Removal Of Capacitor When Fitting DCC Decoder

[rpjallan]

Hi Everyone.

After purchasing my first ever DCC fitted loco (and first ever sound loco) - R3382TTS Holland-Afrika Line and being pretty impressed with the TTS sound, I decided to fit TTS decoders to a couple of my other newer locos. These are R3320 Golden Eagle and R3410 King Henry III.

Now, after some running problems I had with a certain B@CHM@nn loco some years ago, I am a firm believer in removing capacitors when fitting decoders. So, I was quite surprised when I couldn't find one on the King Class. The A4 had one prominently soldered to the motor contacts. Has anyone else noticed this - missing capacitors?

I have not dis-assembled the Merchant Navy to look for a capacitor at this stage as it runs so well straight out of the box.

Richard

[Bulleidboy]

I'm not sure that I'm answering your question, but it's another one of those "some do some don't".  I have never removed a capacitor and all of my DCC loco's run well. Some will say snip it out. As to whether the latest loco's have them fitted or not - I don't know. Not sure of the technical term, but the "workings" of the capacitor is incorporated in all (?) decoders. An expert will come along and give you the difinitive answer.

[2e0dtoeric]

The capacitors in the TRACK CONNECTORS need to be removed - because they are effectively between the controller and decoder, and can (not always!) scramble the coded data beng sent.

The capacitors soldered across the motor brushes are AFTER the decoder, so don't affect anything.

-

These capacitors date back to when tv was 405 line vhf, and an electric motor running nearby drove the tv daft! Much the same as when you are listening to an AM radio station (if anyone does, these days!), and next door starts drilling something, you hear a terrible whining noise on your radio - or an old car drives down the street, and you can hear the sparkplugs firing - pop - pop - .

With modern tv (satellite or cable, and uhf) the capacitors are really a waste of space, but as they are about 10 for a penny, they are still installed on a just-in-case basis.

-

So - to recap, capacitors in the motor can be left as is, capacitors in the track connectors need to be removed for DCC

If you are running dc/analoge, don't bother removing them!

[Flashbang]

Motor capacitors have to be fitted by all manufacturers to comply with current Radio Frequency (RF) interference legislation set by the EU and any country they export to. If you decide to remove them is up to you!  You wont get arrested for snipping any out. But if you're a manufacturer you have to fit them by law! 

As stated any capacitors across the rails or inside a power clip or power track section MUST BE REMOVED as they will corrupt the DCC data.

Some makes of decoder do not particularly like having motor RF capacitors in the feed between decoder and motor as they can cause Back emf settings to become unstable. Decoders use the Bemf reading to try to keep the motor turning at a constant set speed. Bit like a car uses Cruise Control to set a constant speed. 

If your locos have poor running performance on DCC then most times it will be caused by the RF capacitors interfering with the decoders Bemf ability to read and set a constant voltage.  Two ways normally of overcoming this... 1) Remove all the RF capacitors and there is frequently more than one, three is not uncommon.   Or 2) Disable Bemf if your DCC system allows CV adjustments to be made. What CV controls the Bemf? It is often in the 50's but do check the decoders leaflet or manufacturers web site as some use Cv 50 to 57 or other CV numbers.  Setting the CV value to 0 (zero) disables Bemf. But really doing this then defeats the option of using Bemf control! 

[Flashbang]

To add to the above... Normally decoders have their own RF suppression devices built in.

Many locos with a PCB have the RF capacitor (often with inductors [Chokes] too) mounted on the PCB rather than on the motor

[rpjallan]

I don't use track connectors. The wires from my Elite controller are soldered directly to the track. I find Bachmann locos often have three capacitors fitted but I have never seen more than one on a Hornby loco. The loco I had problems with was an LMS G2 and basic Bachmann 21 pin decoder - see here:

https://www.modelrailforum.com/forums/index.php?autocom=custom&page=Bachmann-G2-Super-D

 

I had to completely dis-assemble my DP1 Deltic Prototype (which was no mean feat) to remove the capacitors but it runs beautifully now. Anyway, as I said, I couldn't find any on King Henry III (it doesn't have a PCB) which was a bit of a surprise!

 

Richard

[eyemdee]

I have just fitted a TTS sound decoder to a new Railroad Class 31 loco purchased off a well known auction site. Being new to this game I could not understand why it ran perfectly in analogue mode before fitting the decoder, but then with the decoder fitted it kept shooting forward about a foot at anything above 1/4 throttle at regular intervals around the track. I read a couple of posts about the motor capacitor so thought it was worth a try. I managed to drop the power bogie down far enough to get at the capacitor and just snipped through one of its legs in such a way as to be able to resolder it together if the experiment failed. Problem solved! Speed is now perfectly steady at all throttle openings and the surging has totally disappeared. No evidence of any TV or radio interference anywhere so it will remain snipped! 

[ColinB]

Well, if you read my posts, on my 6 month old class 66 the suppression capacitor failed short circuit and took the TTS decoder with it. Thinking it was the TTS decoder gone wrong, I managed to damage another TTS decoder before I tracked it to the capacitor. On the Hornby class 66 it is hidden on the side of the motor within the bogie, so I could not even see it. Neadless to say, I now remove them. 

[Flashbang]

Which actually doesn't say a great lot for TTS decoders!  If a motor short causes it to blow (two decoders in fact!). Any decent decoder whether sound or non sound should be able to withstand a motor or capacitor short and cut off the power via its own built in protection saving the electronics.  What if the motor / loco stalled? = Instant decoder death then!

 

It's not very often (in fact I don't know of any other occurrence!) where a motor suppression capacitor has gone on short.

[ColinB]

Stall current is totally different to a short circuit and they can definitely cope with that (the motor has some resistance). In the specification is also says that they are short circuit protected, which I think they are. I suspect that this is only for a short time as they tend to heat up, so probably for about 30 seconds you are ok. I checked with another manufacturer and they said roughly the same, they don't have "fold back" current limiting (the comment was there is only a certain amount you can achive on electronics this small and I assume cost). As for the capacitor going short circuit, I have worked in electronics for about 30+ years and I have nver known it happen, but then I always used decent capacitors with a voltage rating way in excessive of the expected running voltage. I never used disc capacitors, as I remember reading once, that they were not reliable. Unfortunately, the one I removed was a ceramic disc.

[NormanQ4]

ColinB

I think you may be trying to teach your granny how to suck eggs.

Flashbank is actually Brian Lambert, seen as some sort of guru by many on this forum.

See his web page.

https://www.brian-lambert.co.uk/

[ColinB]

Well unfortunately, he may be, but in this case he got it wrong. I probably could say the same having been a digital design engineer for a great deal of my life. How then did he did stall current mixed up with short circuit current, one invovles resistance the other the resistance is zero. As I said in my original post, I did contact another Supplier whom I buy a lot of modules from (they are very good ones) and that is where I got the info from. I doubt there is enough circuitry to do "foldback" current limiting and as I said it is expensive to impliment. I don't know about Lenz modules, perhaps they do.