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When a motor contact touches a rail pickup as you describe, the train runs away at full power because it is now getting full voltage from the track. It also burns the motor output of the decoder rendering it useless. this is why isolation of the motor brushes (and the orange and gray decoder wires) is so important. Unfortunately there has never been a practical way to protect the decoder's outputs when this happens. Same outcome can be expected if one or more of the function wires makes contact with a rail pick up. Minimum, that function is destroyed. Maximum, whole decoder is fried.Martin Myers
On the RDC's, I filed two clearance notches in those brass strips that run the length of the floor on either side. The notches are filed where the motor contacts come up. one on each side. The floor already has the squared notches, just make the brass strips match the floor where the motor contacts come up.This lets them pass the brass strips without making contact. Gray and Orange wires are soldered to the contacts.I also Kapton taped the four brass strips- two at each end - so that they continue to provide spring pressure on the truck pick ups. The tape insulates the four strips from the light boards so the functions are isolated from the rails.White wire goes to both boards (W) mark. Yellow wire to both boards (Y) mark. Blue to both boards (B) mark. Screws are removed from each of the holes. IIRC, there are two to get removed in each board. One screw goes back into each board where the holes are marked "DCC".My installation veered from this by using a four function decoder. The extra two functions control the rear light boards so that I can have independant control of the lights when operating multiple units. No changes to resistors were needed at the time. These were the very first release RDC's. Not sure if there has actually been additional releases or KATO just selling off the boatloads of leftovers they reportedly had.Years ago, a guy from Digitrax asked me to write this up but a couple of months later, they came out with a semi drop in replacement decoder. I never got a roundtuit. There are photos in the photo section of the Yahoo NDCC group. They are in the "KATO RDC" album.Martin Myers
Is the DCC track voltage supposed to be 22 v AC?
Tho I guess there's a drop-in now.
The touching motor lead to pickup makes it run away, ok , and mine does that -- except once put on the track again the decoder works fine, it's not burned out. It's an impossible flaw. I don't think even anybody on this forum could find what's wrong and fix it.
Umm... don't think you can read that way, unless you have a peak reading meter, as opposed to an averaging....
There's nothing wrong with what Martin said, it's what Digitrax recommends. (btw, ignore any negative signs on the results and just add them as if they were two positive numbers. Or else keep the negative probe on ground for both measurements.) This apparently works because 'ground' is always the negative side of the circuit compared to the rail, which for the record is not at all anything like the relationship of hot to ground in your AC house wiring. If you have a true RMS meter then you can get an accurate AC voltage reading rail to rail without going through those steps. ( What this won't do is tell you if there is an imbalance due to a DC signal on the track, e.g. address 0.) It's true that a cheaper meter that is only properly designed to read a sine wave will give an inaccurate reading. One thing that is inconvenient about the method that Martin described is that it is not very good for measuring the voltage accurately somewhere out on the layout where you don't have easy access to the boosters ground terminal.
I thought that OldEastRR was measuring the DCC voltage across the track using his meter's AC setting. AC range on the mulltimeter has no polarity - there should never be a minus sign displayed.
I have a Fluke, a RRampmeter, and a couple of $2 Harbor Freight meters. They all read within a half a volt of each other using the AC scale. None of them is anywhere near 22VAC.
Reread the thread and note whose post I was referring to. Measuring both rails to ground with a DC meter is what Digitrax recommends. They don't say anything about the polarity of the probes though, so I was pointing out that one should ignore that. Looking into this made me realize that I don't really understand how a DCC power source is switched with reference to 'ground'. For that matter, I'm not entirely sure if Digitrax recommedation applies to other brand systems.
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Topic: Decoder goes dead for no reason (Read 3532 times)
