DCC 8 pin issues with Hornby Large Prairie

[Warren-368364]

I have a DCC Hornby Large Prairie which has run fine for 12months using a Hornby Select controller. The Prairie is now not working properly. All of my other DCC locos work fine,but the Prairie starts and runs for up to about 2 meters and then stops. If I switch of mains power then back on again it again runs for a small distance before stopping. I have re coded it several times but again no difference.when the loco stops there is a slight buzzing noise from the engine. If I put a 9v battery on the wheels the motor runs fine. Has anyone any ideas on how to solve the problem?

[SteveM6]

TTS are susceptible to stop/start if the track, wheels or pickups are dirty. If they lose power they have to go through their whole startup procedure again.

I would start by eliminating the obvious by giving the loco a service and a clean of the track.

[Brew Man]

Which R number is it? If it is one of the series R3719-3723 then I have one of these that exhibited the very same issue. It turned out to be a faulty decoder socket. Replacing the socket solved the problem for me.

[Warren-368364]

The number is R3725Xb. How do you replace the socket?

[P-Henny]

According to this SS449B

https://support.hornby.com/hc/article_attachments/360016355420/hss_449b_51-61xx_large_prairie.pdf

The socket is part X7507.

Track this part down, I suggest trying Hornby Customer Services first, then it is a case of unsoldering the old one and soldering in the replacement. The SS shows the socket held in place by 2 screws.

If still under warranty (unlikely as you mention 12 months use in your post), then send the loco to Hornby to do it.

[ColinB]

Looking around it is a standard 8 pin DCC socket, now looking at Peters Spares the X7507 is incredibly expensive and out of stock. Hornby for some unknown reason gives them all different part numbers but X9084 part is virtually the same. If you go to New Modellers web site it looks like you want the "L" variant. Looking at the X7507 it looks like it is a 8 pin socket on top of the normal circuit board which usually are very reliable, so check the soldering underneath it. Where they solder the socket to a PCB with large holes you can get issues. It could also be the decoder, that is not unheard of, I normally find another one to substitute. There again you may fix the fault by just taking out the decoder from the socket and pushing it back (it cleans the pins by this action).

[Brew Man]

Colin's suggestion is a good first step, swap decoders first. If no improvement revert the loco to DC with a blanking plate. These are widely available from the likes of New Modellers, Peters Spares etc. If that doesn't work, go for the decoder socket replacement re P Henny's advice. You will need to be able to use a soldering iron. This is all assuming your track, wheels and pickups are all clean. good luck.

[96RAF]

Sometimes the pins are a slack fit in the socket giving intermittent contact. Try gently bending the decoder plug pins together as a bank of 4 pins using pliers. Not much, just enough to improve the grip twixt pins and holes.

[Dukedog.]

could the motor be drawing more current than the decoder can handle?

[Warren-368364]

Thanks for the suggestions.I’ve tried using another decoder that runs fine in another loco butI still have the same problem. I’ve tried the other suggestions and the Prairie runs smoothly for about half of a large oval then slows down and stops. Whennstationery there is a slight buzzing noise from the motor. If I remove the power wire from the select then put it back in the loco sets off for half an oval then slows and stops. My other dcc locos work fine.

[P-Henny]

Does the motor feel particularly hot to the touch. The symptoms you describe could possibly fit with this scenario if it is hot to the touch.

1. Motor cold, loco runs normally for a lap or two.
2. Motor heats up and some physical expansion takes place, possibly in the bearings.
3. Expansion causes drag that slows the motor down.
4. Drag increases heat, thus a positive feedback loop (heat, expansion, drag) is created.
5. Motor finally stops, but buzzes because an electrical circuit path still exists.
6. Power removed, motor starts to cool.
7. Power reapplied, heat, expansion, drag cycle begines again.

Just a thought.

Excess motor heat could be an indicator that motor windings are starting to break down with a loss of winding turns due to compromised winding insulation bypassing a number of turns, with less turns in a winding then current would also increase further exacerbating the heat creation.

If windings are all sound, dry bearings & drive train (gears etc) could be the culprit.

[Rog RJ]

Could just need a clean and a bit of oil?

[Going Spare]

For how long did the motor run when powered by the 9v battery - for longer than it takes the loco to complete the half oval? As a non-DCC user, I would also suspect a motor fault.

[Brew Man]

While I agree, it could be any of the things suggested, I can only say that the symptoms are identical with those that I experienced, and the problem turned out to be the decoder socket.

[ColinB]

I don't disagree Brew Man, I have had many issues with duff sockets.

[Warren-368364]

I sent it back to Hornby who repaired it for free. It turned out to be a faulty resistor.

[ColinB]

So where was the resistor in the decoder or on the loco?

[Brew Man]

I'm willing to bet it was on the decoder socket. There are several surface mount components on those sockets for some reason. I bypassed the socket all together while testing and it ran perfectly. I did replace the socket though after that with a new one.