Fault finding - NEM 652 8 pin socket

[ellocoloco]

I'm sure I've seen somewhere on here in the past, a thread regarding what to look for when checking if an eight pin socket is fitted correctly.

I have a loco with an eight pin which I believe is responsible for cooking one of my decoders. Visually from the top, the connections to the PCB look good and before I remove the PCB to look underneath I would like to put a meter across it. Any suggestions as to what to check for would be appreciated.

[ColinB]

Ok, this is what I do. Go down adjacent pins with a multimeter, if you get a resistance near zero then something is wrong, so test 1 to 2, 2 to 3, 3 to 4, 4 to 5 and so on. Then test 1 to 8, 2 to 7, 3 to 6, 4 to 5. Then finally with the loco attached if the socket is in the tender measure 1 to 5, that is the only one that should give you a resistance less than a 100 ohms, which will be the impedance of the motor.

[morairamike]

In the picture the bottom row right-hand end socket and the one next to it look like they are linked by the solder on the PCB. It may be worth checking that out.

Could be the angle of the photo but all the others have a gap between them.

[ellocoloco]

Thanks to both Colin and Mike. I have spent the morning making sure that there is no direct current flow between the pins but there is continuity between A, B & C despite my sadistic use of a small drill on the PCB. I checked all pins as per Colin's reply above.

The locomotive is a Roco DB 211 BoBo - 43635 - with the service sheet dating from 1999, so an earlier DCC ready model. I have had the Loco for around 15 years but only recently got round to wanting to chip it. It runs fine on DC and has directional lighting.

The PCB is currently removed from the loco so I could inspect the underside for any solder overflow on the socket. Can anyone suggest where my current leak is coming from?

[Rog RJ]

The readings you are getting are from the lights. If it was me I'd remove the bulbs and fit LEDs and resistors instead. The bulbs are probably taking more current than the decoder can handle. Removing the pcb altogether and hard wiring a socket may be a better option.

[96RAF]

If keeping the PCB I would give it a good scrub to remove any whiskering, which seems to be evident in a few places.

Four diodes would suggest a bridge rectifier, but it's not needed for bulbs, so maybe it's a simple blocking circuit for the directional lights.

[ColinB]

I assume you are using the standard Hornby decoder, if it has bulbs as lights then this may my be your issue. Buy a Zimo standard decoder which has a much higher current limit on the function outputs and the motor drive. After Hornby's latest price rise, I think that they aren't much more expensive than a Hornby one.

[ColinB]

Another thing to do is buy a LaisDCC 8 pin decoder, the cheapest one you can get. These are wonderful for testing out DCC as they work and if you blow them up you haven't lost much.

[Brew Man]

I'm finding LaisDCC decoders are getting harder to find, especially the 8 pin direct plug ones which seem to have gone out of production.

[ellocoloco]

Thank you all once again.

@rog - I think you may have hit the jackpot! I remember back in the early days of DCC, Roco catalogues had mentioned that if you were converting locos then you also needed to change the bulbs from 12v to 16v. I have other Roco locos with 8 pin sockets and bulbs which have never had any problems but they are just from the other side of the millennium and a lot happened in DCC in those few years. The spare bulbs listed for my 211 are rated at 12v60mA, the spare bulbs listed for the other Roco locos I have are rated at 16v30mA. I think my plan will be to disconnect the bulbs and live without them. For all the time the loco is likely to be used, they won't be missed.

@ColinB - pretty much all my locos are running Lenz St+ V2 or Lokpilots that come fitted to the Hornby Int'l models. This one, however, needs a direct decoder as it sits in the cab. It was a Rails Connect that smoked on 'installation' and that was rated at 1A. I'll put it back together, test it with the Lenz I have and pick up a direct the next time I'm near a model shop. Your idea of having a cheap 'test' decoder sounds good.

I have a few older Roco models that are also DCC ready and will look closely at them if I ever decide on converting them. I had , wrongly, assumed that as the model had a socket, then the bulbs would have been suitable.

[ellocoloco]

That's it back together and tested with the Lenz St+V2 I had spare and just need to get a direct decoder for it and fix the step I broke whilst messing about. On the plus side, I won't be too worried about unsoldering and then reattaching the main board if I need to do so with other locos in the future. That job wasn't as bad as expected! All lights disconnected at one side so it will be not too much trouble for them to be reconnected in the future if anyone wants to.

Thanks for all the suggestions and pointers.