Non Responsive Decoder

[Smudge617]

Hi

I have Castle class loco which I had fitted with decoder early last year, for some reason this no longer seems to work. It was running fine until it decided to stop on its own accord

I have tried to reset the decoder to the factory setting but receive the error message "No ACK pulse" whenever I try, and all CV's read 00 except for the long address. I can find no fault on the decoder (nothing melted anyway) but it now causes a short on my layout. Lastly, as I did not install the decoder I have no idea which or who the manufacturer is, I'm assuming Hornby. Any help would be appreciated.

[96RAF]

The round thing with a 3 on it in the bottom picture looks like a TTS decoder sound amplifier and the red and black wires attached to the side terminals in the top picture should connect to an 8-ohm speaker.

See this picture of Class 66 TTS decoder.

Try a reset in Reg mode if your controller supports it, else keep bashing value 8 into CV 8 for a full reset. It could take a few goes.

Despite all advice to the contrary have changed a decoder address by writing to CV1 using Operational mode on the main (PoM) - again if your controller supports this.

If there is a speaker attached then select F1 on address 3 and see if it makes a noise. F2 should play a whistle or horn.

[ColinB]

From what you describe, it sounds like the decoder has died. Normally once you get the short you get the burning smell as the input diodes on the board just fry themselves. Recently though I had a LokSound decoder where the diodes were shorted but not drawing much power, so I was lucky and just replaced them. If it presents a short across the DCC there isn't a lot you can do as the DCC is never going to see a reset command. It sounds like it is under a year old so return it to Hornby under their no quibble guarantee. This has happened to me and to several others looking at the posts on this forum. I would really like to know what causes it. Generally as the decoder is happily installed in the loco there should be no way it should blow up unless it overheats, but listening to your symptoms it doesn't sound like you did this. The only other thing that could be causing an issue is if there is a capacitor across the motor. These have a habit of going short circuit and taking the decoder with them.

[Smudge617]

Thanks, Guys, very helpful. I now have sound, but nothing more, I'm still getting "No ACK Pulse" response from my controller, and the Loco won't move, however, weirdly I only get a short if I apply power going forward, but not when in reverse (it doesn't move in either direction tho'). Could this be the motor has seized?

[Chrissaf]

That is something that points to the H bridge output of the decoder having failed. The H bridge is a network of four semiconductors in the shape of an H where the motor sits in the H cross-bar part of the H. If one of the four semi-conductors fails, then a situation can technically arise where the decoder output goes short circuit but only when either forward or reverse is selected.

The quickest way to instantly kill the H bridge is to allow an electrical contact between one of the red / black pickup wires on the decoder and one of the decoder orange / grey motor wires. A common way to achieve this wire to wire connection is due to an issue with the wiring attached to the 8 pin decoder socket or poor wire terminations on the decoder itself i.e where wire insulation has ridden up and exposed bare decoder wires. If the H bridge has been damaged, then the only solution is decoder replacement, the damage is terminal.

Just as a flavour, here is a typical H bridge motor control output circuit.

[Smudge617]

Update to this thread.

I've disconnected the decoder, (hard-wired in unfortunately) and tried DC direct to the motor, my controller cuts out, so the short is somewhere within the motor, so given that the motor has a short the decoder won't find any motor load, that would, I believe, give me the "No ACK Pulse" response on my controller, or have I got this all wrong.

[96RAF]

Your other thread shows the motor is a ring-field which must be hard-wired correctly or it will short.

Follow the advice given over there initially.

Given it shorts when disconnected and power is applied direct is likely pointing at brushes or an armature fault.

[ColinB]

If you have a short across the motor that would explain the TTS decoder failing. They are supposedly short circuited protected but in my experience not for very long. That would explain the H bridge circuit failure. If it is a ringfield motor the current consumption is on the limit for a TTS decoder, although I did find the 5 pole variety was a lot better. Check if there is a capacitor across the motor, this could have failed.

[96RAF]

There is a picture of the motor on the other thread Colin.

https://uk.hornby.com/community/forum/mystery-hornby-motor?ccm_paging_p=1

GS reckons it is likely an Airfix ring-field.

[Smudge617]

Thanks, guys you are correct, it is a Airfix motor, the decoder was fitted by a model shop last July and has worked fine until yesterday. And I'm happy to say the motor is now fixed, one of the small gears had come adrift (loose housing screw), I disconnected the decoder and put DC to the motor and it went like the clappers, so I know the motor is ok, and that was the cause of the problem of it not moving and the short, so I wasn't happy when I reconnected the TTS decoder and I was back to having a short whenever I try to get to move forward, no short in reverse, but won't move in either direction.

So I think Chrissaf is correct and I have an H bridge problem, I don't think a plastic gear coming loose in the motor housing would cause this. So as I didn't buy directly from Hornby their website says I must take it back to the retailer, and as I didn't install it, I assume I will have to take the whole loco back so they can take the speaker out and send it all back in one. Hornby will have to decide if I caused the fault or not. Oh, Such Happy Days.

[ColinB]

I have never bought a decoder from Hornby, but I always return my broken TTS Decoders to them. You probably didn't get the no quibble guarantee that comes with it. Usually you have to tell them where you got it from with a receipt and they like the batch number on the end of the box if you have it, but I am assuming your one is near a year old so even if you had bought it off them you are very unlikely to have kept the box. The issue you will have is that it needs to have the original 8 pin connector on it. Next time find somewhere to fit an 8 pin socket. On my tender driven locos I put it in the loco with a 4 pin connector to the tender as Hornby do on their models except the other way round ( Socket in loco not tender).

That is the trouble with ringfield motors the stall current is in excess of the max current limit of the TTS decoder (near about 800 mAmps), hence when it jammed it took the decoder with it. That shouldn't happen if you have the right decoder with the right current limit. I did think about putting a 500 mAmp reset-able fuse on the motor output of mine when I was having issues. In the end I just didn't use TTS decoders on motors with high stall current.

[ColinB]

Sorry the system won't let me edit my response. I was going to add you imply that a Retailer fitted the TTS decoder, I find that a bit alarming as Chrissaf is always telling everyone to measure the "stall" current before installing a decoder, so whoever it was should know better. Mind you, I have spent several hours fixing a loco for a friend, that a so called loco repairer (he charges serious money to fix locos) had messed up and it wasn't the first one he had done this to.

Moderator message - Colin, when you find the edit button not responding, reload your browser page and you should find the edit button will start working again.

[Smudge617]

Thanks ColinB, After reading your post, sending it back to Hornby is out the window, I doubt the Shop would have kept the box for the decoder and they cut the plug off too so Bye Bye 30 odd quid.

I do now, when I fit a decoder myself, wire in a harness (this one was fitted when only just got back into modelling and had no idea about DCC). And apart for reasons you already noted, if there's no plug chances are slim to none, if it then goes wrong, you'll get a replacement.

Just saw the last bit to your post, Yes, Model shop fitted the TTS, Ah well live, learn, and get it wrong in this Hobby, mind your wallet.

Now to find a new decoder, credit card, where are you

[ColinB]

I don't know, I am sure I read a post on this site where someone sent back the decoder with just the flying leads. I don't know how good you are at soldering, I have been known to resolder the 8 pin plug just to send it back. You could try you have nothing to lose. It is a gripe I have with most DCC decoders, they are supposed to have short circuit protection, but from my experience it is limited. I recently got fed up with dragging out the DC power supply to test DC powered locos, so I setup a DCC decoder where the motor feed fed the track. I was amazed when the decoder blew when I had a short on the track. So much for the short circuit protection.

[Smudge617]

I know what you mean about dragging a DC supply around I use the piece of track which is usually my programming section, permanently connected up a DC controller via a Hornby power clip, but because I have only one plug to supply power to my DCC controller I have to use the same plug for the DC, (everything else is plugged in so far under my layout I have to crawl to get to them), so I can't have both DCC and DC plugged into the power at the same time on my layout. And even if I accidentally leave a loco on my track, DC is disabled on the decoders, so none will suddenly shoot off somewhere. not the best way I know, but it's effective.

There's no way I could solder the wires back onto a plug, not only are my soldering skills good enough, I don't have a plug unless I cut one off of my spare decoders, which are my go-to "is it the decoder or the loco" spares, so not going to do that, it's still in the loco, not going to remove it till I get a replacement, as its quite fiddly and it's a really nice model in pristine condition which considering its age, (apparently mid 80' I've been told on RMWeb) I'd like to keep handling it as little as possible trouble is trying to find a replacement, a Castle TTS is like trying to find 'hen's teeth' at the moment, I've been looking at maybe Loksound or Zimo, but at the moment, not being able to work, it's a bit out my pocket range. I'll contact Hornby, explain the problem (no plug etc), and see what they say.

[Smudge617]

Thanks ColinB, your correct, took it back to the dealer that fitted it, and was told, nope, no plug no chance. I did do a stall test after I'd removed the decoder and it was .75 so within the range that the decoder was built to withstand.

[Chrissaf]

I did do a stall test after I'd removed the decoder and it was .75 so within the range that the decoder was built to withstand.

 

 

.75 is 750mA if the decoder is a TTS decoder as the other replies infer. The continuous current rating of a TTS decoder is only 500mA.

 

 

[Smudge617]

Hi

Just an update, I have managed to get another TTS decoder, but I won't be fitting it into my Airfix Castle Class, as Chrissaf points out the decoder won't take the load. I have a Hornby Loco that I might fit it to, but I'm definitely doing a stall test first.