I picked up an old MRC Tech 3 9500 from my club for free. When I hooked it up to some Atlas track and put a loco on the rails, it behaves as if the momentum is on, ramping up very slowly to full speed and slowing at about the same rate. The momentum switch is off. When the momentum switch is turned on, the indicator light comes on, but there's no change in the loco's movement. I opened the unit up and had a look at the circuit board under the switches and there's a small capacitor in the corner that's blown. On what remains of the plastic cover, I can see the brand name, Maxcap, and 50V 10uf. Seems like repairing the unit should be as easy as replacing that capacitor, but while I'm pretty good with a soldering iron, the finer details of electronics are not yet in my wheelhouse. So, the real question is this:
Will any 50V 10uf capacitor of similar size work or do I need to be on the lookout for other differences between them?
I see some on Digikey that include ratings for the Ripple Current, for example. Will this be important in selecting the right capacitor? Are there other considerations I need to be aware of?
Thanks for any insights.
The short answer, IMO, is maybe. It depends on what the circuit is using the capacitor for. An expert could probably tell by inspecting the circuitboard, or a schematic if one is available.
I don't know anything about the mrc tech 3, and I've never seen an mrc device in my life, but no one else has replied, so I'll share what I can about capacitors.
For a 10uf cap, I think there's a good chance it's being used in a switch mode power supply. If so, there should be a decent sized inductor nearby, recognizable as a coil of copper wire, and you may see a schottky diode nearby too.
If it's indeed part of a smps, there's a bit more to it than just capacitance. The equivalent series resistance (esr) is an important value. If the esr is too high for the frequency of the smps, the 10uf cap won't actually provide 10uf, and the cap will get hot to the point of damaging itself or popping. So for replacing this cap I'd use either 10uf of ceramic caps (they tend to have low esr) or a 10uf 50v low esr electrolytic cap, depending on what you've got available. After soldering it on I'd check that it's not heating up too much during operation by taping/gluing a thermistor onto it to keep an eye on its temperature, and since I have an oscilloscope I'd use that on the capacitor leads.
Another question is, though, why did this capacitor blow in the first place? Faulty cap? Hard use? Or is there another failure in the device that caused it?
Unfortunately, as this is a second-hand unit, I don't know what caused the failure. My guess is that the capacitor has something to do with the unit's momentum feature, since that is the apparent malfunction.
I don't see an inductor on the board, but the capacitor was soldered to the same path as the object shown in the photo.
I can't see the photo you uploaded... This forum needs you to use an external hosting site for photos, I use imgur, and there's a thread on the topic in the general discussion.
But regardless, if you're looking to replace the capacitor, I'd say pick a 10uf 50v electrolytic capacitor that says low esr, or 10uf of ceramic caps
For example, I went on mouser and searched "10uf 50v low esr electrolytic" and a 39 cent Panasonic capacitor showed up that'd probably fit the bill. Any 10uf 50v ceramic capacitor would be a good bet too. Without knowing exactly what the capacitor is supposed to do, the only way to know would be to try it out. Better fully assemble the case after replacing the cap and test it in a well ventilated area in case the new one pops too.
If were me, I'd use a 10uf 50v ceramic because I have a bag of a zillion that you can get on ebay for a couple bucks. The thing with buying electronic components on ebay rather than a more reputable source like mouser or digikey is the worry that you get sent counterfeits rather than the thing you actually wanted. Anecdotally, myself I've ordered thousands of electronic components from ebay over the years and only once have I received a bad item: I ordered n-mosfets and was sent NPNs. Otherwise I've had no problems at all. I only order from well rated sellers that are established sellers of only electronic components. YMMV
I'll give it a try. Thanks for the advice!