Method to prevent sound dropout on Athearn FGE mechanical reefers?

[nscaler711]

Roof came off, just wiggle it a bit... I did this when I got mine... I am at work, so I just went back into my PB and found that picture. 
I will pull mine apart in the morning and take better photos!

[bbussey]

Taking an educated guess, looking at the blurry photos, the large silver cylinder marked 470 25V is the DC filter capacitor.  Adding more capacitors in parallel with that component would make the circuit more resistant to power dropouts. But I also assume that it is a surface-mount component so its leads are not easily accessible.

I would also not just add more capacitors without also adding a circuit which will limit the inrush current and also allow the additional capacitors to supply the energy quickly to the rest of the circuit (2 diodes and a resistor).

Pete, wouldn't it be easier to simply replace the existing capacitor?  Un-solder it from the circuit board and solder in a new one?

[jagged ben]

Well, I have to eat some crow on the roof, they are indeed not glued and can be gently wiggled off.

It does appear to be a 470 microfarad capacitor, similar to this one but with different markings:
http://www.goodluckbuy.com/10pcs-25v-470uf-smd-electrolytic-capacitor-470uf-25v.html

To replace with a larger capacitor you might have to put the capacitor sideways inside the car.  No big deal.  However I would note that it's already a sizeable capacitor, about the size Pete recommended at the beginning of the the thread.  So I'm not sure how much good it will do to add a larger one.   I think the real problem on these cars is the pickup technology which looks like it comes from 1970. 

Do you mean to hook up a large polarized (electrolytic or tantalum) cap between the left and right rail pickup?  Really?  That doesn't seem like a good explosion-free solution.    Bad advice, especially to modelers who aren't very familiar with electronics.
The cap really needs to be installed somewhere on the sound circuit board after the bridge rectifier.

You're right.  My point was that one does not need to be concerned about filtering a DCC signal. 

[nscaler711]

Better?

well photobucket is ********.... I rotated the photos 90 degrees to the left, saved them.. and they still popped up portrait mode.... (that and my phone is crapping out so the accelerometer is dying so... its a PoS as well... )

[jagged ben]

Ha, you beat me to posting photos.  They appear correctly oriented though.

Here's one more picture that shows how the wires are attached.  The stranded wire is just wrapped around the terminals and then those plastic caps keep it in place.  Probably would be a bit of a pain to re-install when all done.

[jdcolombo]

. . .
However I would note that it's already a sizeable capacitor, about the size Pete recommended at the beginning of the the thread.  So I'm not sure how much good it will do to add a larger one.   I think the real problem on these cars is the pickup technology which looks like it comes from 1970. 
. . .

Well, actually, 470uf isn't much.  If the pickup system isn't great, then this amount of capacitance won't help much.   My Model Power Mikado has a not-very-good power pickup system, and I ended up putting 1000uf in it to avoid sound dropouts on an ESU LokSound decoder.  Now, it does have a motor that it's powering along with the sound, but still . . . 470uf just isn't much if the electrical pickup sucks.

So, I'd find the biggest 25v cap I could fit in the space (as you note - sideways, wired to the board).  A minimum of 1000uf, but more is better.  And don't forget that you can wire caps in parallel to increase the capacitance - You could probably get 20 150uf 25v tantalum caps in that space, which would give you 3000uf (of course, it would also cost $50 - in this case, a cheap electrolytic is the ticket). 

John C.

[peteski]

Brian, sure you can add a humongous capacitor directly in parallel with the existing one (or just replace it with much larger one). The problem is the inrush current.  At least it worries me.  When a discharged large-capacitance capacitor is hooked up to a source of 12V, it initially presents a close-to-zero ohms resistance across its terminals. Since it is "empty", it is very current-hungry, like a dry sponge absorbing water (to put this in non-techincal terms).  So for several milliseconds (length of time depends on the capacitance and the total series resistance of the circuit) it will be drawing most of the current available form the throttle or DCC booster.  This is no only detrimental to the tiny bridge rectifier which passes all the current going to the sound board, it is also detrimental to the pickup-wipers and the tiny contact area between the rail and the wheel.  It can also add noise to the DCC signal.

But when the capacitor is already fully or partially charged then the charging current will be much lower. That is why when a very large capacitor is installed in this sort of circuit it doesn't hurt to add a simple inrush-current-limiting circuitry.  Either a small choke (coil) in series with the large cap or a circuit using 2 diodes and a resistor.

Just adding a humongous cap straight to the circuit will work too - but it is something I wouldn't do.  I can also see that I might just be a bit overly cautious. 

[bbussey]

An inductor choke?  What specs do you recommend if three 1000uF capacitors in series are installed in place of the stock 470uF capacitor?

[peteski]

An inductor choke?  What specs do you recommend if three 1000uF capacitors in series are installed in place of the stock 470uF capacitor?

Borrowing from an excellent ZIMO decoder manual, a 100 uH  (micro-Henries) choke rated for at least 0.5A is recommended. As an alternative a 33 ohm 0.25W resistor can be used.  Either one will limit the inrush current. If you use electrolytic caps, you should be able to use ones rated for 20V or maybe even 16V (16V is cutting it really close) caps because they should be physically smaller than ones rated 25V. Electrolytic caps are much more tolerant of running close to their rating voltage than Tantalum caps.

[nscaler711]

Ha, you beat me to posting photos.  They appear correctly oriented though.

Here's one more picture that shows how the wires are attached.  The stranded wire is just wrapped around the terminals and then those plastic caps keep it in place.  Probably would be a bit of a pain to re-install when all done.

Lol  I see that now! Dont know why but when I was on my PC the top two were portrait.

As for the clips, if you have ever played around with Athearn HO stuff, its the same just pop the caps off, and run the wires through the holes and slide them back on... Though can be tough if using solid wire, or more than two of the wires Athearn uses... (best to use needle nose pliers and some patience if you have any... Which I dont some days...)