Hornby class 92 "Brahms" short circuit when decoder fitted

[sonofjamie]

Hi All,

Wonder if any of you can help me here.

I have been converting my locos to DCC over the last few months with a lot of success and great satisfaction - very pleased with how well it has all been running!

However, I attempted to fit an 8 pin decoder to a recently purchased Hornby Class 92 "Brahms" (which incidentally runs very smoothly on DC). On switching on the DCC power to the service track the Dynamis displayed the "STOP" and "SHORT" symbols. I removed everything and started again, but to no avail, the "STOP" and "SHORT" symbols appear instantly.

Thinking the decoder might be faulty I tried a different one but likewise, to no avail, same thing.

I'm assuming that if there was a problem with the wiring/pick-ups it would have "shorted" on DC also, so as the loco works fine on DC I assume there is possibly something wrong with the PCB but I can't see anything obvious.

Is it possible to do away with the PCB completely and just hard-wire the decoder?

[richard brown]

Try putting some insulation tape under the pcb as the pins on the chip might be touching the metal under it.

 

Richard

[RFS]

Firstly I would strongly recommend setting the address on the programming track first.  If there is a short, you risk blowing the decoder if you place it straight on the track at full DCC voltage. 

 

As Richard says, you could first check that the pins of the decoder are not touching the metal underneath.  Try putting some insulation tape underneath, or easing the decoder up a little so it's not pushed fully home. 

 

To run a loco on DC, a blanking plug simply joins pins 1 and 8 at one end, and 4 and 5 at the other.  Due to faulty manufacture, one of these pairs of pins could be connected underneath by a blob of solder.  However this will have no impact on DC as that's all the blanking plug does anyway. But it will cause problems on DCC when you fit a decoder.  If you have a meter, you could try removing the plug and testing that these pins aren't electrically live to each other.  If they are, it means you'll have to get underneath the PCB and break the false connection with a small file etc. 

Edited July 27, 2017 by RFS

[sonofjamie]

Thanks RFS and Richard for your replies.

Unfortunately, neither of these things are the problem. I have tested the DCC socket as you suggested and none of the pin sockets short with any of the other pin sockets, and the underneath of the PCB, where the socket is, is nowhere near touching anything.

I have just successfully hard-wired a decoder into a class 86 (as it didn't have any kind of socket or PCB) and I can't think of any reason why I shouldn't just by-pass the PCB on the class 92 and hard-wire a decoder into it too. There is the addition of directional lighting on the 92 but the wires are clearly marked where they connect to the PCB, so connecting them to the correct wires on the decoder shouldn't be a problem. Does anyone know exactly what the PCB does, other than connect the relevant pins on the decoder to the appropriate terminals on the PCB?

[Andy Mac]

If you've recently purchased it, return it for a warranty repair?

[tamperman36]

what sort of motor is it fitted with. If it is fitted with the old Hornby ringfield it may well have some parts of the outer casing which will short out the DCC chip but not the DC running of the motor. These contacts pass the current from the wheels on the motor bogie to of the brushes on the motor.  It may require that you strip the motor and insulate the outer casings and solder the wires directly onto the brush contacts.

[RAF96]

what sort of motor is it fitted with. If it is fitted with the old Hornby ringfield it may well have some parts of the outer casing which will short out the DCC chip but not the DC running of the motor. These contacts pass the current from the wheels on the motor bogie to of the brushes on the motor.  It may require that you strip the motor and insulate the outer casings and solder the wires directly onto the brush contacts.

 

Unlikely as the OP says it has an 8-pin socket as standard.

 

I would check with a meter from each socket pin to its intended connection on the loco to see if there is continuity on that link path only and not to any other link path - e.g. the normal practice of ensuring the motor is not connected to the wheels or the lighting wiring is not awry. Models have been know to come ex-factory with duff wiring, only spotted when the decoder is fitted and fries.

 

Rob

[Izzy]

Thanks RFS and Richard for your replies.

Unfortunately, neither of these things are the problem. I have tested the DCC socket as you suggested and none of the pin sockets short with any of the other pin sockets, and the underneath of the PCB, where the socket is, is nowhere near touching anything.

I have just successfully hard-wired a decoder into a class 86 (as it didn't have any kind of socket or PCB) and I can't think of any reason why I shouldn't just by-pass the PCB on the class 92 and hard-wire a decoder into it too. There is the addition of directional lighting on the 92 but the wires are clearly marked where they connect to the PCB, so connecting them to the correct wires on the decoder shouldn't be a problem. Does anyone know exactly what the PCB does, other than connect the relevant pins on the decoder to the appropriate terminals on the PCB?

 

I believe that the PCB will have resistors and diodes in the lighting circuits to protect the LEDS and decide which ones are lit according to the direction of travel.

 

Izzy

[RAF96]

If bypassing the pcb then pay attention to the polarity of the leds as some three legged leds are positive anode and some positive cathode and there are two legged leds, some single colour and the bi-colour which switch colour by swapping polarity.

And as stated you will need separate resistors.

Rob