MTH Searchlight Car Questions

I can't say with 100%  certainty this is the answer, but I have a suspicion.

Back when DCS/Proto 2 first came out, I noted that an MTH Searchlight car I had triggered the bell while running one of my Proto 2 engines.

I suspect that diode is to address that situation so that engines don't start ringing their bells just because a searchlight car is turned on.

-Dave

The AC voltage stimulates the electromagnet 120 times per second...or twice per 60 Hz cycle - a positive pulse and a negative pulse.  A diode would allow only, say, the positive pulses through.  As it turns out, some vibrating mechanisms work or vibrate more effectively when stimulated at the lower rate.

The diode of course would cut the power going to the vibrator mechanism (and lamp if it's also after the diode).  I don't know if the searchlight car was originally designed with the diode, but I know some guys add such a diode to operating cars with similar mechanisms especially with the higher track voltages used for command operation.

Dave45681 posted:

I can't say with 100%  certainty this is the answer, but I have a suspicion.

Back when DCS/Proto 2 first came out, I noted that an MTH Searchlight car I had triggered the bell while running one of my Proto 2 engines.

I suspect that diode is to address that situation so that engines don't start ringing their bells just because a searchlight car is turned on.

-Dave

I've also got a searchlight car (of modern, 2016 catalog vintage) and can't run it with the light on. It seems to cause a DC leak, causing random horns/whistles in all of my engines. No issues - so far - with the bell randomly activating. I've got Modern and Postwar Lionel, MTH PS2 and PS3 and Williams TB2 and TB+ locos, all have issues with the light. Glad to know it isn't just mine car!

I don't really care if the light doesn't spin but I'd at least like it to turn on. It will get an LED conversion...eventually!

So do the same problems occur with engine horns and bells if the older postwar searchlight cars are on the track?

It's a  vibrating mechanisms vibrate for the Generator.

Good luck , JOHN   

Because the diode only drives the vibrating electromagnet with positive pulses, it can unbalance the AC track voltage ("DC leak"); it's a your-mileage-may-vary based on what else is on the track, your transformer, etc.  Some engines are more/less susceptible depending on the type of DC detection circuit used for horn/bell triggering.

As to how to modify a searchlight car to vibrate without this "DC leak" would be an interesting discussion for the Electrical Forum.

SJC posted:

I've also got a searchlight car (of modern, 2016 catalog vintage) and can't run it with the light on. It seems to cause a DC leak, causing random horns/whistles in all of my engines. No issues - so far - with the bell randomly activating. I've got Modern and Postwar Lionel, MTH PS2 and PS3 and Williams TB2 and TB+ locos, all have issues with the light. Glad to know it isn't just mine car!

I don't really care if the light doesn't spin but I'd at least like it to turn on. It will get an LED conversion...eventually!

OK, sounds like the diode wasn't to address the problem then, since they still seem to be making the cars that way!  

Mine was the Big Mo version of the car, not that it probably matters much.  I probably never took it apart, but it may have the diode too, I guess.

-Dave

"Because the diode only drives the vibrating electromagnet with positive pulses, it can unbalance the AC track voltage ("DC leak"); it's a your-mileage-may-vary based on what else is on the track, your transformer, etc.  Some engines are more/less susceptible depending on the type of DC detection circuit used for horn/bell triggering."

So that's why I was asking about running a Lionel 3520 searchlight car with modern engines.  I don't remember anything on the 3520 that would alter the AC voltage used on the vibrating mechanism.   So would a Lionel 3520 postwar car cause problems on the track with the modern electronics engines?

I'll see if the local club can run it and has any problems with it.

I have two of the NYC versions, I like them they operate well, one thing I did notice was that it rotates the opposite way of the Lionel version.  Probably due to the coil being wired the opposite way...

Marty

For vibrating motors I've seen rotation direction is determined by the rubber driver washer.  The little fingers are angled so when it vibrates, the ring "wants" to rotate away from the lean.  In other words I don't think swapping the wires to the vibrating element will change direction.

Again, I'm speculating the diode was added to modern cars to lower the power going to the vibrating motor as more layouts have 18V AC command voltage on the track.

A few years back, before LEDs became practical to install in passenger cars, guys would add a diode to reduce bulb power on 18V AC command layouts.  Otherwise the cars were too bright and bulb life was reduced.  Same issue.  By only applying positive or negative voltage (depending on diode orientation), this introduced the aforementioned "DC leak."  Anecdotally, there were reports of random whistle and bell triggering.  The solution was to flip the diode direction in half the cars...so some cars would drive the bulb with positive pulses, others with negative pulses.  This restored order to the universe; if only life were that simple! 

I'm wondering if anyone has solved the diode/whistle problem for these cars.

Does the diode affect the light too or only the motor?  Is the wiring for the motor separate from the light such that a switch could be easily added to disable the motor but keep the light on, or is the coil integrated with the bulb socket?

I'm thinking a second diode oriented the opposite way feeding a resistor with the same impedance as the coil or coil+bulb would allow the coil to still see only 60 vibrations/second while eliminating the unbalanced DC effect.  Does that sound plausible?

I'd think by removing just a few screws, or releasing the truck from the under-chassis you should be able to determine by inspection as to the wiring.  I found this diagram that suggests separate wires run to coil and bulb.

Your idea of balancing the load with another diode makes sense...though if you add a on/off switch for the coil, in principle you'd want that switch to select the balancing load too... or maybe a load that straddles the two choices probably biased to coil "on" value as I figure the coil draws more current than the bulb. 

In any case, measuring the resistance of the load would give you the DC-impedance which would give you an idea what kind of power rating you need.  Watts = Voltage x Voltage / Resistance, but half that since the diode steers power only half the time. 

Don't know what your parts "stash" looks like, but on eBay you typically get a dozen or more of some component (diode or resistor) for 99 cents with free shipping from Asia.  Note that the single diode design was an expedient way to cut power in half.  But if you're going to mess around under the hood anyway, and have a bag of diodes, you could employ the "string of diodes" method to drop AC voltage.  You'd be pulsing the vibrator at 120 Hz as originally designed.  And no "DC leak". 

If it turns out that you have separate wiring to the coil and bulb, an added "feature" of the string-of-diodes method is you can fine-tune the vibrator power as, anecdotally, these can have sweet-spots of operation.  You could separately "tap" the diode string at a different voltage to drive the bulb.

Thanks!  Hopefully MTH copied the Lionel design enough that MTH used two wires also.  Looking at the OP's photo, it looks like there are enough wires there that separate wires for the coil and bulb are likely.  Can't tell if the diode is for the coil only or for coil and bulb.

I found a set of instructions for an MTH car (not sure what the date was) that said not to run it over 14 Volts (I think it was 14) or the coil would overheat.  I wonder if the version of the car corresponding to those instructions had the diode (which might suggest the diode is there because the motor wants to run at 60 Hz) or if the diode was added to a later version (which might suggest that the diode is there to decrease the power).  But yes, two strings of diodes might be something to try.

My layout is still in the planning stages, but my daughter wants to learn about electronics and these cars are pretty inexpensive so I think this might make a fun project.

I found this pair of photos in a folder titled "MTH Searchlight" so odds are it's what you have!  It shows 2 wires going into the vibrator/lamp "shell".  One wire goes to the centered screw/nut which releases a wire eyelet suggesting you can separate the connections to coil and bulb.

Other photos show the diode board under the generator in the recessed bed so the diode is before the coil and lamp - again for the version I had.  Now that I think about it, and it's been while, but other photos show what I was doing was adding a remote-controlled on-off switch.  Here's a photo of a board that sits under the car in a molded recess.  A latching relay is activated by a pair of reed switches.  Position one or other switch over uncoupler magnet and use DCS remote and AIU to turn car on and off.  Obviously the on/off could have been done with a 10 cent slide-switch....

Thanks, Stan, that's very helpful!  I don't have one of these cars yet but I think I will get one.