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Author Topic: Did my DC powerpack corrupt my locomotive?  (Read 16077 times)
Len

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« Reply #15 on: February 26, 2015, 06:01:06 PM »

The older MRC Tech II power packs used a built in pulse width modulation scheme that worked great for older DC locos, but is known to cause problems with newer decoder equiped locos.

I used an MRC 1300 in my repair shop to verify decoder equiped locos would operate on DC before making any DCC changes. While it does have a 'chopped sine wave' output, it does not use PWM. And it never caused any decoder problems in the 14 years I used it.

Len
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jbrock27

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« Reply #16 on: February 28, 2015, 09:18:37 AM »

Thank you for clarifying and confirming what I had thought and had said Len.

When you say "older packs", how old are we talkin?
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Len

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« Reply #17 on: February 28, 2015, 10:16:27 AM »

Sorry, I should have been clearer. By "older Tech II's", I just meant older than the MRC 1300/1370 power packs. Not that there was a difference between early and late Tech II's. All of the Tech II's I know of had the built in PWM for "smooth starts".

Len


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jbrock27

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« Reply #18 on: February 28, 2015, 08:33:07 PM »

That is interesting bc I have seen some in the Tech II line have the pulse switch and some w/o.  Like I alluded to earlier, I would find it strange that one Tech II model with a pulse switch would be identical to a Tech II model w/o one.
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Len

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« Reply #19 on: February 28, 2015, 10:27:14 PM »

The "Pulse" switch added in spikes of higher voltage at lower throttle settings on top of the PWM to get the old open frame motors started. This was a holdover from ealier straight DC throttles.As the throttle advanced, the spikes would get smaller until at some predetermined point they would go away.

With PWM the voltage comes in pulses (not higher voltage spikes) spaced farther apart at low speeds, they get wider and closer together as speed increases, until they merge into essentially a flat DC line.

It's easier to visualize if you think of narrow square waves, with wides spaces between them at low speeds. As the speed setting increases, the square waves get wider, and the spaces between them get smaller. A meter attached to the output shows an apparent voltage increase, as the time voltage is present gets longer.

The Tech II's don't actually use square waves, it's more like  partially rectified AC to create the PWM. At some point, someone at MRC figured out you don't actually need the "Pulse" voltage spikes on top of PWM, and the "Pulse" switch went away.

Len
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jbrock27

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« Reply #20 on: March 01, 2015, 09:23:34 AM »

Ok Len, thank you for taking the time to explain the physics of it.  So, are you saying that a Tech II 2400 with a Pulse switch, will operate the same as a Tech II 1400 w/o the Pulse switch?  Both are Tech IIs. 

Since you mentioned the Railpower 1370 earlier, I have to point out that there is a fellow (Glen on MRR) on the net who claims his oscilloscope found a manufacturers defect (really a lack of including a resistor) in them, affecting their operation.  A GOOGLE search should turn it up.
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Len

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« Reply #21 on: March 01, 2015, 10:54:04 AM »

The Tech II's operate the same, using PWM, with the pulse switch off. Turn the pulse switch on and you get the spikes on top of the PWM on the ones with the switch. Since the PWM used has very steep sides to the pulses, not good for motors that don't have a lot of mass, as it generates heat.

Current DCC systems, and later MRC power packs, also use PWM, but the slopes of the pulses is more like / \ instead of | |, generating much less heat. But still more than you want to put into a coreless motor without a decoder using address '0'.

I tracked down a thread on Trainboard from 2009 where a fellow named Glenn said:
Quote
While the MRC 1300 is OK, avoid the MRC1370 at all costs. A manufacturing defect on the MRC1370 results in full wave power at all speed settings, unlike the MRC 1300 with correct half wave output at low throttle settings.

That's not a manufacturing defect. All it means is the MRC 1300 use half-wave rectified AC on it's track output, like the older Tech II power packs, while the MRC 1370 uses full-wave rectified AC, resulting in a smoother DC output on it's track output. There's a fairly decent explanation of the difference here: http://www.radio-electronics.com/info/circuits/diode-rectifier/full-wave-rectifiers-circuits.php

All it really means is the MRC 1370 is actually providing more power at a given throttle setting than the MRC 1300, while the pulsing effect of the half-wave output of the MRC 1300, like the Tech II's, may give better starting performance to older locos.

Why someone who doesn't understand something as basic as half-wave vs full-wave rectification is looking at things with an O-scope beats me.

Len
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jbrock27

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« Reply #22 on: March 01, 2015, 01:26:08 PM »

Thank you Len, I appreciate your sharing all this knowledge you have on this subject.

So getting back to my question, does this mean then that simply bc a model is a Tech II, does not mean it has the same Pulse feature, unless it has a Pulse switch?: The Tech II's operate the same, using PWM, with the pulse switch off. Turn the pulse switch on and you get the spikes on top of the PWM on the ones with the switch.

Why someone who doesn't understand something as basic as half-wave vs full-wave rectification is looking at things with an O-scope beats me.

To answer your question: a) bc he is nuts; b) bc he has nothing better to do with his time   ??

I would not know about the time, I'm one of those "10 minute" modelers.... Wink
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Len

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« Reply #23 on: March 01, 2015, 01:55:14 PM »

Thank you Len, I appreciate your sharing all this knowledge you have on this subject.

So getting back to my question, does this mean then that simply bc a model is a Tech II, does not mean it has the same Pulse feature, unless it has a Pulse switch?: The Tech II's operate the same, using PWM, with the pulse switch off. Turn the pulse switch on and you get the spikes on top of the PWM on the ones with the switch.

Yes. Pulse switch = PWM with spikes when on, No pulse switch = PWM only.

The main difference between the Tech II's and the later, e.g., 1300, power packs is the PWM on the Tech II's had the straighter sides (| |), while the later ones are more sloped (/ \).

Len
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jbrock27

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« Reply #24 on: March 01, 2015, 03:43:30 PM »

I know, I have one of each, which was why it was important to me to get clarity on this.  At first, it appeared you were saying that all Tech II models were alike, Pulse switch or no.  Thanks again.
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zeeglen

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« Reply #25 on: December 03, 2015, 01:18:09 AM »

I tracked down a thread on Trainboard from 2009 where a fellow named Glenn said:
Quote
While the MRC 1300 is OK, avoid the MRC1370 at all costs. A manufacturing defect on the MRC1370 results in full wave power at all speed settings, unlike the MRC 1300 with correct half wave output at low throttle settings.

That's not a manufacturing defect. All it means is the MRC 1300 use half-wave rectified AC on it's track output, like the older Tech II power packs, while the MRC 1370 uses full-wave rectified AC, resulting in a smoother DC output on it's track output. There's a fairly decent explanation of the difference here: http://www.radio-electronics.com/info/circuits/diode-rectifier/full-wave-rectifiers-circuits.php

All it really means is the MRC 1370 is actually providing more power at a given throttle setting than the MRC 1300, while the pulsing effect of the half-wave output of the MRC 1300, like the Tech II's, may give better starting performance to older locos.

Why someone who doesn't understand something as basic as half-wave vs full-wave rectification is looking at things with an O-scope beats me.

Len
[/quote]

Len,
You are correct that half-wave gives better starting performance.  Also better slow speed performance.  Because there are only 60 half-wave sinusoidal pulses per second (with intervening gaps) as opposed to 120 PPS full-wave (assuming a 60HZ AC power source) each pulse must be a higher voltage and current for the same motor speed.  This causes the motor to output a higher torque at that speed than it would have at 120 PPS.
 
The MRC1370 did have a manufacturing defect.  This was back in 2002 for the circuit board identified by the numbers PC-1370M-1  and 16-01-01.  MRC might have fixed the problem since then; I don't know.  What I do know is that a MRC representative told me in a telephone conversion that the 1370 should should be half-wave output at low throttle settings like other similar MRC products, and seemed surprised when I reported that it did not.
 
If you have one of these units you can determine (and fix) any defects by opening the case, unplug it from the AC outlet first.  The 'security' screws are easily overcome by filing a notch in the blade of a regular flat screwdriver to make it fit the screw.
 
Look at the circuit board to see if there is an empty component location labelled 'R1'.  The defective boards have an error in the copper etch that connect one end of R1 to the output of the bridge rectifier.  This resistor should have been connected to one of the AC terminals of the bridge rectifier instead.  Since the manufacturer chose to leave out this resistor it is obvious that the manufacturer was aware of the problem and decided to ship the product anyway.
 
If your R1 is missing, fix the problem by installing a 5% 5.6 kilohm resistor, 1/4 watt is fine, one lead into the pcb hole that connects to the wiper (center terminal) of the speed control.  Leave the other lead of the new resistor out of the pcb, instead use a wire to connect it to one of the AC input terminals of the bridge rectifier.  Which AC input terminal does not really matter unless you want ALL your power packs to be phased the same way, so that when the locomotive crosses between blocks that are each driven by separate power packs it does not temporarily speed up by receiving out-of-phase pulses (temporary full-wave while stradding blocks) from the two separate power packs.  If you do not have an oscilloscope you will have to determine the phasing by experimentation.
 
Now your MRC1370 should output half-wave at slow throttle setting, increasing gradually to full-wave as the throttle is advanced to full speed like it was designed to do.  By gradually I mean that the suppressed sinuoidal pulses in the 'gap' between the main pulses will increase in amplitude relative to the main pulses.  Your locomotives will now run even smoother at slow speed.
 
And by the way, I am the 'Glenn' you refer to in the above quote; and my occupation for the last 40 years has been electronic design engineering.  I do know the difference between half-wave and full-wave rectification
 
Regards,
Glen
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jbrock27

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« Reply #26 on: December 03, 2015, 08:00:56 AM »

Glen, do you happen to have any pics of your fix?  You know what they say Wink
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zeeglen

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« Reply #27 on: December 03, 2015, 09:33:11 AM »

jbrock27
The original pics were lost in a hard drive crash long ago, but was able to recover the .doc file from a forum.   Being new to this Bachmann forum cannot figure out how to attach it.  However, the file with the pics can be seen at either

https://groups.yahoo.com/neo/groups/Z_Scale_Electronics/info

https://groups.yahoo.com/neo/groups/MTL-ZScaleLocomotiveMaintenance/info
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jbrock27

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« Reply #28 on: December 03, 2015, 01:37:57 PM »

Use Photobucket to then get the pics here.

PS-Sorry if this is a silly question, but how come I only am seeing 2 pics?  One is sine wave kinda stuff and the other looks like the bottom of RR truck.
« Last Edit: December 03, 2015, 01:54:23 PM by jbrock27 » Logged

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zeeglen

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« Reply #29 on: December 03, 2015, 02:47:29 PM »

but how come I only am seeing 2 pics?
Sorry, was in a hurry this morning to get to work.  Those are the groups home pages.  Click on 'Files' near the top and then to the MRC1370 file.
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