Overheating Bridge Rectifier on TAS SAW Board

if a full wave bridge rectifier gets hot it is because it is drawing too much current, you have to locate the short or whatever problem is causing the high excess current causing the rectifier to over heat.!

Alan

If it is the part between the power transisters, I had the same problem with a few. Too small. And if I remember, no way,(or very difficult) to repair or replace.

Thanks. I did a search but came up empty on any kind of wiring schematic for the board.  Seems likely that an upstream resistor has failed.  I will see if  can find identify the problem. I guess the worst case is that I can salvage the sound board and sound power board as well as the receiver board if I can't find the short and replace with an ERR upgrade.

I have carefully checked all wiring connections, making sure AC Hot and ground connections to p/up rollers are in good condition and not causing the short. Checked all peripheral connections, lighting, couplers etc, still can not isolate a short.  So I have to conclude its something on the board (TAS DC SAW Board).  I have continuity on all the resisters.  I'd really appreciate any helpful hints. The Bridge Rectifier is hot, but its not burning like it was the other day.  Is it worth trying to replace the BR.

I know I will probably have to upgrade but I really like to solve these issues if possible.

regards

Ralph D

Some years ago?....how long has it sat before you ran it this time? Alan is correct, if a rectifier is getting hot, the load pulling through it is high. Is it possible you have a mechanical issue increasing load consumption? Dried up grease, or something binding? Don’t be so quick to pluck out a component until you verify and diagnose the issue.....could be something simple not requiring major surgery.........Pat

Measure the current the locomotive is drawing from the track.  If that bridge is getting hot, chances are that something is drawing a lot of current.  The hot bridge is likely to be a symptom, not the failure.

If Ralph's problem is the same ones I have encountered, The rectifier or transistor got hot with no load, just bench testing. No load out of the board.

I'd be looking for shorted triacs in that case.

Harmonyards, the engine is regularly run  and the roller p/ups were replaced about a year ago. I removed both motors to check that ground and hot /p/up connections were properly insulating. Grease is fresh.  Its not a mechanical binding issue but thanks.

Gunner, thanks I am going to measure the draw at the rectifier later today.  Not sure where the triac is on the board but will try to check that as well. Is there a way I can test it with a multimeter?

Thank you

Ralph 

Just put this on a test bench and measure the power going to the locomotive.  If you don't have test rollers, just make sure the smoke is off and put it upside down and connect power/ground.  When you run it freewheeling, you shouldn't draw more than around half an amp.

Ralph, attached photo showing the Triacs. There are 4 of them. The triacs that came on the board are TECCOR Q4008L4 if my memory is correct. The bridge is a 8A 100V  GBU802. The bridge ran hot enough that on later boards TAS moved it to the bottom of the board and attached it to the heat-sink. I think they also bumped it up to 10A. TAS sold a version of the SAW which was rated at 15A the only parts different were the triacs and the bridge. The triacs were TECCOR  Q4015L5 might be L4 can't remember for sure.  I don't have the number on the bridge but would assume it is a 15A ~20A  100V.  This board was designed to be used in locos with 4 motors like an AA centipede or MTH F3AA with 4 motors.  I have installed  Q4015L5 triacs in a SAW and a 15A bridge and it seems to work correct on the bench but I have yet to install it in my centipede. A wild guess you could have one fried triac or one of the diodes in the bridge might be. Any 8-10A 100V bridge would work though unless you can find the exact triac that is on the board I would replace all four.      Good luck,        j

I really appreciate the input.  I did a freewheeling bench test.   With 18 volts from the transformer,  the engine seems to be drawing about 3.8 ma ( if I am reading this correctly).  Rectifier gets really hot.Johnacton, the pictures are helpful.  The bottom one with the BR on the topside is the one that I have.I am guessing that I did the upgrade about 10 years ago, so it may be an older board as you suggest. The engine is a two can motor SD 45, originally PS 1.

Like everyone else,  all my more recent TMCC upgrades are ERR.  I may use this one as a learning experience to see if I can replace the triacs.  One more question, is there a way to test the "diode in the bridge"? And just for clarification is the diode integral to the bridge or upstream/downstream in the circuit.  Thanks for all the help.

Well, there ain't no way that 3.8ma is heating that bridge, are you sure that's not 3.8 amps?

Since it's a PITA to remove the triacs individually due to the construction, I'd replace all four and the bridge.  What I do is reach under and cut the leads to all the triacs and then remove the leads from the PCB one at a time.  I then mount the triacs on the heatsink and form the leads to slide into the PCB.  Get the assembly all positioned with all the leads poking through, and then solder them all up.

FWIW, I'd also move the bridge to the bottom and bolt it to the heatsink as well.

Ralph,  replace the bridge with a 10A. The bridge is a group of four diodes and yes you can test it but as John says "just cut it out" and put a new 10A on the bottom and attach to the heat sink. Again as John says attach it to the heat sink then bend the leads to line up with the holes in the board. BTW.  before you cut out the bridge carefully bend it up enough to see which lead is marked +  and mark the board so you put the new one in correctly.          j

You know there's another option here...  "Band-aid"-ing the problem for sure, but consider:

I'm not an EE, but I can tell you that reconnecting your motors in SERIES instead of parallel will reduce your current consumption by half.  It'll also allow you to use more of the throttle range, and increase smoke output (if your loco is so equipped.)  Most folks are still happy with the top speed after this popular modification.  It's certainly easier to execute than changing components on the PCB's, and you'll get improved performance from your loco in the bargain.

Ted S posted:

You know there's another option here...  "Band-aid"-ing the problem for sure, but consider:

I'm not an EE, but I can tell you that reconnecting your motors in SERIES instead of parallel will reduce your current consumption by half.  It'll also allow you to use more of the throttle range, and increase smoke output (if your loco is so equipped.)  Most folks are still happy with the top speed after this popular modification.  It's certainly easier to execute than changing components on the PCB's, and you'll get improved performance from your loco in the bargain.

This works well for most locos and most layouts it does limit the max power output of the motors.  If you have a loco with twin motors that is capable of pulling 40 average freight cars on level track. This may limit the max number to 25 ~30.  Hardly a problem for most of us, me included.  However in this case I think that Ralph may have problem in his TAS SAW board or a motor gone bad. That is something none of us hit upon yet.  I think the first thing I would do is disconnect each motor from the harness and hook a DC source to each individually and see that they run approximately the same RPM on the same voltage. It is possible to scorch or have an open winding without completely killing the motor and it will run but pull a good bit more current.  So Ralph before you start pulling parts off your SAW board it is easy enough to make this check of your motors.  Sorry I did not mention this yesterday.  Once you have diagnosed and repaired your problem Teds suggestion does have merit.            j

There should be no reason to consider serial motor wiring, and it will only compromise performance.  The problem with series wiring is when either power truck loses traction, you go nowhere.  It's bad enough that happens with the truck with the speed control, no reason to make that problem worse. 

Things getting hot on the TAS board means something is broken.  John's suggestion to test the motors individually is a solid one, I totally spaced out on a separate verification of the motors.  I was kinda' coming in the back door on that with checking the total current draw on the bench.

Thanks all, but I am committed to replacing All 4 Triacs and Bridge rectifier. Parts are on the way and I have started to remover the triacs.  Decided I needed to upgrade my soldering kit as well.  

Great input and discussion and much appreciated.  I will return once repairs have been effected.

Ralphd

If you intend to replace the components on the board, it would make sense to first diag the problem, both GRJ and John Acton are trying to help you save some grief....you may be replacing components only to find out the problem still exists.....might not be the case, but confirmation that the motors are true to one another would be good first step to a thorough diagnosis....dart board diagnostics just leads to  frustration.........Pat 

GRJ, the original poster said this was a PS1 loco, and I didn't think the SAW board had speed control.  I know that two motors wired in series acts like an open differential.  But as John Acton said, most of these locos are such strong pullers, the risk of losing traction is acceptable for home layout-sized trains.  In most cases wiring the motors in series gives the loco slightly better slow-speed performance.  (The real solution would be a lower gear ratio and/or back-drivable gears, but we know that's never going to happen )

I wholeheartedly agree that he should determine and resolve the root cause of his component failure.

The TAS SAW board is a standard TMCC, that's true.  However, even so, I still never wire stuff in series for command locomotives, there's really no point.

SAW, was Sunset, Atlas, and Weaver, boards with licensed Lionel plug-in boards.  There was a SAW Gen2 board that had a lot of problems. Eventual the EOB (Engineer On Board)Boards, from TAS, (Train America, Studios, Mike Reagan, Youngstown, Ohio), solved most of the problems, and added speed control.   Today, we can do similar upgrades from ERR with Lionel approval.   One of TAS's first major achievements was a unique bottle board that fit the Atlas SW models.  Getting all that stuff, under an SW hood, was a major achievement, IMO.   Electro-couplers on a Switcher model, what a novel idea at the time.  

I had an EOB board damage both can motors in an Atlas  model.  I thought the engine was off, but was in one of the low speed steps, not moving.  Smelled bad, was hot to touch.  Motors never did work right again.  I replaced both can motors, EOB board was fine.  SAW boards were not as robust as the EOB boards. IMO.  

One of the unusual characteristics of the EOB boards was a non-linear acceleration pattern.  Once you got uses to the ramp-up and ramp-down, it was a pretty good experience. 

Had sound, electro-couplers, head and rear lights,  no gingerbread lighting. 

I didn't like EOB at all, the one example I had, I gutted.  Noisy and too hard to control with the vague knob on the CAB-1.

One reason I suggest wiring the motors in series, is the original TMCC has only 32 speed steps.  In certain cases there's a big jump in voltage between steps.  We can't do anything about the magnitude of the voltage change or the shape of the curve.  But wiring the motors in series means that the loco isn't as sensitive to changes in voltage.  So the speed change between steps won't be as noticeable.

I encourage the original poster to try the series wiring.  If the loco is too slow or prone to slipping it's easy enough to change back to parallel.

Ralph D posted:

Thanks all, but I am committed to replacing All 4 Triacs and Bridge rectifier. Parts are on the way and I have started to remover the triacs.  Decided I needed to upgrade my soldering kit as well.  

Great input and discussion and much appreciated.  I will return once repairs have been effected.

Ralphd

 

Ralph sorry we came in with a solution that did not consider the motors first which would have been the easiest diagnosis.  Please realize that we all keep up with the forum from a third eye and have a life outside the forum so the obvious can sometimes fly over our collective heads. I am afraid you cut the triacs and heatsink off the board so you could get the part number and there is no going back. If not DON'T.  You'll need the parts you ordered eventually but for now it is easy enough to test the motors. Even if you eventually install new parts you don't want to try and power up a bad motor with the board. If you do test them mark them so you get the wires back on the correct side.  I bought a kit of 8 colored sharpie like pens at the dollar store just for this task. While your getting up to speed order yourself an anti-static mat and grounding bracelet. Their cheap on eBay and some sellers have US warehouses if your in a hurry for a couple more $.   Make sure the soldering iron is ESD (electrostatic discharge) safe.  NEVER use a soldering gun on modern electronics.  They are induction devices and the tip usually has a potential to ground over 30VAC.         j

Another point I just thought of.  If you unplug the motors from the board and hook leads to one motor their parallel and both will run.  You can prop the chassis up or lay it on it's side so the wheels and flywheels can turn freely.  I use an old LGB transformer when I need variable DC and they dial down to very nearly O vdc most any good HO powerpack would be acceptable.  Most of these will also dial down to nearly 0 volts. This is useful in that one of the motors will almost certainly start running first.  This is OK however you want the second one to start in less than a volt more. If it doesn't, with the lightest touch you can muster, turn the flywheel on the motor which started first to feel the level of resistance then turn the flywheel on the reluctant motor and make a mental comparison of the effort. It should not take significantly more effort. If it does start looking for mechanical problems. And don't forget the SNIFF test sniff the holes near the top of the motor for any hint of burn. After you have determined that both motors turn equally free yet one starts at more than 1V more disconnect one lead from the reluctant motor and hook up your VOM set to read amps and take a look at the current draw it should free wheel with no load under 1amp at 12v  If both motors match and test OK your back to the board.           j

Thank you.  I do have a variable voltage DC transformer so I will test the motors.  Don't think that is the problem though.  I will try to verify.

Again, input is appreciated. Testing the motors will be important because my plan B is to invest in a new ERR  cruise commander and sound system.

RalphD

So here is my update.  I did test the motors with a variable DC power source (a small HO transformer).

When I tested the motors in tandem I got the same labored performance that I was getting with the board in place. I then tested each motor separately  and got a labored and sluggish performance from the front motor  and apparently normal performance with significantly greater RPM from the rear motor.  So it appears that the front motor may be the source of the problem, but no smell of burning.

I guess the good news is that I think I have two timken can motors that might work and as of now I have only removed one Triac and the Bridge rectifier, so it shouldn't be too difficult to get the TAS SawBoard back in order when the parts arrive.

I do need to test the motors again after I remove them from their respective trucks to see if there is any change in performance.  There was an earlier post from Harmonyards that suggested a possible mechanical /binding problem that would increase the load.  I thought I had checked , but never disengaged the motors from the trucks since they both seemed to be exhibiting strained performance.

Regardless of the final outcome, this is a pretty good learning experience.  The saga continues...

t 

Ralph, with the motors off the trucks spin the flywheels with the lightest touch you  can and compaire the difference. If one is a bit tighter than the other pull it up and down against the can and the shaft should move in and out of the can freely about 1mm no more than two.  With the motor running slow look at it from the side do you see any wobble from the flywheel ?  The solution may be as simple as a drop of oil on the motor shaft at the bearings.   The worms are a bixxxxch to remove and just as bad to press back on. If you have a VOM run the motors 30 sec or so to remove oxidation from the commutator. Then check the static resistance of both motors they should both read about 3-3.5 ohms for a twin motor MTH loco. Spin the shaft and test 3-4 times for an average.   You can find MTH motors on eBay cheap. Make sure to buy two that came from the same loco.  ask the seller up front about this.  Don't buy two motors that came from single motor locos.  While it's apart clean and relube the trucks. I don't know what the chinese use for lube but have noticed gum deposits on the shaft where it exits the bearing. Common 20 or 30 weight motor oil is excellent on the motor bearings it is gum free I have loaded a pin point oiler with it and used it on motor bearings for years it doesn't gum up.          j

Those motors should only draw a couple hundred milliamps running free.

What John Acton said... not just any motor will work.  If it turns out that one of yours is bad, you'll need to find one with the correct worm gear to match the worm wheel in your SD45's truck block.  You should try MTH first, because otherwise you'll be playing a guessing game on the aftermarket about which ones have the same worm gear as your SD45.

Ted, I totally forgot about the different length of the worms on the MTH Mabuchi motors used in diesels. Luckly they are easily identified.   I have to state I have not seen every MTH motor and the worm on them but I have seen a bunch and the only difference I find on the ones I've seen is the length of the worm. The worm wheels they mate with are all the same diameter and pitch. Of those I have seen the motors used in the 3 axle trucks have the long worm. Two axle trucks have the short worm.  Ralph make us a photo of your motors while out and lets see the worm. I have not seen the ones in SD45s.  Attached photos of GP-9 truck and motor and a Alco DL-110 truck and motor. I see the point in buying new motors however if I had to re-motor a 5 year old loco no doubt I would put new motors in it. But Ralphs SD45 is over twenty years old and if I could get a couple of used guaranteed motors for the price of one new one, or less, I'd likely do so.          j

Timko has the ability to install your existing worm on a new motor.  

In many cases, but he will tell you that occasionally they are a bugger to get off.

Thanks,  unfortunately I have to set this project aside on the bench. Its model RR month and I have an open house on Sunday, so I am shifting focus to getting ready for that with some other relliable motive power.  I did not have a chance to test the amp draw but when I tested the motor(s) independent of the trucks they both seemed to run ok.  There seemed to be resistance in the front truck, so that is my next target.

So just to close the book on this one,  I ended up replacing the triacs and bridge rectifier mounting it on the underside of the board so it sits on the heat sync. Unfortunately I don't think the board survived my surgical intervention.  When I reinstalled and checked all connections, the lights come on but the engine is otherwise dead in the water.  So for now this engine is a shelf queen.  I will check the motor truck that was the original source of the resistance.  If I can eliminate the resistance with a thorough cleaning,  I will probably upgrade to a newer ERR cruise commander after the holidays.

Ralphd

Bummer, you'd have thought all of those parts would have fixed it.

So I have a different conclusion than anticipated.  Having upgraded several Williams engines to TMCC I kept several electronic e units including some true blast boards.  Since I run a couple of conventional loops with TPC's I decided to use what I have on hand.  So this SD 45 started out as a proto , became a TMCC engine with TAS boards and now it runs great as a Williams True Blast and is back on line hauling freight consist and running great.  Its not command but heck...

cheers

Ralph D