R3725 BR51XX Motor Failure

[Teditor]

I have been given a DCC Ready R3725 Late BR 51XX Large Prairie 2-6-2T to repair, the right rear coupling rod screw kept coming loose, the last time the owner put it back in, when he tried to run the loco it would start and then no go.

I have pulled the motor out and resoldered the brush wires (removed the capacitor), the motor runs fine in or out of the chassis with power applied direct to the brushes, the chassis free-wheels no problem but when power is applied to the pick-up pins or the left side drivers the motor spins freely then slows and stops, change direction and it again spins and stops. Any effort to power the motor through the pickups results in the start-slow-stop syndrome.

I have metered the wires and blank plug and all checks out for continuity, I am in Australia so sending it back over the big pond to Hornby isn't practical.

I'm out of ideas.

Ted (Teditor) Freeman

[Topcat2018]

The problem sounds like either the pickups are only making intermittent contact or the motion work is jamming. Use your meter to measure the current drawn by the loco when power is applied to the wheels though the track. If it draws a high current and the motor doesn't run (more the a few hundred milliamps) then the motor is stalled and you may have a mechanical jam or excessive friction somewhere. If it draws no current then the pickups may not be making contact with the wheels reliably.

[Brew Man]

I'll be very interested to learn the outcome to this because I've got one of the same series of Prairie with exactly the same fault. I've had it to bits, cleaned contacts throughout replaced the under carriage pickup unit for a brand new one. I've checked electrical continuity from wheels to motor with blanking plate in situ, swapped decoders for known good ones, but still it starts, runs for a few seconds then stops. If I apply DC directly to the motor brushes it runs indefinitely. I ended up buying a pre-owned one that someone had removed all the livery labels and numbers from (why?) but was otherwise immaculate for a very good price. I swapped the bodies over just to get a working model.

I'd say it has to be a problem with transmission between wheels and motor but can't for the life of me discover what. In face, narrowing it down I'd say between the decoder socket and motor because I think I've been able to program address and CVs. I say, 'think' as it's a while ago now.

[What About The Bee]

Hi Teditor

Just a thought, but may prove worthy of consideration.

Suppose the gear mesh on a set of gears is too tight, two pieces of junk in the gear mesh which line up or gear teeth with a bit of damage. The model runs until that particular alignment of the gear train/mechanism is reached, and, representing a retarding force to great for the motor to over come, it stalls the motor. Topcat's current draw test will show this. Gear ratios being what they are, it may take some time for the particular alignment to come around each time. That is, the model will run for a bit, and then stop when it hits that alignment. Reverse, and it runs for a bit then stops upon that alignment. Freewheel rotate the wheels until the entire gear train is cycled multiple times, end to end. Looking for a high point may take several cycles to appear.

Just a thought.

Bee

[Brew Man]

The OP has said that the motor runs fine in or out of the chassis when power is applied direct to it, as mine does. I have therefore ruled out a gearing problem.

[What About The Bee]

Hi Brew Man 👋

OP suggested that he is using a blanking plate, so there is no decoder. It is a DC model.

You wrote that it seems to be a power transmission problem. That is what I was after.

A suggestion of something binding as it runs. OP states that it runs for a bit, and stops. Reverses, runs for a bit and stops. Perhaps that is consistent with your experience?

I make no claims of clairvoyance or ability to diagnose an issue without examining the evidence first hand, I merely provided a possible mode of mechanism binding.

Could it be other bits? Sure. Have you tried removing the slide valve and connecting rods, and treated it like a diesel? That is, gears rotate wheels and nothing after that. Divide and conquer!

Bee

[Teditor]

Many thanks for the thoughtful replies, the chassis rolls freely with the motor out and no binding with motor in the chassis if power is applied direct to the brushes, I am going to try another blanking plate, when I can find my stash.

You have all been great with your ideas, I really appreciate your time to reply.

Ted (Teditor) Freeman

[threelink]

A puzzling problem. I had a not dissimilar experience with a different loco many years ago. It transpired that one of the driving wheels was slightly loose on its axle. It would run fine for a while and then, when the wheel slipped on the axle, the quartering was sufficiently upset to make the whole thing lock up. On reversing, the wheel reverted to its correct position and the loco would run until the wheel slipped again. I have no idea whether this information is of use but hope that you find the cause of the problem soon.

[atom3624]

That's interesting and from what I've seen perfectly believable!

Al.

[threelink]

Having re-read the opening post I cannot help but think that the problem is related to the coupling rod mentioned. It sounds as if the rod is binding or is upsetting the quartering in some way. Bee also makes a good point about gears. I had a loco on which two of the pinion teeth became partially detached on a flap of material split from the nylon gear wheel. When the flap lay against the wheel all was fine in a forward direction but in reverse the flap would lift and cause a jam. Removing the motor for inspection allowed the flap to fall back into position so it took forever to spot the problem. I do not suggest that this is the cause of the particular problem here, but it is indicative of the sort of fault that can arise and be devilish difficult to see. If the gears and motor are indeed in proper order then I think the problem has to be with the coupling rod.

[Brew Man]

I am reluctant to post further in this thread as I appear to be hijacking it. But the problem with the OP's and my locos are so similar that I feel there is some legitimacy.

As an additional note, I find that after the loco has stopped and while still under power, it can be pushed along by hand and everything appears to be working as it should with no binding or jamming, just not under its own steam. I wonder if the OP can check if that is the case with his.

I haven't yet done a current draw test, which I will do - when I get round to it.

[Teditor]

I have tried a plain blanking plate, with this I am able to get power to the motor brushes and everything runs great but still a slight turn and stop trying to pickup through the wheels.

It appears to be a problem in the PCB with the DCC Ready properties, I am going to hot wire the pickups to the motor, eliminating the PCB and see how I go, if successful I am going to say the PCB and original blanking plug are faulty, if the owner wishes to go DCC at any stage it will have to be a hard-wire job.

I will advise the final outcome after I complete the PCB by-pass wiring.

Again, many thanks for the enthusiasm and thoughtfulness of everyone on this forum,

Ted (Teditor) Freeman

[What About The Bee]

Brew Man, you wrote:

I am reluctant to post further in this thread as I appear to be hijacking it. But the problem with the OP's and my locos are so similar that I feel there is some legitimacy

I agree with the commonality, and I absolutely do not think you are hijacking any thread. We are troubleshooting an issue. Positing theories may lead to resolution. Brainstorming may take many twists and turns. Your idea may be the one. Never give up!

If we are to discard the mechanical binding theory, and are focused on the DCC ready PCB, then I would consider intermittent shorting. A bit of kapton tape around the PCB will completely isolate it from any arc to ground. Teditor is about to eliminate the PCB with direct wiring. This will prove instructive.

Bee

[Teditor]

Trials and tribulations have me concluding that the chassis mounted PCB and/or the blanking plug are faulty, there seems to be a lot of components on these boards for a loco that doesn't have lights and only motor control. Just changing the blanking plug to a plain no nonsense one allowed me to operate the motor via the brushes but not via any pick-ups, hardwiring from pickups to the motor seems to have solved the problem, I will be doing a complete layout test at the club today.

Fortunately Hornby have followed documented wiring colours which made wire selection easy, short of any more problems I will sign off on this one and as said previously, DCC will require hard wiring as opposed to Plug & Pray.

Again, you have all been wonderful regarding input and ideas, many thanks.

Ted (Teditor) Freeman

From the land down under.

[Brew Man]

That's really good to hear, Ted, I'll give it a try. It's one of those faults that makes you think you're going mad having tried everything you could think of.

[Brew Man]

Bingo!! I've just bypassed the 8 pin socket by direct wiring the pickups to the motor and it works as intended grin.

Thanks for starting this thread, Ted. I'd have otherwise probably consigned this loco to the, "might have another look at it sometime never" box.

I shall now see if I can get a new socket for it, but otherwise hard wire a decoder in. The beauty is I now have 2 Prairies. Well 3 actually. I also have an old Airfix one circa 1975 that is also in need of attention.

Happy days.

EDIT: thinking further about this, I have one or two 8 pin sockets in my spares box, but they are straight through connections as it were, whereas the original has other components on-board which I am guessing are speed conditioning components. However, since the loco appears to work when direct wired, I am guessing it will also work with a simple socket installed. I'm saying that but I did that with an OR Adams Radial once and cooked a decoder.

Any thoughts, guys?

[What About The Bee]

Hi Brew Man

Firstly, permit me to retract the commonality remark and insert a "it was the identical problem" statement.

PCB boards typically have nomenclature on them which serve to identify them. Given we now have two PCBs with this issue, I would urge you to post up that nomenclature for others who might experience the same issue.

And yes please, Teditor, the nomenclature on your PCB as well.

Bee

[Brew Man]

Not sure what you mean by 'nomenclature' Bee. I mean I know what nomenclature means, but don't know what you are referring to here. The PCB in question is the DCC socket into which either the DCC decoder or blanking plate is inserted. Apart from component labels and wiring designations there are no other markings on the board. Hardly any room for any either.

The socket is not not like many where the pins are directly connected through to the respective wires. There are components, two coils, a resistor and a capacitor in play. If I had to guess I'd say one or more of those components has either failed or changed value.

[What About The Bee]

My apologies Brew Man. I will mark this down to the great aluminum / aluminium debate!

Your PCB board will have printed text on it. Some of that text will be for manufacture, chip orientations and the like. There will be some text which identifies the board, and may include a revision number, etc. The name of the board, as used by Hornby, for internal purposes. It will simply be a string of characters which have no other purpose.

Bee

[Brew Man]

No Bee, I can assure you there is no ID whatsoever on the PCB other than that mentioned, oh and pin designations. These are tiny boards - 2cm x 1.25cm. There is hardly room for the socket let alone additional text. For that matter I can't recall having seen such information on any other DCC socket either.

[What About The Bee]

Perhaps we can trace it a different way.

Does it have a Hornby spare parts number?

Bee

[What About The Bee]

Referencing Teditor's model number, we have X7507 PCB holder and blanking plug.

Bee

[Brew Man]

Yes, that's the Hornby part number. That does not appear on the PCB. Pictures in following post. I've ordered said part to replace the faulty one.

[What About The Bee]

Excellent!

For others with X7507, a searchable record is created.

Thank you Brew Man!

Bee

[Brew Man]