Farish Class 31 pcb failures

[EMP]

Firstly as this is my first post, a sincere thanks to all the contributors here, many times the advice and knowledge here has been a great help.

 

My Cl. 31 (6 pin PCB) has now had 2 PCB failures on DC, the most recent of which was with a brand new PCB spare purchased from Bachmann. Although purchased in 2016, this loco has run for all of 5 mins, literally. 

 

Bachmann has refused to help me, stating that spares aren’t covered by warranty, despite the PCB arriving here in Aus with lighting wires which detached from the PCB upon opening the bag!
 

I’ve traced the fault to what I think is Diode D3 on the PCB as the loco will run in direction A only with forward lights and no rear lights. Direction B I have no movement and no lights at all. Testing the PCB lights with my multimeter, both PCB mounted lights are functioning.
 

I think I might be able to resurrect this PCB if I can replace diode D3 as this diode is displaying zero ohms resistance. But I’m happy for any guidance you can provide on what I should be testing on the PCB and how to test it.
 

Given Bachmann has taken my money but refuse to help me, I’m not too keen on buying yet another spare as this PCB seems to be very flaky. I’ve many other Farish locos up to 10 years old that run just fine.

 

Does anyone have any advice as to what specification of diode I would need to source if D3 is the culprit and how I might go about desoldering and resoldering it.

 

Apologies if this question has been asked/addressed before - I did have a good search before committing to join and post.

 

Thanks in advance to the community for your kind assistance.

 

All the best

Nick

[97403_Ixion]

Hi Nick / EMP,

 

Firstly, welcome to the group!  This is a good place to get advice from others as no doubt, someone will have had a similar issue at some point.  A very useful place.

 

I am not aware of the details of the PCB inside the 31 but I am an electronics engineer by profession, so may be able to advise...

 

Did the loco once work but has now failed, taking out the PCB and also the spare PCB?

i.e. did the fault occur after some previous use?

I ask because zero ohms for a diode is not my normal experience of a failed diode.  It sounds like there may be a short circuit somewhere else beyond the PCB.  It's not likely, though not impossible, that both PCB's could have been faulty from the start... small components can become shorted between the pads underneath during the soldering process.  However, I suspect this is not the case, or luck has not been favourable!

 

I imagine to fit the spare PCB, you must have connected wires to the PCB somehow - are they all plug/socket connections?  It sounds like you haven't soldered anything yet...  If this is the case, can you remove the PCB, then re-check the diode resistance?  Measure it in both direction too - there will often be a difference - it may even read open circuit one way around.  If, having removed all the wires to the loco, the diode does not measure short circuit anymore, then clearly, there may be something in the loco itself.

 

What may also be of help to me/us is a photo of the PCB... it may be possible to see the context of the diode with regards to other components on the PCB and/or connections to the loco.

 

It may take a little time to work things out but I am sure it probably will be solvable.  A new diode may be required, though there are a number of different types (power, current, forward voltage, etc.) but it most likely will not be necessary to source  the exact same type - there are a myriad of manufacturers, all competing to sell their own!  However, if the diode has failed, it's best to try to understand why, first.

 

Let us know how you get on with the PCB isolated and if you can supply photos, that may help the diagnosis.

 

Cheers,

Ixion.

[EMP]

Thanks for your reply Ixion. I have solved part of my mystery as I confirmed diode D3 (in a bank of four) has failed.

 

When tested off board D3 allows current to pass in either direction with only a few ohms resistance.
 

For comparison, I removed diode D4 from my other failed PCB (this too has a failed D3 diode) and tested it in resistance and diode test modes:

 

In resistance mode there is 133ohms fwd biased and infinite reverse biased.

 

In diode mode I get readings of 169 and infinite. 

 

The reading in diode test mode is peculiar (to me anyway) as I was expecting to see a fairly low value representing the voltage drop. So I wonder what type of diode it is…

 

Photos attached.

 

Thanks again

Nick

[Izzy]

I have read that this is a fairly common issue with these locos. The diode failure which then prevents movement in one direction. I am not sure if it is the same particular diode - D3 - that fails but obtaining and fitting a replacement has been the answer. As you have spares from the original board, D1/D2/D4, using one would seem to be a solution. Why these particular components keep failing is another matter.

 

[meil]

What do switches S1 & S2 do?

[EMP]

Thank you Izzy. If it’s a common issue, I hope they addressed it with their latest release of the 31 for the sake of others. I was going to buy a few but when they refused to support the replacement they sent me, I decided against it. So they lost a few sales to me and damaged their reputation with me.

 

Hopefully our electronics engineer will chime in with some possible diode specs as I’d really like to buy some spares and repair both PCBs if I can.  I will try and fit the one I removed to the newer board in the meantime.

 

On the plus side I am learning how to fix PCBs so who knows, this thread could end up being quite helpful to others.

 

Meil- switches S1 and S2 allow for forward running in either direction with or without the respective rear red lights.

 

Thanks gentlemen

Nick

[Izzy]

10 hours ago, EMP said:

If it’s a common issue, I hope they addressed it with their latest release of the 31 for the sake of others.

 

The spec is now Next18 with a pre-fitted speaker so I would presume so. Most probably a duff batch of components on these problematic PCB's rather than any kind of design issue though.

[97403_Ixion]

17 hours ago, EMP said:

Thanks for your reply Ixion. I have solved part of my mystery as I confirmed diode D3 (in a bank of four) has failed.

 

When tested off board D3 allows current to pass in either direction with only a few ohms resistance.
 

For comparison, I removed diode D4 from my other failed PCB (this too has a failed D3 diode) and tested it in resistance and diode test modes:

 

In resistance mode there is 133ohms fwd biased and infinite reverse biased.

 

In diode mode I get readings of 169 and infinite. 

 

The reading in diode test mode is peculiar (to me anyway) as I was expecting to see a fairly low value representing the voltage drop. So I wonder what type of diode it is…

 

Photos attached.

 

Thanks again

Nick

 

Hi Nick,

 

I've had a really long day with CAD problems on work stuff, so can't provide a good analysis today but regarding the 169, did it say 169 with the suffix 'mV' on the diode test?  Diode tests are in volts (or millivolts), not ohms, so the reading of 169 may look a bit odd if misinterpreted.

 

If so, this could well be right for a working diode (i.e. your D4 from your spare PCB) and suggests it is what they call a Schottky diode, which has a lower forward voltage than regular silicon diodes.  This type is likely to be used to prevent voltage drop in a rectifier circuit which basically turns AC (such as a DCC powered rail) into DC... and with four diodes side-by-side, it looks like it could be what they call a full-wave rectifier.  In DC mode (which I think is what you are using), the diodes work in the same manner but only one half of them conduct when running in one direction; the other pair in the opposite direction.

 

I found another PCB photo online and it looks like the two feeds from the DCC connector's middle contacts are the Rail power and feed into this diode arrangement.  This would be to create a DC supply of some sort - probably for the LED's/lights.

 

Now, if diode D3 or even D1 were to fail short circuit (or low value), it would mean the track would be trying to drive the supply across the other, non-failed diode directly which would limit the voltage across the track... this tallies with you not getting movement in one direction as it effectively short circuits the motor's supply of power.  In the other direction, the diode effectively conducts due to its shorting fault but as such, is acting like a more perfect diode - hence the track would not be short circuited and the motor would run.  Hopefully that makes some sense?!

 

I'm wondering if the switches control the rear lights?  i.e. so light loco (front and rear lights) or train setting (front lights only) can be set - perhaps one switch for each end?  So, the rear lights may not show when the model moves if the switch for that direction is in the wrong position.  However, you would get no lights at all in one direction if the diode in the rectifier circuit has fused short circuit - but this only applies in one direction.  All your comments do seem to suggest this.

 

So.... the question is why did it fail?  I hate to say it but production costs may be a cause - i.e. cheapest possible that does the job - not saying this is it but it could be a factor.  A diode can fail through a few means - mainly over-current and over-voltage.  It is probably unlikely that over-current is the cause, as the rectifier probably only supplies the LED's, which generally draw low amounts of current, though capacitors may exist which need charging rapidly and could cause a failure.  However, another failure mode is over-voltage where the diode junction just cannot cope in reverse bias, so fails (breaks down in a non-recoverable way) and allows a current flow in either direction.  This is a possibility!  What kind of controller are you using?  I have a Digitrax DCC controller and it has specific settings for N, OO and O gauge... though I would imagine DC controllers are not specifically rated for N or OO but more generic (I only have H&M or Bachmann's trainset controllers otherwise!).  It is possible the diode ratings are a little near the mark and the controller is just a bit over the mark.

 

Another possibility is a short has formed somewhere else - loose wire perhaps?

 

As I say, it may take a little bit more thought to resolve your issue but what would be useful is to know the size of the diode - length and width.  This will help identify the package type, though it looks like it may be what is called an 0805 (80 thou / 2mm by 50 thou / 1.25mm).  This should help identify a diode that would be suitable.  Rated to enough current and voltage with some margin should ensure it works properly for years to come!

 

I must get some rest now (as I say, a long day but helping out is always a good thing!).  I'll see if I can find anything more out in the meantime... but I suspect a badly rated type of diode or an over-voltage controller may be the cause.

 

Cheers for now,

Ixion.

[pauliebanger]

Thanks for taking the time to give such a comprehensive response to the OP.

 

Best regards,

 

Paul

[EMP]

Thanks for taking the time to reply Ixion after what was clearly a tiring day for you. I’m grateful for such detail, especially given your circumstances today - a sincere thank you. And, I hope the following days are not so gruelling for you.

 

Re your questions:

The controller I’m using is a basic gauge master combi and it delivers pretty much spot on 12v across the tracks at full power. So I don’t think the controller is the culprit, although I guess I can’t rule out a surge, though I do run the controller from a surge protected board. Many years ago I had a Bachmann train set controller but stopped using it when I realised it was delivering 22v across the tracks at 100%. I have only used the Combi with the cl.31.
 

The wiring on my layout is super simple, I have only a single feed to the track, soldered to the underside of the rails. The layout is a 1.2m long inglenook with 2 points/sidings. The points are peco electrofrog with no wiring to them or polarity switching, they just route power to the active track as supplied out of the box. All very simple. All my other Farish and Dapol locos run beautifully on this layout and have done for years.

 

The diode is hard to measure precisely without digital calipers but with my steel rule the measurements you quoted seem to be on the money at around 2mm x 1.25mm. 
 

Thanks again for all your help, It is very kind of you to give up your time to provide such comprehensive assistance.

 

All the best

Nick

[EMP]

Oh, regarding the diode test reading, it doesn’t provide a measurement unit on screen (very basic digital multimeter) and the manual was not helpful. But it would appear to be millivolts as I tested an axial diode in my spares box and it read about 500 which suggests millivolts as that’s in typical range for standard diode voltage drop I believe.

 

Cheers

Nick

[EMP]

A pic of part of my layout to break up all the text…sorry for the digression.

[97403_Ixion]

Those 33's are good, aren't they?!  I like the layout  - looking good!

 

Ironically, I was adjusting a PCB symbol for a diode after a big design review today - funny how life is!  You are right about the 500 likely being mV for a regular silicon diode (I could probably go into the maths but I would sound a bit... Sheldon like [Big Bang Theory TV programme])

 

If the Gaugemaster gives 12V, it's not very likely to be the culprit as you say... that sounds quite a good unit to me - will have to look them up.

 

There is a remote possibility of damage due to Electro-Static Discharge (ESD), which could be down to poor static control... though most models are pretty robust - at least the DC ones!  The grass in the photo does look rather good and I would guess it was applied using a static process?  I'm not saying the grass would be a problem itself but I take it the models weren't out near the applicator when the unit was used?  Since you have two failed PCB's, this probably rules this out... unless both got near the static at different times... clutching at straws already...

 

Hmm, it's a puzzler!

 

I've done a quick check of what I suspect may be a suitable diode.  It's a schottky, rated at 40V, 500mA, so I would suspect it is more than capable of a few LED's... for which I am thinking the motor does not form part of the circuit beyond the diode bridge rectifier.

The part is: PMEG4005AEA - link to RS Components (other suppliers also available!).

It looks to be in a SOD-323 package (~1.7mm x 1.25mm), with small leads at either end.

The pack size is 20 @ ~26p each, so plenty spare and not too expensive to try...

 

Another possibility, would be to use a sacrificial diode from the spare PCB... at least this may get a quicker response.  Though be careful to fit it the right way around, or it could blow it...

 

Now, I'm not saying it will solve the fault that causes the problem but if it is inherent in an under-rated diode, then it could.  It may also give you a chance to get the model up and running.  There may be a fault or short elsewhere on the PCB, so don't despair if it fails again... If you can run the model with the bodyshell off, carefully touch the diode while running the 31 up and down a few times - if it gets hot, there is likely a short elsewhere which may need sorting too.  If it runs cool (or luke warm) then it's probably OK.

 

Now... my turn for a photo...

These N gauge motor vehicle models by Oxford are great!  I was blown away though when I saw one with lights fitted by Kytes Lights - so much so I had to buy it!

 

I'll keep thinking of other possible causes for your model and let you know...  I think I have a 31 in my collection but will need to track it down - I think it went into store in a large cardboard engine shed with other locos  

 

...and no problem helping out - I'm not so good at getting my own layout up and running but I do like seeing other people's layouts at exhibitions and on here, so helping others out is always good.

 

Cheers,

Ixion.

[EMP]

Thanks Ixion. Yes the n gauge 33s from Dapol are very good. I also have a Dapol 26 and mine run super smooth on DC even at very slow speed, very quiet too compared to most of my GF diesels. 
 

Those Oxford models are excellent too, I seem to add to my collection whenever I’m supposed to be buying something else! Impressive that Kytes managed to fit lights into that Ford Transit, at least I think it’s a Transit?

It’s funny you mentioned Sheldon because I just happened to be flicking through the TV channels last night and they were showing some re-runs of that show and I had it on in the background while wiring up an led - see below…

 

Re the static, the locos were all safely boxed up in the cupboard when I put the grass down.
 

I appreciate the link to the diode spec - I will see if I can source some locally here in Oz, if not I’ll just use the link you kindly provided. Thanks for having a look into this for me. I’ve learnt a lot through our dialogue and am nearly ready to solder the spare ‘sacrificial’ lamb/diode to the PCB to see if I can get this thing up and running again.

 

Yesterday evening I fitted a waterclear 2mm gull wing smd led to one my peco buffers as a trial. Bit of .25mm magnet wire and 45k resistor to get the brightness down. Rated at max 70mcd but this thing was blindingly bright at 20ma. Made by Kingbright, they are true to their name!

 

All the best

Nick

[EMP]

Success! Diode D4 from old PCB transferred to new PCB to become D3 and loco back in action, at least for now. Will see how it goes…

 

I have sourced some more diodes and when they arrive, I’ll have a go at fixing the old PCB.

 

Thanks to all for your contributions, but especially Ixion. 

 

I sincerely appreciate the time everyone took to help out.

 

All the best

Nick

[97403_Ixion]

5 hours ago, EMP said:

Success! Diode D4 from old PCB transferred to new PCB to become D3 and loco back in action, at least for now. Will see how it goes…

 

I have sourced some more diodes and when they arrive, I’ll have a go at fixing the old PCB.

 

Thanks to all for your contributions, but especially Ixion. 

 

I sincerely appreciate the time everyone took to help out.

 

All the best

Nick

Hi Nick,

 

Glad it is sorted for now!  Keep an eye on whether the diode gets hot in anyway, which could indicate a partial short or similar.  It may just be an under-rated diode (voltage-wise) in the design, so if it does fail again, I would have thought the 40V (reverse-bias) ones suggested should be suitable.

 

I like that buffer-stop LED!  The trouble with LED's these days is they are so bright, so putting a larger resistor inline should help quite easily.  Alternatively, pulsing it rapidly with variable on-off times can yield the same effect but a lot more electronics is involved!

 

The Network Rail transit van in my picture was not the Kytes Lights one but I do have a very similar sized one by them in yellow with the AA (Automobile Association) branding on it.  It has yellow lights if I remember right but the real impressive ones are their fire engines and ambulances which have very well sequenced, high brightness flashed blue LED's, along with other lights where appropriate.  How they get the wires in is a masterpiece of work without them showing but they are great in cameo scenes on a layout... the only downside is no sound (though it may be possible to modify the OO ones to fit it?!).

 

Always glad to help people out... it's what forums like this do well... but I do appreciate your thanks    Keep enjoying your modelling!

 

Thanks,

Ixion.

[EMP]

Hi Ixion, I’m running with the bodyshell off at the moment as you suggested to monitor that pesky diode. No noticeable heat detected so far.

 

Re the vehicle LEDs, I think I’ll have to source one of those fire engines or ambulances with flashing lights, my younger son would love it - the cameo could involve a certain class 31 if it doesn’t last the distance haha

 

I know the peco buffer light isn’t exactly prototypical where Its mounted but it looks alright. I just painted the back of it and the magnet wire with some black acrylic paint to block out the light bleed at the back.

 

Next time I’ll get a bit of 1.2mm tube, mount a couple of lights to it and run the magnet wire down through the tube.

 

I might make up an LED dimmer circuit next just for a mini project. I was reading up about PWM control but I think I will start by wiring in a cheap potentiometer to see what that does. I run all my LED off a regulated 12v DC supply. All in parallel and each with their own resistor. Although I can switch the output down to only 3V, I also have a colour signal on the layout and don’t want to dim that. Being able to vary the brightness of my building interior lights and the buffer lights from my mini control panel would be an interesting little experiment as I seem to enjoy playing around with leds at the moment.

 

All the best

Nick

[97403_Ixion]

Just found this video online of an ambulance and a fire engine - Emergency Vehicles - Kytes Lights.  Judging by the track this is OO gauge but they do N gauge ones too.  I vaguely recall that they couldn't fit one or the other light, maybe the headlights, but they are pretty impressive things.  If I can, I'll try to get an N gauge one out over the weekend and see if I can get a photo.

 

PWM control will allow you to technically do any kind of light animation but the potentiometer way is easiest to start with.  Glad the 31 is still running... hope the emergency response vehicles won't need calling out

 

Have a good weekend,

Ixion.

[EMP]

Hi Ixion, I’m glad you put me on to Kytes - they do some really good stuff at reasonable prices, though the n gauge lighted vehicles were quoted at about $50AUD which is heavy…but I can imagine the labour and skill needed.

 

I know we are off topic now and to any readers looking for the class 31 info, it’s all above here, apologies.

 

50k ohm pots purchased today - got the log version so will see how they perform with a range of resistors to protect the led. I know I could do the math but brightness is a bit subjective so out with the breadboard.

 

Have a good weekend too. I’m off to watch the Dragons v Raiders (NRL rugby league) otherwise known as footy! I know, I know…lol

 

Cheers

Nick

[97403_Ixion]

In essence, the brightness is down to the current magnitude - basically, the number of electrons dropping the energy level per second   As the diode changes little in forward voltage for fairly significant changes in current, a potentiometer will do the trick to a first order... though using a log pot may make it a bit sensitive at the one end.  Also worth putting a fixed resistor in there too so when the pot reaches near to 0 ohms, it limits the current to the maximum brightness of the LED.

 

I tried getting some photos of the Kytes models but they are not quite what I was after - if I can, I'll try to get some better ones tomorrow.  The models are quite expensive but then they are quality models with a lot of manual work behind them.  They often supply a small 2 x AAA battery box to power the model but also do a proper regulator if running off 12V or another voltage.  I remember them telling me not to use rechargeable batteries in the AAA box though as they had heard of cases of the LED's blowing when the batteries were found to be running above 1.5V...not heard of that myself but I bow to their experience!  They often put warnings on the boxes saying '3 Volts DC Only' so using one of their regulators should protect over a wide range of voltages.  As I say, they are good models with very bright flashing blue lights.

 

Just checked the AA van - the two yellow lights on the rear flash in quick succession then off for a brief period before repeating.

 

I might have to create a 'On The Workbench' like thread... I don't post too much myself but more than happy to share comments, ideas and maybe one day details of a layout!!!  Will look into that and let you know...

 

Cheers,

Ixion.