New Loco....Decoder issues

[guslcp]

Been away a bit (life gets in the way), but I just got me a Bachmann 2-8-4 with DCC installed. What a beauty of an engine..!!.
This is the first loco I buy with the goodies already installed, so I'm kind of hesitant to mess too much with the CV's.  I've set CV's 2, 3 and 4 to values that make it start & stop quite nicely. But I've got a couple of questions.
For starters, the loco seems to respond much faster going forward than in reverse. This is kinda' strange, since all my other locos behave exactly the opposite (faster in reverse than forward), but by a very little difference. On this loco, it's like 2 or 3 times as long to respond...
Also, since the decoder can only be programmed in OPS mode, I have not been able to change the default address (3), to the number it should be (22, in this case). Also, can this decoder be reset to its default settings by entering 8 to CV #8..??
Since all my other decoders are D'trax I don't feel too comfortable messin' around with this one. Any help y'all can give will be much appreciated....

[the Bach-man]

Dear G,
Your loco should work on the programming track with the addition of a 1000 Ohm resistor across the rails. CV 8 to 8 will restore defaults.
Have fun!
the Bach-man

[guslcp]

Update....I tried everything I could think of, and, as a final resort, I thought of cutting off the capacitors, so I popped open the tender and find a board which does not have the "normal" capacitors I'm used to seeing (little yellow pancake thingies...).  The are three thingies which are labeled C1, C2 and C3 in the upper left.  Are these the modern day capacitors?  If not, where are the caps..??

Note: I disconnected the decoder so you can see the whole board.

[lescar]

Yes C1,C2,C3 are capacitors, there called SMD's (surface mounted devices).  The technology is getting smaller so they can do more on a smaller circuit board.

Les

[Jim Banner]

If your locomotive has the usual Bachmann decoder, you can leave the capacitors in place.  Bachmann decoders use a lower pulse repetition rate to drive the motor which means that there is less or no interference from the capacitors.  In any event, the usual problem with the capacitors seems to erratic operation rather than a difference in speed depending on direction.

Have you thought of trying one of your Digitrax decoders in place of the Bachmann decoder to see if it is a decoder problem?  If you do, then you will likely have to remove the capacitors to keep the ultrasonic ("silent") drive output happy.

The "faster one way than the other" problem usually turns out to be a mechanical problem.  But it can be a decoder problem too.  The motor output circuit in most decoders is an H-bridge.  The four arms of the H represent transistors while the cross bar in the middle represents the motor.  The top left and bottom right transistors work together to control speed in one direction and the top right and bottom left transistors control speed in the other direction.  If one of the transistors is a little weak, the motor drive in one direction can be less than in the other direction.  This is easy to see with an oscilloscope but a 'scope is not usually a tool found in a model railroaders tool box.  So substitution of the entire decoder is a more practical test.

Jim

[guslcp]

Thanks for the info, guys.  I've been thinking of putting in one of my D'trax decoders in there to see if it's in fact a decoder problem....Just waiting to see what input you all had, and which loco I want to "sacrifice", even if it's just temporarily...

I'll go ahead and do that and see what comes up.

If I decide to remove the capacitors, I'll need to take a soldering iron to it...right..??

[Jim Banner]

Right.  A soldering iron with a small tip - 1/16" or smaller - and a pair of fine tweezers.  Grab the SMT in the middle with the tweezers and touch each end of the SMT with the clean and tinned tip of the iron.  Lift off the SMT.  You may have to touch ends alternately until the solder is melted at both ends.  This is a technique that works for me.

Jim

[richg]

I do not know how good you are at this but below is what I use for the SMT capacitors. I just snip them off. I usually use small side cutters like this, not the bigger ones. I get the jaw right up at one of the solder points.
A soldering iron with a fine point tip, conical shape, also works very well with the tip of an X-Acto blade tip under the capacitor to lift it.

http://www.micromark.com/SIDE-CUTTING-MINI-HOBBY-PLIER-WITH-SPRING-RETURN,9269.html

Rich

[guslcp]

Well...It's done....I took one of my D'trax decoders and set it in the board....Small to no improvement.  Soooo...Out came the snippers (flush cutters) and off came the capacitors...Well, not off, really. I chopped them in half.... and just made sure there were no remains between the two halves.  Back on the track....Smooth sailing.  I didn't want to mess with the decoder settings since it's going back to where it came from.  But once I get my new decoders (bought them yesterday...) I'll pop one of them in there and set it up so it runs like I want it to...

On a side note...I placed the B'mann decoder into the loco I took the D'trax decoder from (no capacitors on it), and it ran OK (just..), but I still can't read the decoder address....   Are these addresses set in stone..or can't they be programmed in anything other than OPS mode..??

[ABC]

What DCC system are you using?

[richg]

Ok, Digitrax decoder type and DCC controller type?

Rich

[guslcp]

Hi Rich...Long time no see...

Decoder: B'mann 44915 (?) It's the only number I can see..It's the "stock" decoder that came with the loco (Bachmann 2-8-4).
Controller: D'trax Zephyr on a programming track using Page mode.  Unable to neither read CV#1, nor write to it-either in Page mode or OPS mode.
All my D'trax decoder behave themselves (D123H's & D163H's.

[richg]

I cannot comment on the issue as I use the NCE Power Cab. I do use the Program track option with my controller and I can read the CV's.
I am not familiar with the Zephyr.
What the Bach Man said about the resistor across the tracks works for some people from what I have seen.

The C3 on the PC board is for the LED's headlight on my Spectrum 4-6-0 along with a Zener diode and resistor. I am quite sure I see the Zener diode on your PC board. The Zener diode limits the voltage, not the current to the LED. Zener diodes usually have a glass case for the smaller sizes. This C3 does not have anything to do with the motor control.
The PC boards I have also have a diode in series with the LED voltage path along with the resistor. Never did figure out why Bachmann does that.

Rich

[Jim Banner]

A regular diode in series or in reverse parallel with an LED is to prevent reverse breakdown in the LED in case of reverse voltage being applied.  Virtually all semiconductor diodes will conduct reverse current if the applied voltage is high enough.  With Zener diodes, this reverse breakdown is used to regulate voltage.  In rectifier diodes, it can result in catastrophic failure.  In Light Emitting Diodes, it can shorten their life and change their properties.

In some applications, the current limiting resistor may protect the LED from the bad effects of reverse voltage.  In others, it may not.  A couple of examples:  A high efficiency white LED used as a locomotive headlight might be used with a 1000 ohm resistor.  With 12 volts applied, the current is 9 mA in the forward direction.  The LED's power dissipation is 9 mA x 3 V = 27 milliwatts.  This is fine as the diode is probably rated at something like 33mA maximum current and 100 milliwatts maximum power dissipation.  Now apply the voltage backwards.  The LED probably has a maximum reverse voltage (a.k.a. reverse breakdown voltage) of about 6 volts.  So now we have only 6 volts across the 1k resistor so only 6 mA is flowing.  But the power dissipation in the LED has risen to 6 mA x 6 volts = 36 milliwatts.  Our LED is still safe.

Let's now replace that headlight with a mid efficiency white LED.  To get the same light out of it, we might try using a 270 ohm resistor.  With forward voltage applied, it is carrying 33 mA and dissipating 100 milliwatts, right on its limits but still safe.  With reverse voltage applied, the current drops to 22 mA  BUT the power dissipation rises to 133 milliwatts.  Now that 33% overload is not likely to cause catastrophic failure but it is enough to cause slow degradation of performance.  The LED grows dimmer and the colour starts to shift.

The above calculations were done based on 12 volts applied to the lighting circuit.  Increasing the voltage only makes the problem worse.  Putting another diode in reverse parallel limits the reverse voltage applied to the LED to about .6 volts.  Putting a rectifier diode in series with it limits the LED's reverse voltage to about nothing.

That's the story behind those extra diodes you may find.

Jim

p.s. if you are laying a resistor across the programming track when using your Zephyr, make sure the resistor leads and the rails are absolutely clean.  Better yet, solder a small alligator clip to each resistor lead and clip them onto the rails for positive contact.