Julian B

Hi All Thought i would post a little update on the Hornby decoder i managed to break Firstly i removed the rectifier diodes hopeing that these might be the cause of the issues . Alas this wasnt the case . When removed they read correctly . Anyone doing the same, these tiny thing have a very low foward bias voltage , only 0.2-0.3V Whilst the diodes were off the board i hooked in a bench power supply and set it to 12 volts . No significant current draw was noted On the decoder board there is a surface mount 7805 voltage regulator and the 12 volts were being correctly regulated down to 5 volts for the processor chip on the other side of the board and that is exactly what i measured there . So thats about it . i did return the model to the track with the bench supply still coneected ( rectifier removed ) but it still failed to respond to the id number that was previously programmed Ive put the rectifier diodes back on the board and put the chip into the " requires more attention " plie . Unfortunately i only have a Hornby Select controller ( funds are tight ) and that doesnt allow you to get to the cvs i believe . Ive read that CV8 needs to reset to reset the chip? Anway its been a learning experience and again thanks for all the help oh and if anyone can reccommend a dcc controller that can program CVs and that isnt the price of a new kitchen i would be interested to hear Thanks again

Hi All Thanks so much for all the info Yes to the bit about limiting the current ( school boy laziness on my part ) although im not sure what the revesed biased diode across the resistor does although it wont hurt i guess , also i see the valid point of the zenner across the cap just to keep the supply limited to a sensible level. The one thing that i didnt see much mention of on the internet and forums was the fact that if you have a massive batttery like "stay alive" then the DCC decoder chip will happily work away regardless whether there is any connection to the host devivce , this means in effectt that if you issue a speed setting and then connection is lost, that speed setting is the one that is acted upon and any other command like" stop" is obviously not recieved and therfore not processed. Am i right with this or are the decoders looking for a heart beat signal ? With my very limmited knowledge i would therefor suggest that going for the smallest possible value of capacitor is the way to go. Not sure if you will agree with that Also thanks for the info on the decoder chips , i see that there is a lack of love for the Hornby boards I think the comment that they are nasty rather harsh but again i am new to this so alternate information on non nasty electronics boards that you might reccomend is greatly appreciated Thanks again

Hi All This is my first post so please excuse any mistakes i make. Im a returnee to the hobby after rediscovering my childhhood model railway equipment in the loft durning lock down , Anyway to cut a long storry short and a lot of Peco code 75 Bullhead track , endless replacment wheels and the culdron of fire that is the change from DC to DCC i have a working layout Some of my converted Lima and Hornby models really struggled to work with the combinaltion of the scale track and their poor pickups but i finally have them behaving . A few days ago i fitted a 4700uf cap to the R8249 decoder and it worked a treat. The converted loco would crawl across un powered Frogs ( thats a term you dont hear too often ) without problem but on day 2 of testing it failed . I think ive recked the rectifier as i didnt put a current limiting resistor in series with the cap, i know, daft mistake . The dc output of the rectifier is only measuring 5 ish volts accross the cap and i seem to think it was measuring arount 17volts DC when it was working. I will probably replace the decoder but at £25 a go i want to see if i can fix the broken one . I do have some electronics knowledge and looking at the board it looks like one side is the power supply and the other is dedicated to black majic and voodoo Any help much appreciated. If you have a circuit diagram for the R8249 could you let me know ?