This may be a hard one, but has anyone used a scope to check out a decoder. I would like to know where to connect the probe Gnd, if anyone knows. Thanks Bruce
Yes there is a way. This assumes you understand the typical layout of a decoder. They all have SMD components. They usually have values on the top. The sound decoders will be more complex. Determine the point for the common on the decoder. It will be the minus side of the main filter cap. Techies will insist on using a differential probe but I have never found that necessary. This is not rocket science. That is usually the largest SMD cap. The tip of the Scope probe is applied to a spot for measuring. That should be obvious. lol Make sure the Scope is not grounded via the three wire power cord. That can affect how the waveform will look. I use a Widow Maker with my Tek 475 Scope. It is much easier with my Velleman battery operated Scope. The DC voltage on the output of the main cap should be in the vicinity of 12 to 14 VDC but that depends on the voltage of the booster. Some might be a little higher. A light board decoder will be easier to Scope out, versus, one in a shrink wrap. Remember, there are many different styles of decoders. Good luck. Rich
Thanks Rich, thats a start, I don't have a window maker, that wont make it worse will it, I'm just trying to see what kind of activy is going on. The decoder is an MRC 1644 sound decoder. it wont move the engine, but has some sound. I tryed to get info on the chips, and can't come up with anything on a 6CW71 Trans?. It's sits just befor the BA6428 motor drive chip. You by anychance know where you can sch. of decoders do you. Bruce
I meant Widow Maker. Not really lol of course. That is a two prong to three prong for AC power cords. It isolates the ground in the house outlet so the Scope frame is not ground protected. That term is been used for many years ever since companies went from two prong to three prong. There have been instances over the years that some would cut off the ground pin and some have paid the ultimate price, usually with an old electric drill with metal case. OK, back on subject. Maybe the wave form will be a little distorted but not a big deal. This fellow is very active in some of the Yahoo DCC forums. There is a MRC Group also. http://www.vimav.hu/download/dcc.pdf I am including a link that will give you a little more info on what can be on a decoder. No idea on your level of experience. The decoders put out PWM to the motor. Full voltage pulses, positive or negative depending on direction selected. The pulse get wider or narrower, depending on speed requested. No pulse, no speed. I believe the decoder have 5 VDC for operating the IC's. Been quite a while since I have looked at a decoder with a Scope. Rich
Yeah, I tired to search out 6CW71 and came up zero. You are sure that is the label? It could be specific to MRC. I have seen a few times over the years that a device could be specific. I wonder if it is a transistor or a motor driver which are different. How many pins on the device? Two companies that I have death with are, Mouser Electronics and Digi-Key. Both have huge paper and online catalogs. You could check them online and see if any SMD components comes up, at least similar. Rich
Have you tried returning the decoder for a working one? I am not familiar with the MRC return policy. Rich
What model Velleman scope do you have, and how do you like it. I was looking at portable scopes this morning, and would like fairly good cheap one.
While you may well be able to track some signals around, all of the interesting stuff is going to be done in programmable logic, and you are not going to be able to break into that. The best you can do is to look for obvious opens and shorts, cold solder joints, etc. I'd put the odds at even an expert being able to actually debug the decoder at very near to 0. I have read that the best repair tool for an MRC decoder is a sledge hammer. Just sayin'!
Hi Jeff, Thats all I want to do, is look for opens and shorts, and if I find something, and the part can be found and does'nt cost and arm and leg replace it and see what happens. I've heard MRC is junk befor, but if I can somehow get it fired up again, cheap I'll learn more about decoders. Bruce
I have the HPS5, fairly old. Mine has rechargeable batteries. Ok for general testing but no sync port for more detailed analysis. My Tek475 is what I use if I needed more details. That dual trace Scope goes back many years. About eight years ago I had to replace the electrolytic caps as the ESR was quite high when I looked at them with my ESR tester. Rich
For finding possible opens and shorts, good lighting, a magnifier, and an ohm meter will be a lot more useful than a scope. If you find something, you probably won't need to replace any parts, just do a little (very small and precise) solder work. You can also look for black spots where something may have burned out, but if you find it, you might not be able to tell what it was! If if was me, I'd find a more interesting way to use my time. There are schematics on the web for making homemade decoders, including code for the processors. Much more informative for learning how they work, and maybe useful for some kind of stationary decoder.
