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MRC Tech II 2500 problem

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MRC Tech II 2500 problem
Posted by Redvdub1 on Friday, February 15, 2019 2:41 PM

One of our club's MRC Tech II 2500s has "lost it's zero"...we cannot shut the power off to keep (most) engines from continuing to run.  I see no external zero adjust...is there a way to open the pack (I know URL doesn't like that) and reset zero.  Or do I just have to figure out a circuit to shunt some current? 

Thanks for any help on this...

 

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    December, 2015
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Posted by BigDaddy on Friday, February 15, 2019 3:28 PM

Your problem is different than this

http://cs.trains.com/mrr/f/744/p/274270/3126023.aspx#3126023

 

I don't think you are going to find anything adjustable inside.

The screws on my 2400 look like Torx.  You would need something like this to open it up. 

https://www.harborfreight.com/3-pc-saemetrictorx-folding-hex-key-set-61921.html

 

Henry

COB Potomac & Northern

By the Chesapeake Bay

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    June, 2002
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Posted by drgwcs on Friday, February 15, 2019 4:15 PM

Redvdub1

One of our club's MRC Tech II 2500s has "lost it's zero"...we cannot shut the power off to keep (most) engines from continuing to run.  I see no external zero adjust...is there a way to open the pack (I know URL doesn't like that) and reset zero.  Or do I just have to figure out a circuit to shunt some current? 

Thanks for any help on this...

 

 

I have the same issue- It had been in storage for a while. When I hooked it up it is still sending a little bit of current to tracks when it should be at 0. My first guess is that the rheostat has went bad. I suspect itis the rheostat and I would imagine that these are not going to be something we can find. With two people having the issue there must be others- wonder if anyone has been able to fix one? Jim

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Posted by Attuvian on Friday, February 15, 2019 7:13 PM
Where are you Mel?
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  • From: Reading, PA
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Posted by rrinker on Friday, February 15, 2019 8:57 PM

 IF it's just the main potentiometer, you can buy one of the same value and possibly repair it. The biggest problem is there is no schematic available to help troubleshoot it. It could also be the main drive transistor gone leaky, or parts of the pulse system, not suppressing the pulse at 0 throttle.

                                               --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

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Posted by Harrison on Friday, February 15, 2019 9:24 PM

drgwcs

 

 

I have the same issue- It had been in storage for a while. When I hooked it up it is still sending a little bit of current to tracks when it should be at 0. My first guess is that the rheostat has went bad. I suspect itis the rheostat and I would imagine that these are not going to be something we can find. With two people having the issue there must be others- wonder if anyone has been able to fix one? Jim

 

I had this happen to one of my MRC Railpowers, between the two wiring guys in our club, they got it fixed. It is great to have experts like them. The actual part doesn't cost more than a couple of bucks though.

Harrison

Homeschooler living In upstate NY a.k.a Northern NY.

Modeling the D&H in 1978.

Route of the famous "Montreal Limited"

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Posted by RR_Mel on Friday, February 15, 2019 10:13 PM

Attuvian
Where are you Mel?
 

I’m here.  My grandson borrowed my Security Torx so it took a bit to get mine open.
 
The pot measures 1KΩ, most likely a linear taper.  It appears to be a regular pot not something special.
 
I ended up breaking the pins out of the screws.  The inside is about what I expected.  Not real easy to work on.  The printed circuit board is held in place by one screw into an aluminum frame and a second screw doubling as a heat sink for the power transistor into the aluminum frame.  The circuit board appears removable with out soldering.  Mostly wires off the board, one dangling 27K ¼ watt resistor from the board to brake switch.
 
The transformer is not anchored, it’s floating in plastic brackets top and bottom.
 
 
“That’s all folks” for now.
 
 
EDIT:
 
There appears to be board mounted pot in series with the low side of the throttle pot, possibly a 0 adjust.
 
EDIT:
 
The board mounted pot is on the high side of the throttle.
 
 
It still works after I resembled it, that's one in a row!
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
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Posted by SouthPenn on Friday, February 15, 2019 10:17 PM

If it has a potentiometer you might try cleaning it with an electronic parts cleaner. Make sure the cleaner is plastic safe. I have used CRC QD electronic cleaner.

This cleaner also works great on wheels and electrical pickups.

South Penn
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  • From: Reading, PA
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Posted by rrinker on Saturday, February 16, 2019 9:54 AM

 Bunch of parts off the board - there's another flying resistor on the direction switch, whatever that could be for. And another across 2 of the terminals of the momentum switch.

 Hardest part about figuring it out would be getting a look at the other side of the board. Can;t just pull it out, with all those components soldered right between the board and a switch, and those switches aren't coming out, the plastic is melted in the mounting holes to secure them. I suspect it's a fairly simple momentum throttle circuit though, nothing super special (MRC's marketing nothwithstanding). Brake switch connects the momentum capacitor in the normal position, in brake it discharges the capacitor through that resistor. Color coding of the wiring, what color coding, it seems all over the place. Each of the pilot lights justs used the white wires the lights come with, you can see the two from the power light just parallel across the variable DC output, which are yellow and black wires. Another pair of white wires going to the momentum switch is the momentum indicator light. Be interesting to see what's going on witht he momentum circuit, as the left side terminals of that switch have the white wires for the indicator and also 2 yellow wires, one of which appears to go to one of the AC terminals ont he back, and the other to somewhere on the board. The other side has a green wire which goes under the board near the green component - probably the cap. And a black wire that is soldered to the board near the bottom center, next to the hole, and the large red wire coming from the transistor. Looking at the direction switch, it looks like the variable DC comes off the board in the upper left, the red and white wires.

 It does look slightly familiar - I must have opened up my 1500 at some point.

                                  --Randy


Modeling the Reading Railroad in the 1950's

 

Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.

  • Member since
    December, 2012
  • 127 posts
Posted by Redvdub1 on Tuesday, August 13, 2019 7:57 PM

Redvdub1

One of our club's MRC Tech II 2500s has "lost it's zero"...we cannot shut the power off to keep (most) engines from continuing to run.  I see no external zero adjust...is there a way to open the pack (I know URL doesn't like that) and reset zero.  Or do I just have to figure out a circuit to shunt some current? 

Thanks for any help on this...

 

 

 

This thread got split up so I will add to this thread segment my final findings. 

Opening up the 2500 seemed to cure the "leakage" or "residual voltage" problem. I was able to reassemble the unit and it now works well..time will tell.  I did findthe Darlington Tx  literally submerged in heat sink grease and I cleaned it off and reapplied grease upon reassembly..but I hardly think sink grease could cause leakage across the TX leads.  Upon reassembly (and after I ascertained the unit was working ok) I cleaned off the circuit board with isopropyl and a Q-Tip.  

I believe the MRC Tech II 2500 utilizes a non center-tap Xformer, a 4 diode bridge rectifier, and a 2500-3000 ohm wire wound potentiometer (to feed the base of the Darlington Tx) as the essential "guts" of the power supply.  There are two trim pots and two electrolytic capacitors to filter the rectified DC signal.  The pulsing is (reportedly-not by my measurements) 60 Hz and as the throttle (aka potentiometer) is rotated from zero to full throttle the filtering is increased so as to remove all/most of the ripple.  

My best guess as to the fault mechanism is that a leakage path developed across the Tx so that the potentiometer could not turn off the Tx completely.  The most obvious causes would be either dendrite or whisker formation on the circuit board or the Tx leads.  I feel a board defect is more likely.  

Thanks for everyone's help and suggestions...much appreciated.  

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Posted by RR_Mel on Tuesday, August 13, 2019 10:24 PM

I was going to make a schematic of the MRC 2500 circuit board but stopped when you got yours working.  I think it would be a mistake to take a working power pack apart simply to make a drawing.
 
Here is the drawing I came up with, I used the picture as a go by.
 
 
 
 
 
Mel
 
 
My Model Railroad   
 
Bakersfield, California
 
I'm beginning to realize that aging is not for wimps.
 
 
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Posted by gregc on Wednesday, August 14, 2019 6:38 AM

thanks for posting the diagram.

That along with the picture in the other thread may be enough to create a schematic

greg - Philadelphia & Reading / Reading

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