Bachmann Spectrum Model # 81705

[Wincoll]

I just purchased this model, a DCC ready Russian Decapod. it ran fine on the test track in the hobby shop, but it won't run on my old dc layout at home. It seems that the five large engine drive wheels short out the power supply and pass current directly from one track to the other. (I checked with a volt-ohm meter.) Any thoughts please?

[RAM]

Does it run at all.  If it does not, check the tender trucks to make sure one of them did not get turn around. 

[Wincoll]

Hi RAM:

Many thanks for your reply. I believe the tender trucks are correctly aligned. The loco does not run at all and as soon as I place the loco on the track and start to increase the throttle control the dc power pack circuit breaker trips out. Really strange since it ran fine on the test track in the hobby shop. Any further thoughts would be greatly appreciated. Thank you.

[Joe Satnik]

Dear Wincoll,

My "lazy" answer would be to tell you to take both your loco and power pack to your hobby shop and let them trouble shoot them.

Digital, or old analog Ohm-meter?

What range is your Ohm-meter set to? (Should be on its lowest setting.)

How many Ohms just touching the two probes together? (For meter and probe "zeroing".)

With the loco off your track, How many Ohms do you read touching a left, and a right drive wheel? 

If you want to continue, take a reading across every L-R pair of wheels on the loco and tender, with the tender harness plugged in.

Thanks.

Sincerely,

Joe Satnik

[Wincoll]

Dear Joe:

Many thanks for your reply. Regarding my volt ohm meter - it is a commercial grade digital Fluke (manufacturer) model, accurate and calibrated.
With the tender connected I get these continuity readings across each pair of loco drive wheels, starting with the front pair: 6.5 ohms, 7.5 ohms, 13 ohms, 6 ohms and 7.5 ohms. I'm thinking that this is a problem. If I put the tender on the track on its own the rear reverse light comes on with the power pack reversing switch thrown, and the power pack circuit breaker does not trip. Anytime the loco is on the track the circuit breaker trips.
Any further suggestions would be much appreciated. Thank you so much!

[bapguy]

Since the tender doesn't short the power pack, they could still be a problem. If both trucks were turned 180 degrees, they are our of phase with the drivers. IE: the front truck gets power from the right rail, while the driver gets power from the left rail and the other way around. This will cause your short. With just the tender on the track, the front truck gets power from the right rail and the rear truck form the left rail. This completes a circuit to light the back up light. When the loco is put on you get the short.   Joe

[richardl]

Make sure both trucks did not swivel. Doubt that could happen but you never know. Not all Bachmann tenders allow 180 swivel.

Rich

[Joe Satnik]

Hi, Wincoll.

I was expecting 9 ohms or so, but 6 Ohms is not out of the question. 

I suspect your power pack circuit breaker may be faulty (trips too soon), and/or something is binding or needs lubrication in your loco.

Just to be assured of your electrical pick-up situation, please take these readings:

1.) With the loco and tender off the tracks and connected to each other, touch one of the 6 Ohm wheels with a probe.

Take a resistance reading with the other probe to all other wheels on that side of the loco and tender, one at a time, non driven wheels included. 

Keep readings in order, front to back of the loco-tender consist.

2.) Repeat for the other side.

Thanks.

Joe Satnik

 

[richardl]

A DCC ready loco with DC adapter drawing about 500 ma at 12 vdc would be a resistance of around 24 ohms. A calculation I made some years ago.

My Bachmann Spectrum HO 0-6-0T measures about 150 ma at 9 vdc which is about 60 ohms. Only a small open frame motor. No lights.

Rich

[Joe Satnik]

Rich,

Please re-read, as I have made clarifications and corrections.

The Back-EMF of a spinning motor reduces the current (I) it draws, thus making it appear a higher calculated resistance (Rcalc).

(Power supply voltage - BackEMFrpm) / I = Rtot

Rtot is the total resistance of: the wiring to the track,  the track,  track to wheel interface, wheel to pickup interface,  loco internal wiring and motor winding.  

With "ground" at the Power Pack (-) terminal, here is the circuit:

G----(-)PPK(+)---Rtot----(+)BackEMFrpm(-)------G

What you are seeing is:

Power Supply Voltage / I = Rcalc, which is higher than Rtot.

Hope this helps.

Joe Satnik

Edit: Changed Rcalc to Rtot in 3 places.
         Added Italics

[Wincoll]

Hi All:

Thanks for your continued suggestions. Still no luck, though the loco did finally run half way round an 8 x 4 oval then the power supply circuit breaker tripped. No more action. Starts momentarily and then the breaker trips.

Regarding the tender trucks, I believe they are correctly aligned as they could not rotate without breaking the truck bodies. I'm pretty certain that power for this loco comes from the tender trucks as each truck is "hot" to the opposite track.

Here are the ohm readings that Joe requested, front to back:

Left side:
first drive wheel to front 2 wheel truck - open
first drive wheel to second drive wheel - 0.9 ohms
first drive wheel to third drive wheel - 2.5 ohms
first drive wheel to fourth drive wheel - 1.5 ohms
first drive wheel to fifth drive wheel - 1.9 ohms
first drive wheel to first tender truck wheels - 2.5
first drive wheel to second tender truck wheels - open

Right side:
first drive wheel to front 2 wheel truck - open
first drive wheel to second drive wheel - 1.1 ohms
first drive wheel to third drive wheel - 1.1 ohms
first drive wheel to fourth drive wheel - 0.9 ohms
first drive wheel to fifth drive wheel - 1.0 ohms
first drive wheel to first tender truck wheels - open
first drive wheel to second tender truck wheels - 1.7 ohms

I am using a "Tech II Railpower 1400" power supply with 14 vdc and 18 vac (for switches) and a total capacity of 13 VA. No other power is utilized when I'm trying to run this loco, and I have good voltage at the track (14.4 vdc).

Any further thoughts, or should I return to Bachmann for repair? Can't really go back to the hobby shop as I bought the loco in Maine and live in Connecticut.

Thanks to all for your thoughtful replies - much appreciate the suggestions.

Will (Wincoll)

[Joe Satnik]

Hi, Will.

Your pick-up situation looks good.  

Sorry that took so much work.  

But hey, that's what you get for letting me know you had a good Ohm-meter.

Possibilities:

1.) Something is binding in the loco's drive train.

2.) Loco needs cleaning and lubing.

3.) The motor winding is shorted.

4.) There is a weak (defective or worn out) circuit breaker in your Power Pack.

5.) Your loco does normally draw more than 900 mAmps (=13VA/14.4V),

 meaning your power pack is too small.

Can you set up a current test circuit with a dummy load and meter

to pull 600mA, 700mA, 800mA, then 900mA to see if the power pack's

circuit breaker holds?

Thanks.

Joe Satnik

Edit: Eliminated possibility #5, as new research shows much lower loco current draws.  

[richardl]

Since the OP has a meter, put it on the highest DC amps scale. Put the meter in series with one lead between the pack and track.
Standard procedure when anyone does a loco amps test before installing a decoder.

Sounds like you have only one meter but this will still work.
Below is what I do with two meters. Has worked for many years, even when all I had were analog meters forty years ago.

http://www.trainelectronics.com/Meter_HF/index.htm

Rich

[Wincoll]

Hi Joe and Richard:

Thank you both for your further replies and suggestions. Sounds like my power supply is maxed out when I start the loco thus causing the circuit breaker to trip. I greatly appreciate your help and will start with the motor amp draw test, and if it is close to power supply capacity then obtain a larger capacity power supply as I'm expanding the layout anyway. I like Rich's idea of the two volt ohm meters wired up to measure voltage and amperage simultaneously - probably find something at Radio Shack as there're having closing sales.

I'll let you know how I make out in a few days.

Thank you again.

Will (Wincoll)

[Joe Satnik]

Hi, Will.

Found a review for your Frisco 2-10-0 loco by Jim Hediger in the April, 2002 Model Railroader on pages 25 and 26.

Scale MPH   Load    Volts    Amperes

9.5             Free     5.5       0.20

13.6            Free     6.0       0.21

36.9            Free     9.0       0.23

55.0            Free    12.0      0.24

                Slipping  12.0      0.32

                Stalled   12.0      0.51

So, your loco running free shouldn't draw more than 1/4 amp. 

This eliminates my previous #5 possibility. 

Other possibilities:

6.) Short in tethers,

7, 8.) Short in loco wiring or circuits,

9, 10.) Short in tender wiring or circuits.

Hope this helps.

Sincerely,

Joe Satnik