Bachmann DCC decoder buzzing

[ColinB]

I am just after some advice. I bought a Bachmann DCC decoder off EBay ages ago, it didn't cost much so no big issue, but when it first drives the motor in the Loco there is a buzzing sound. Once it gets going it behaves normally, it is just this buzzing at "start up". I did once see a video on YouTube about someone complaining about cheap Bachmann DCC decoders, so I am wondering if it is this type. I will add I have other Bachmann decoders but a different type and they behave normally so I am not complaining about Bachmann. It is just intriguing, I bought off someone in France. I only use it for testing with the view "if I am going to blow up a module, let it be this one", although sometimes I forget and think the decoder is about to blow up (the sound is the same as ones that have blown up). I would just like to know if there is a bad batch of these around, they have got to be several years old. The decoder is properly wired so it is not that. 

[2e0dtoeric]

Which controller, Colin?

Is it the decoder or the motor that is buzzing?

Try a factory reset of the decoder. The previous owner might have been messikng with the cv's.

[Flashbang]

Bachmann decoders are reset to factory default settings via CV8 entering a value of 8.  All CVs will go back to their original settings including address number to 03.  

Buzzing at very low speeds is often caused by the pulse width modulation (PWM) output from the decoder to the motor.  The motor is being feed with a stream of 12 volt pulses at around 20KHz (20,000 times a second) or more per second.  For low speed these pulses have longer Off periods than On periods, so the motor is quite likely to buzz. As speed is increased the Off to On ratio is reduced so the buzzing goes away.  

A very simple example of how PWM works is shown here, if its of any help.... https://www.brian-lambert.co.uk/DCC_Page_1.html  just scroll down a little to the PWM information.

[ColinB]

Yes, I know all that, but thanks for the data and as for PWM, I used to write actuator software so yes I can probably write a book about it, I even designed my own PWM controller in the past. What I wanted to know was is it an undesired feature with this particular decoder and had anyone else seen the same thing. I suspect that they probably got their software slightly wrong on this design probably not initialising the port properly or lack hysteresis on low value commands, as none of the other decoders do it and sorry a software reset is not going to cure it. I use an Elite and Fleishmann twin track and it does it with both, but I doubt if either would cause buzzing, possibly the way the decoder interprets the signal could be an issue. As I say I have other Bachmann decoders of a different design and they are OK. 

[Flashbang]

AFAIK Bachmann decoders were cut down versions of ESU ones, at least they used to be.  What make they use currently I have no idea?  The very early Bachmann decoders (call them version 1) were not that good, but the ones introduced to replace them seemed to me to be OK and I've several of these and no buzzing issues.  However I'm unsure when they replaced version 1 with version 2 and where they are currently?

 

I wouldn't worry about the buzzing at all if its only one loco.  A reset could reset some CVs that are having an effect on how the Bemf is being read and may?? reduce the buzzing??? Always worth a try IMO.

[96RAF]

Does it still do it if you swap the decoder to another model or a test rig. It could be a combo of decoder and motor characteristics. My thinking is the laminations on the motor armature behaving like those in an invertor. The ones on the old Victor2 aircraft electrical system used to literally sing.

[ColinB]

@RAF96 Yes it does, it does it with every loco I try. I think Flashbang has probably answered my question, I suspect it was a very early one hence why I got it cheap. As I said I usually use it to test out old locos where I have added a DCC socket, working on the assumption that I would rather blow it up, than a decent one. The thing that prompted it, was I had put it into a Hornby Ringfield A4 I modified and initially I thought it was the tender driving wheels slipping until I remembered. Once running at reasonable speed it is ok. I bought a lot of the 6 pin right angled ones to fit into my early split chassis Bachmann A4s as they were the only ones that fitted without a lot of hacking and they are perfectly OK, so I suspect it just an early release.