Debugging a DC Athearn SW7 help sought

[webbcompound]

Purchased an Athearn SW7 which claimed to be a reasonable runner. It just staggers for a second or two then siezes up on my temporary test track (Guagemaster Combi controller fastened to the rails with crocodile clips). When it is just sitting there there are occasional blue shorting sparks which seem to be associated with the top of the power bogies. I have no idea how these things work (I'm guessing the chassis is live). Any suggestions as to what might be wrong, and how to rectify it?

 

[alastairq]

I Seem to recall the current collection is via the right angled bits of metal on top of the trucks [to which it appears someone has hard wired, to the motor?]

The problem is the likelihood of intermittent contact between the two bits of metal?

I suggest soldering a flexible wire from the steel pieces on the trucks, to the steel piece where the motor wires are soldered...bridging the gap which can sometimes make & break..or get obstructed with muck...

Return should be via chassis.

[PaulRhB]

Sounds like poor contact or an intermittent short. As said above one side is live to the chassis and the other goes up through the bent metal strips on the bogies connected to the motor with those wires. Check the upright contacts aren’t bent out and catching the chassis then check there’s no dry joints on the added wires on top.
 

If that fails pop out the motor by pushing up on the four plastic circles underneath the fuel tank and test motor out of the loco. Then use a meter to find short in chassis. 

[PaulRhB]

The motor looks to be at a slight angle to so might not be seated right, just push down firmly, there are contacts to the chassis underneath it that might have got dirt in. 

[long island jack]

Early model, but usually bomb proof, have a look here, this applies to all Blue box

 

https://model-railroad-hobbyist.com/magazine/mrh2018-10/dcc-blue-box

 

Ray

p.s some times the metal couplers short on the frame, if the tail hangs to low

Edited March 17, 2020 by long island jack

[webbcompound]

Excellent. Thanks for all this. Looks like I can rescue this loco.

[Zunnan]

As pointed out, very simplistic models these, they disassemble almost as easily as old Triang models. If memory is correct, the bogies contact with the metal chassis for one pickup path to the motor, which is via a shaped phosphor bronze or similar clip running the length of the motor that retains the brushes and springs. The other motor feed is via the 'L' brackets atop the bogies then contact with a sprung wire which bridges over the motor and contacts another phosphor bronze retaining clip. This arrangement may be the slightly newer blue box models, but the principal is pretty much the same with the even older models; I have seen the bridging spring sometimes replaced with wire that has been soldered to the brush retaining clips or also sometimes attached with a small phosphor bronze clip.

 

If I were in this position, my course of action would be to take the bogies out and thoroughly clean the bogie to chassis contact patch, a fibreglass scratch pen and some IPA is ideal. The motor does look a little out of alignment, which I would think also affects the contact with the chassis. As the motor torques and twists in its slightly unseated position it might well be moving momentarily and possibly leading to a further intermittent contact issue. I'd do a total strip down to get the motor out and get it remounted more firmly, and while its out I'd have a check of the condition of the brushes and commutator. Given its age, I would automatically assume the commutator could be a bit grotty and be in need of a bit of TLC.