K-Line Heavyweight Observation Car Circuit Board - Drumhead and side marker lights not working

That board is using the diodes as "regulators" but depends on the incandescent large bulbs as load, to then push the diodes towards the maximum 0.7V silicon diode drop https://en.wikipedia.org/wiki/Diode.

Again, here is the concept. You have the load of the regular large incandescent bulbs in the car for main lighting. These diodes are in series to those lamps. Across the diode, as you load it, the voltage increases to a maximum 0.7V per diode.

Forward threshold voltage for various semiconductors

• Silicon diodes: 0.6 V to 0.7 V

I count 6 diodes.  Let's say they wired them in pairs so one always conducts the AC waveform. That logically puts 3 diodes in series.

3*0.6= 1.8V and expected- many grain of wheat bulbs are only 1.5V

So again, you only see 1.5V when the circuit is loaded by the larger incandescent bulbs. If one or two burns out or are not connected- then you get near nothing out the terminals for the grain of wheat bulbs.

Again, a betting person says, given that picture of the board nothing wrong. The diodes are not burned and unlikely to be in any way faulted. the only way they would burn is a dead short on one of the larger bulb sockets.

In order to get any voltage out of the low voltage contacts- there has to be sufficient load and source voltage (because the load bulbs are incandescent).

Suggest testing the drum head and marker lamps with a 1.5V battery.

Another thought, some incandescent and small bulbs like Christmas lights have a self shorting mechanism, so when the bulb "burns out" the filament breaks and the little arms swing out and short (so in a series string- the rest stay lit). If that happened in this circuit on the smaller bulbs- then it shorts all the other small bulbs. So test each one individually (the small red and black wires). I should also add, that again, a short on the low voltage output of this board is non damaging. You simply are shorting around the diode for the main load. That said, the same short causes all the other small bulbs to be out. No damage, but no operation either.

I also should add, they way I've seen this come up before is Williams and others used similar 1.5V marker lamp "regulation" using bridge rectifiers (4 diodes) shorted + and - with the 1.5V bulbs across the ~ AC legs of the rectifier.

The problem came when someone put LED bulbs into the main original incandescent sockets that form the main load. Back at no load (minimal current LED bulbs) results in less voltage drop across the diodes and no incandescent marker lights.

Again, this is like a peak regulator. The voltage output on the markers can rise up to a "limit" and that rise only happens when there is sufficient main load current through the diode to push the diode into the "knee" of the voltage drop curve.

Thanks very much for that thorough explanation. 

I tested each bulb individually with a 'C' cell that registered 1.6V on the voltmeter. No light from any, so it looks like I need to refresh the bulbs.

Now to research how to do that without damaging the housings.

It occurs to me that I may have tested the low voltage drops on the board with 'AC' selected.

That my be why I saw zero voltage from the board.

Thanks again for sharing your knowledge.

I'd put a 1.5V bulb across one of the outputs and see if it lights or blows before you put a bunch of labor into replacing all the bulbs.

Thank you, John.

Doing so would ensure that:

a) the bulbs work prior to install

b) the bulbs are not being blown by the circuit board?

It appears that the bulbs in the side marker lamps at the rear of the observation car may simply pull out of the housings. I'll investigate tomorrow in daylight.

I'm concerned about replacing the drumhead light, however. Sure don't want to break anything.

Are you familiar with the older K-Line heavyweights? This one has four incandescent bulbs on the metal chassis, no interior.

Will the drumhead cover pop off, or will I have to remove the entire rear railing section to access the drumhead light?

Thanks very much for any insight that you can offer.

It's surprising to me how much there is to learn about a seemingly simple repair.

I would take a meter and measure the resistance of those bulbs. Again, you did the sane simple battery test. Most of the time even if the voltage was wrong, in theory the bulb might light dimly.

I'm fighting a couple of things:

#1 I don't have similar K-line cars to test on my end

#2 documentation is nonexistant

#3 K-line used multiple configurations for marker lights in cars. I saw this in another LED based topic about modifying K-line cars. Point being there are boards with an obvious regulator attached to a heatsink and 4 diode rectifier- but your car is specifically 6 diodes. There are all kinds of ways they could be wired, but again my educated guess says that typical grain of wheat markers are often 1.5V and other manufacturers followed this. They could be 3V, 6V, who knows?

Examples https://ogrforum.com/...-maker-light-led-mod

This picture by @lehighline shows a true regulator style board that would be constant voltage vs peak voltage regulation with just diodes. Again the major difference, 4 diodes, not 6, and that 3 pin heatsink tabbed regulator.

Missed a link, https://ogrforum.com/...r-led-lighting-again

That may have details down in that topic that answer some questions?

I don't have any of the older K-Line cars.  I also don't take too much note of how those work as when I'm in there, I'm usually ripping out all the lighting and installing LED's.

Thanks gentlemen for your help.

The drumhead on this car is easily removed it turns out. I pulled a dental pick from my toolset and gently pushed on the tabs where the portion of the railing snaps into the floor. The railing cam out easily.

The drumhead has a small hole in the backside. Inserting the pick into that hole caused the cover to pop off with hardly any pressure.

I connected the bulb directly to my old ZW and slowly added voltage. The bulb did not light.

Now I wait for the 1.5V grain of wheat bulbs on order to arrive.

@Rusty Axle posted:

I connected the bulb directly to my old ZW and slowly added voltage. The bulb did not light.

No surprise there, the ZW starts out at over 5V, and it would blow a 1.5V bulb in a flash.  Even if it wasn't bad, your test sealed it's fate!

Going back to your original post...  Did you try contacting Brasseur's Trains in Saginaw, MI??  I believe they bought the remaining service parts of K-Line when they folded their tent.

OTOH, looks like you're well on your way down another road.

I'm with GRJ...once into a K-Line, et alia, passenger car...and especially their last runs of the benchmark aluminum cars...I'd be ditching the incandescents and installing LED's throughout...like the ones GRJ makes (Hennings) for passenger cars!   

I'm counting on winning a Powerball Lottery sometime soon so I can afford the $$$ side of that project...and re-equipping everything with Kadee couplers.  Then there's that issue of time.  The incessant, growing sound of 16 hoofbeats approaching from my 6 may overtake the aspirations,  rendering the ol' proverbial 'bucket list' null-and-void in an instant.  It can happen.   C'est la vie!