# loco moving only forwards. dead decoder?



## eatrains (Jan 2, 2008)

I've got a loco chipped with an old MRC decoder. I fired it up recently and found the loco only runs forwards. When I try to run it backwards, the loco just stands still. I know the loco itself is fine because when I installed the decoder I included a switch to switch between powering the loco via the decoder or straight analog DC. Running off DC, the loco runs beautifully. Has this decoder given up the ghost? Worth investigating a fix or would it make more sense to invest in a new one?


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## East Broad Top (Dec 29, 2007)

It's always worth investigating. At the very least, see if you can perform a decoder reset. If you know the model number, you may be able to download the manual from MRC's web site. 

Unfortunately, MRC large scale decoders don't enjoy the most favorable reputation for durability, so "remove and replace" may be in your future. Keep the MRC decoder for a critter or something that you'd just set on the track to watch run around the railroad.

Later,

K


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## Greg Elmassian (Jan 3, 2008)

Many of the MRC decoders use a relay to reverse direction under DCC... They are not very reliable decoders, I bought a ton of cheap ones with sound, almost half have blown up.

Do you have the model number of the decoder?

Greg


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## Dan Pierce (Jan 2, 2008)

It can be the regulator which is used to energize the relay.
I believe mine had a 12 volt unit (LM7812) and this gets hot on 24 volts (spec by MRC is 21 volts max).


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## Greg Elmassian (Jan 3, 2008)

I've never looked at the schematic, but the ones I have had either a power transistor or regulator in a TO-220 package near the relay, is that what you are talking about Dan?

Greg


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## eatrains (Jan 2, 2008)

Don't have the model number, haven't had a chance to open the loco up. It's probably 10-15 years old.


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## Greg Elmassian (Jan 3, 2008)

Well, in answer to your questions, I think you will have to open it up no matter what, so do that first. It's a long shot, but trying to do a reset via a programming track might be worth it, but if you don't know the model, try a typical MRC reset.

Greg


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## Dan Pierce (Jan 2, 2008)

Greg I thought the older G MRC units had a LM style regulator but no heat sink. I will have to check my unit, I still have 2 not installed.
model AD324 is non sound and has a relay and mosfet. AD322 is the sound version
model AD1818/AD1819 with sound has a heat sunk LM7809 plus a small LM7805 and no relay, uses mosfets bolted together.


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## Greg Elmassian (Jan 3, 2008)

I have about 10 or 20 AD322's 

the manual: http://www.elmassian.com/images/stories/dcc/mrc/AD322.pdf

last page has a pictorial of the regulator... and I have had a fair percentage blow up there..

Greg


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