Operating Reefer Car MTH 20-94220 < FIXED >

Are you willing/able to remove the circuit board and make electrical measurements?  Two small screws hold the board.

The idea is if AC is getting to the board, then check the 3-terminal 7809 DC voltage regulator IC which drives the DC gearmotor.  With the door removed as you show with limit-switch in the "out" position, all you need is track voltage and 9V DC should be supplied to the motor.

Stan,

I am working on it, and will report back tomorrow.

Thx!

Alex

I think you'll be able to track down the problem and repair it with minimal out-of-pocket cost. 

If you're not familiar with the electrical function, when idle the limit switch is "in" or closed.  By applying a few seconds of power to the activation rail, the DC gearmotor moves enough to slide the door and the limit switch opens to the "out" position.  In doing so, the DC power to the gearmotor is now supplied from the normal center/hot rail.  The unit goes through the cycle ejecting one crate and the limit switch closes and the action stops...since you've presumably released the "unload" lever which removes activation rail power.  The smoke heater voltage and lamp if you have one are also derived from the DC voltage but since you are getting no action whatsoever I figure it's easiest to focus on getting the gearmotor running since it drives the door and controls the power cycle.

Stan,

Before I read your second post, above, I did more checking based on your first post.

1. As you said, with the limit switch not actuated (out), and with track power on, the motor indeed runs.

2. With the limit switch actuated (in), track power AND unload (secondary rails) power on, nothing happens.

Here is where I have Track Power and Unload Power when the Unload switch is pressed and the Limit Switch actuated (in):

Where should I check next?

Thanks!

Alex

Whew!  If the motor runs then you're close!

So presumably you measured AC voltage at the 2 "Unload Power" connections in Blue.  That AC voltage goes thru a bridge rectifier to a 7809 regulator IC to generate 9V DC.  That 9V DC drives a relay as indicated in the maroon box with polarity shown.  You should measure 9V DC on the relay coil when "Unload Power" is applied...if the room is quiet you might even be able to hear the relay click. 

On the left side of the board there is another bridge-rectifier-regulator circuit that generates 9V DC motor voltage from track power.  We know that works since the gearmotor runs.  The relay (when active) applies this 9V DC motor voltage directly to the motor when the Unload button is pressed. 

It's a little confusing that you have two 9V circuits but what are you going to do!

Stan -

I confirmed that the power from the unload auxiliary rails is reaching the bridge, and the bridge DC output is reaching the 7809. However, there is no output from the 7809.

I unsoldered and removed the 7809, and just for kicks, I tried powering the relay with a 9V battery, but the relay did not pick up. The room is absolutely quiet, I have my hearing aid on , but did not hear anything.

I measured the current through the battery when attempting to power the relay, and it was 3.5 amps, which means that something is not quite right. According to the relay data sheet, the coil consumes 0.36W, or about 0.04 amps, so 3.5 amps is way off.

To troubleshoot this, I have the unload button on the controller clamped down to provide power at the auxiliary rails of the track. I happened to touch the 7809 heat sink, and it was really hot - too hot to touch, so well over 130°F. Now I am wondering if the relay failed (shorted coil?) and in turn caused the 7809 to fail.

What do you think?

Thx!

Alex

Hmm.  I'd be surprised if a 9V battery could put out 3.5 Amps even into a short-circuit.

If the 7809 is indeed seeing a shorted relay coil, it will get hot real quickly and go into thermal shutdown to protect itself. 

Did you simply measure the coil resistance with the meter?  If it's supposed to take 0.04 A at 9V, then that's about 200 ohms (R = V / I).  I just measured mine and it reads 175 ohms.  Depending on your meter it might initially give an odd reading since there's a capacitor and the 7809 regulator itself across the relay coil but you should not see a dead-short as suggested by your 3.5 Amp reading.  BTW I assume you minded the polarity when applying the 9V battery - there is a polarized electrolytic capacitor across the relay coil that does not take kindly to applying a backwards voltage!

In any event if it is indeed the relay, it appears to be an inexpensive component ... you'll pay more for shipping!

Yes, the current would go down really fast from 3.5 down to 3 in a couple of seconds, before I removed the battery. Yes, polarity was correct. I am going to test the 7809 by itself, as it may be OK, but I will replace it regardless. I have not been able to find a replacement for the relay. I have several, but all are 12v and the wrong footprint. Any ideas of where to look? MTH? 

Thx!

Alex

Another thought.  It could be that the capacitor on the output of the 7809 shorted.  This is right on the output of the 7809 so it would annoy the 7809 as much as a shorted coil.

I think I got the right pads circled on the right photo.  You should be able to pull the cap straight out by heating both pads together with the edge of your soldering iron.  If the short goes away, that sure would be an easier repair!!

Hmm, I just assumed it would be easy to find that relay but you're right, it didn't pop out from a basic google search.   But here's to hoping the cap is the problem; presumably you can buy the entire module from MTH but if it comes to that I think it is more satisfying (and less expensive) to find a relay that fits in the space with some wires added...if you can't find the exact part.  I mean it shouldn't be hard to find a 9V DC SPST relay...don't need the 10 Amp contacts either since it's just carrying the DC gearmotor current which is surely less than 1 Amp...

Mine was a limit switch on a similar car.

Err SELLS a kit to where you can activate with Lionel Cab 1 or Cab1L or is it Cab2L?

Joe

Joe

Trainlover160 posted:

Err SELLS a kit to where you can activate with Lionel Cab 1 or Cab1L or is it Cab2L?

The ERR stuff can be activated by any TMCC or Legacy system, so all of the above.

stan2004 posted:

Another thought.  It could be that the capacitor on the output of the 7809 shorted.  This is right on the output of the 7809 so it would annoy the 7809 as much as a shorted coil.

I think I got the right pads circled on the right photo.  You should be able to pull the cap straight out by heating both pads together with the edge of your soldering iron.  If the short goes away, that sure would be an easier repair!!

Hmm, I just assumed it would be easy to find that relay but you're right, it didn't pop out from a basic google search.   But here's to hoping the cap is the problem; presumably you can buy the entire module from MTH but if it comes to that I think it is more satisfying (and less expensive) to find a relay that fits in the space with some wires added...if you can't find the exact part.  I mean it shouldn't be hard to find a 9V DC SPST relay...don't need the 10 Amp contacts either since it's just carrying the DC gearmotor current which is surely less than 1 Amp...

Well, Releys do fall. But, I would really suspect something else....

I think Stan's top choice was the capacitor.  I'd be surprised if the relay coil shorted, that's an unusual failure mode.

SUCCESS!

While looking for a relay among my collection of such devices, I also thought that the problem would likely not be the relay. I have used hundreds of these over many years for work and home projects, mostly 12V and 24V coils, AC and DC, and have never had a coil fail. 

With that in mind, and before going any further, right after lunch yesterday, I removed the capacitor and tested the relay with the 9V battery. YES - I heard it click! So the relay is OK; well, at least the coil, that is.

The question now was the 7809. I remembered that Stan said, "If the 7809 is indeed seeing a shorted relay coil, it will get hot real quickly and go into thermal shutdown to protect itself." Now we know that it did not see a shorted coil, but a bad capacitor instead; but still, it should have "...shutdown to protect itself", so I soldered the 7809 back on.

Next was finding a replacement capacitor for the 220uf 16V failed cap. I have dozens of small electrolytics, but only one 220uf, and it was 25V, so slightly larger in physical size. It fit in the space, but it blocks one of the two holes for the screws that hold the PC board, but I did not deem to be a critical issue, so I soldered the cap in place.

And everything works fine now!

(I was concerned about putting everything back together, as I have it apart for several weeks, but the reefer went back together nicely).

Thank You, STAN, and everyone else who chimed in.

Alex

Ingeniero No1 posted:

...

And everything works fine now!

And zero out-of-pocket.  That's the kind of DIY story I like to hear! 

I think the most common failure mode for the operating reefer is a mis-aligned or bent-arm limit switch where the reefer runs all the time.  That's why I found your situation interesting. 

Anyway, my familiarity with how the board works goes to my DIY modification to remotely control of the reefer - I don't run TMCC so could not use the ERR module.  About $5 in parts documented here - in essence the wireless remote activates a transistor which simulates exactly what the existing relay does by directly applying the 9V DC motor voltage until the limit switch releases and completes the cycle.

And here she is in action.