DCC. And no noticeable flicker. They seem to be working as designed. 

[Chrissaf]

I too have six Pullmans with lights plus two DIY fitted Mk1s ... eight in total ... again no issues at switch on ... same for years. Each individual carriage lighting draws just a few mA. Plus the two DIY MK1s have 'Super Capacitor' based 'Stay Alives', again the inrush current for the Super Capacitors is not an issue for my system either.

[ColinB]

Possibly since this thread is 4 years old, people didn't understand DCC like they do now, perhaps the diodes in the bridge rectifier were not quite as good as they should be, we know Hornby generally design to the limit with very little thought to redundancy. Also electrolytic capacitor do degrade with age and we also know that Hornby seems to get batches of duff capacitors. I imagine on a modern one, there should not be any simlar issues. Diodes are so cheap, just replace the lot, similarly electrolytics don't break the bank so replace those.