MRC 4-6-2 and 2-8-2 Sound Decoder Selection and RS-11 Install

[RBrodzinsky]

One thing I have noticed on doing a couple of these 73100 installs is that if the board overhangs the speaker, and is too close, you can get this type of distortion. I fixed it by putting some small styrene shims of about 0.060" height between them (use the solid parts of the enclosure).  Not certain if it was just resonance of the board and speaker, or some other interference issue, but as soon as I did this, all distortion disappeared.

In looking at the install photo, there is a lot of the board (and big LED) over the speaker, so this solution might help here.

[peteski]

One thing I have noticed on doing a couple of these 73100 installs is that if the board overhangs the speaker, and is too close, you can get this type of distortion. I fixed it by putting some small styrene shims of about 0.060" height between them (use the solid parts of the enclosure).  Not certain if it was just resonance of the board and speaker, or some other interference issue, but as soon as I did this, all distortion disappeared.

In looking at the install photo, there is a lot of the board (and big LED) over the speaker, so this solution might help here.

How about using a layer (or two) or a double-sided foam tape?  That should deaden any vibrations better than a solid piece of styrene.

[narrowminded]

The volume is at full for the video.  My threshold in that room is about half of that. 

I won't get to check it out until later.  Right now it's pretty tight in there so I don't think there's even room for a .010" shim but I'll see later.  It's spring time which means a lot of outdoor work around here.

Looked at where I'll add caps and think I have a way to do that neatly.  Again, maybe get to work on that later tonight.

[narrowminded]

Yep, that's the Alco 251.  Recognize that anywhere.

This might help explain my strong interest in the 251.

https://www.therailwire.net/forum/index.php?topic=41797.msg524094#msg524094

[narrowminded]

Got a chance to check out the distortion.  It's not speaker distortion but the outer frame of the speaker enclosure vibrating against the bottom of the board ahead of where I put the glue strip.  Pinching down on that point pretty much eliminates the vibration even at full volume.  It's much better at a lower volume and if I redo the glue it should be fine at the volume I'll be using, if not all of the way up to full volume.  This is the problem with this direct board in these smaller units.  No room for the speaker. 

Another day and another application, do folks think it might be better to forgo volume for enclosure wall thickness and rigidity?  I'm thinking that might be the case.  As this is, there's only about .005" between the frame and the enclosure bottom, no room for a shim that would be tight and no rigidity in the board overhang to actually apply a pinching force without just jacking the end of the board up, which also has only .010" before the light hits the shell.  A more rigid box could be more aggressively attached, maybe even screws.  But there goes valuable volume.

Also, if it weren't for the narrowness of these shells that Select Micro could fit and would offer all sorts of flexibility to decoder placement, therefore speaker placement.  Two of my soon to be done/ attempted diesels will be FM H16-44.  That one is just wide enough to take a Select Micro without grinding the shell.   Even though it's DCC ready, from a cursory glance it looks like it will be impossible for a Direct and still a tough one for the Select Micro. 

[jdcolombo]

I'd probably sacrifice a bit of internal volume for more rigidity - or maybe I should say "inert-ness."  Back in the day, I would do speaker enclosures from 1mm sheet lead, because that stuff doesn't vibrate.  It's really not rigid, because lead is a soft metal, but it is very inert.  But if you've got an issue with enclosure vibration, better to tackle that and worry about enclosure internal volume later.

John C.

[narrowminded]

I'd probably sacrifice a bit of internal volume for more rigidity - or maybe I should say "inert-ness."... But if you've got an issue with enclosure vibration, better to tackle that and worry about enclosure internal volume later.

John C.

I don't know that it's a problem on this one because it's so shallow but it has occurred to me after realizing how strong the vibrations are, even on such a little speaker.  Gripping this one on the sides doesn't change anything that I can hear but that distortion type buzz is there and is from the top, vibrating against the board.  On this one I allowed about .010" clearance in the design height but .005" of that went to the glue line thickness, a little more than I thought it would be.  It's not hitting now but as a universal design, one chassis to another, I suspect it should have more clearance because as I see the capability of those speakers to shake the whole enclosure I think I need a more firm attachment and right now there isn't enough room to do that without being too close on the opposite side.  And I can't just thin the bottom because it's already thin as made (.020").  While fixing the one side you'd create the same problem on the opposite one.  I need to leave another .010" anyway.  On this one I'll stay with it as is just tightening up the top attachment but this is my little learning curve, I guess.  

Just to complete this exercise I'm going to go ahead with 4 capacitors mounted to a piece of circuit board and then get on with one of the MP steamers.  That should be a lot easier.   

I've learned a lot already with just this little bit of experience and all of the excellent guidance.  It's being fun so far.

[narrowminded]

Machined a circuit board to fit and mount caps.  Even got the color coding right.   It'll be some time until I mount it.  I'll be curious to see how they work.  I haven't had any dropouts yet but I'll tip it to a side and see if I can tell a difference there.

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[narrowminded]

I've started to look at one of the 4-6-2 Model Power engines.  Space is much better but still a little tight IF... and this is the question, I go with an 18 x 13 which may be marginal on the volume or the 15 x 11 which should be generously sized.  Just roughing in some numbers it looks like I can get about 900 cubic mm for either one.  Better with the bigger speaker with less relative volume or the smaller one where the volume should be generous?

[narrowminded]

I think this is more like it.  Speaker enclosure was buzzing against the board where it wasn't attached.  I still haven't done any adjustments to the sound file but I think I would like the bell, horn, and compressor sounds not as loud and the engine sound a little louder.  That's for later.

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[jdcolombo]

I've started to look at one of the 4-6-2 Model Power engines.  Space is much better but still a little tight IF... and this is the question, I go with an 18 x 13 which may be marginal on the volume or the 15 x 11 which should be generously sized.  Just roughing in some numbers it looks like I can get about 900 cubic mm for either one.  Better with the bigger speaker with less relative volume or the smaller one where the volume should be generous?

Go with the 11x15mm in the larger space.  You can hear what happens to a 13x18mm when you skimp on enclosure size (or don't use an enclosure at all) here:

The first 1:20 or so of this video is a 13x18mm speaker with no back on the enclosure (not sealed); that is followed by a 6mm deep enclosure; that in turn is followed by a much bigger enclosure made by adding 7mm of linear space.  You just can't get around physics - the larger the speaker, the larger the enclosure you need for best sound.  A 13x18mm really needs about 1 cubic cm (1000 cubic mm) to sound best.  With an 11x15, you can get great sound with about 750 cubic mm, but larger would be even better (up to the magic 1000, which is the size manufacturers use to test these speakers).

Bigger speakers are not always better.  Bigger speakers are better IF they can be put in a properly-sized enclosure.  But if you have to skimp on enclosure space, it is better to use a slightly smaller speaker with a more generous enclosure.  Here is my MRC Mike, with an 11x15 speaker in a fairly generously-sized enclosure:

John C.

[jdcolombo]

I think this is more like it.  Speaker enclosure was buzzing against the board where it wasn't attached.  I still haven't done any adjustments to the sound file but I think I would like the bell, horn, and compressor sounds not as loud and the engine sound a little louder.  That's for later.

/>

Yep - now the distortion is gone.

There is a reason we've adopted these rectangular cell-phone speakers: they are VERY powerful for their size, and have extremely long diaphragm excursion for their size.  At the beginning of my speaker test video in my prior post, you'll see how the 13x18 "jumps" because of the large diaphragm excursion these speakers provide.  But the downside is that they need clearance from anything above to avoid causing the "buzzing" you heard on the first go around and they need to be very firmly attached to the frame (I use thick CA to attach mine to the frame).

John C.

[jdcolombo]

This might help explain my strong interest in the 251.

https://www.therailwire.net/forum/index.php?topic=41797.msg524094#msg524094

BTW, I assume you know that the Alco 251 is STILL in production.  Fairbanks-Morse bought the rights to the 251 when Alco went under, and they still make it to this day, mostly for marine and stationary uses.  But if you want to buy a brand new Alco 251 diesel, FM will happily make one for you . . . which says something about the 251's design.

http://www.fairbanksmorse.com/engines/fm-alco-251f/

John C.

[narrowminded]

Thanks again John for all of your shared experience.  It has been helpful. 

You may be able to tell from some of the questions that I'm grasping some of the concerns but am looking for the confirmation of what the seat of the pants tells me.  I was pretty surprised at the force that could be felt, generated by that little speaker, but probably shouldn't have been.   

I saw your video of your RS-11 with the air start.  There's a story about the origin of that air starter.  It's one of many that were told in a fashion that was interesting for the topic alone but told in a way, through subtleties and support anecdotes, that covered a much broader tale than the immediate topic. 

That one originated as an aside to a Navy spec for submarine engines or as it was identified in Alco, the "Snorkel Engine", where an air starter was a mandatory item.  Air starters were not uncommon for diesel engines but they weren't used by most when electric was available because of all of the little parts and pieces needed to make them work as the conventional diesel air starters did.  They pressurized the cylinders individually to turn the engine over and then introduced the fuel to be off and running.  That took a lot of little and not so little parts, levers, cams, etc.  During the engineering meeting reviewing the specs there was a younger, newer engineer (like my father at the time, but not him) who had little experience and didn't really have much of a grasp on the conventional diesel air start methods but did know about vane type air motors.  Well, the spec didn't say how to design it, just that it must be air start.  When that engineer mentioned what he thought could work from his prior experiences the boss listened and decided to go ahead and try it out.  If it worked it would save a LOT of money designing, making, and assembling and setting all of the little components for the conventional air start methods.  It worked and worked so well that it became an Alco standard offering on all of their engines.  Cost effective, reliable, and simple.  At least that's the way I heard it at the kitchen table.

[narrowminded]

A question for the speaker enclosure in the MRC 4-6-2 and 2-8-2 steamers.  Both tenders are the same so the method will be common.  The speakers will be 15 x 11 and the internal enclosure volume should come in right at 1000 cubic mm with any error in that estimate to the plus side.  I could just as readily make the enclosures from .020" brass or .040" styrene or ABS.  So which would you do, brass or plastic?