Our club now has three MRC throttles which don’t “shut off” when the throttle is set to “zero”. Two are pulse power throttles and one is a dual throttle 50 watter which I don’t think is pulse power.
Any ideas as to why the “drift”? Are they “fixable”. MRC was no help…they no longer have any bench technicians on board…couldn’t find anthing on the web. The 50 watter has no “trim pots” anywhere…I haven’t opened the 12 and 18 watt pulse power units.
BTW…these are “old” units…at least 15 years old…probably older.
We will need a bit more information. What voltage do you measure at zero throttle? Do you have a load on the power packs when you check the voltage? Is the Momentum switch on or off? What are the MRC model numbers?
What kind of meter are you using or how are you determining there is voltage with the throttle at zero.
The big ones put out a pulse even on stop, it was enough to drive a constant lighting circuit, but no one’s locos ever moved The regular type, I used to have a Tech II 1500 and none of my locos ever kept moving at stop, and I hav e a Tech IV 220 or something like that which I use for DC testing, again my locos all stop when the knob is turned all the way to stop. Both use pulse power.
If they are older, I’m going to suspect a bad capacitor. At most there are probably only a couple of electrolytic capcitors in those. If any are bulged or oozing goo they are bad and need to be replaced. They don;t always leak though and can still be bad. Replace with equal value, same or higher voltage rating. Both numbers are printed ont he capacitor itself.
My Tech II 2500 has about a volt or two at a zero potentiometer setting. I do have a load in the circuit (a Proto BL2)…Momentum was off. The motor was “buzzing” at the zero potentiometer setting. On the 2500 I was unable to move the recessed screws holding the bottom and top halves of the pack together. Looks like MRC didn’t want anyone to get into the box. Probably a UL requirement. If someone thought it was “fixable” I would drill out those chassis screws to open up the chassis.
On my other “creeper” (currently at the club-an older, 50 watt, two pot pack-no pulsing) I was able to open the chassis up. I don’t see any electrolytic caps, there is a “disc” cap. I will bring that one home to my test bench when I return from a trip (4 weeks).
Curiously, another tech II creeper (tested years ago by another club member) tested out aok on my bench.
My old MRC Amppak can be opened but these Tech IIs seem to have been built so as to discourage opening up the chassis halves.
as far as I know, there is not a variable screw to adjust zero position or voltage … All there is inside that is replaceable are the caps, bad ones can easily be seen to have ‘black gunk’ coming out of swelled ends …
Mind you the best test is unsoldering one end and using a specialized capacitor tester on them, not what the average user has on hand though …
They also have ‘safety’ screws holding them together, either a specialzed socket set, or just somehow manage to get them out, and replace with normal screws …
As most of you know how much I like to draw on my CAD I’m really tempted to take my 2500 apart and make a schematic drawing. My Arthritis is in high gear and I don’t think I could handle the mechanics side of it. Both shoulders are easily a 10 and that severely dings my mobility.
Redvdub because your 2500 has problems I’m game to work with you to fix it as well as making a drawing to help others with problems.
All you should need is a #10 Torx, a soldering iron, a cheapo meter and a camera to post pictures. You do the mechanical work and post the pictures and I’ll make a schematic drawing and beween the two of us we’ll “getter done”.
thanks Mel…I’ve enjoyed your blog for years and have learned a lot from it. I have just ordered a tork wrench set with a #10 so I should receive that in a few days. I return from a trip on Fri. and will get to work opening the box immediately.
I have never posted pictures before but I see that some else has posted a photo of the opened 2500…and there are two trim pots…maybe they will allow us to reset the zero point.
I did get a rampmeter hooked up and it seems that the 1400 and 2500 Tech IIs are not “pulse power” …my Tech II 2400 is “pulse power” (it even has an on-off switch). The 1400 and 2500 do have “momentum” on-off switches…the 2400 does not.
Hopefully, we can fix this by next weekend. If not…I will take my stuff with me (soldering gun, meters, the 2500, etc.) when I go away for another 3 weeks.
It does seem that others have this “creep” problem also and hopefully this thread will help them out also.
Again…thanks for your generous offer to help. BTW…we are of the same generation. I was born in the last year of “peace”.
The “Year of Peace”. I guess that puts you about a year behind me, and you still travel. My only traveling is to my doctors offices. My last venture traveling was in 2008. My old buddy Arthritis won’t let me go very far, a long jaunt for me is the mailbox. If it doesn’t hurt then it died and fell off.
I wouldn’t tweak either pots until we figure what they do. If you are a knob tweaker mark them where they are first so you can put them back. In my 50 year working career in electronics I learned that tweakers can be dangerous and generally not deterred.
The 1500 and 2500 definitely use some form of pulse, they aren’t pure DC, no matter what the rrampmeter says. The 2400 uses classic tap off the mid point of the rectifier 60Hz pulse, the 1500 and 2500 are a bit more sophisticated and the pulse effect tapers off as the throttle is increased. If I still had my 1500 I’d put it on the scope to see the output waveform. No idea if my Tech IV uses the same design or something different, I could test that one and see what it looks like.
I got my Tork set but they didn’t fit into the 6 screws…I think I may have needed a “security tork” with a hole in the bit to accomodate a pin that sticks up from the screw “slot”??? Anyway…I’ll get a set but it will now have to wait till I return from a 3 week trip…
I have a “Tektron” tool kit I got from Amazon for not a whole lot of money that has all the common and alsoo the oddball security type bits, to take pretty much anything that has some sort of removable fastener apart. Actually I somehow ended up with two of them, so I have one at my electroonics bench and one for modeling.
Well…I’m back. Got my security torks and opened up the MRC2500 (see below). The residual voltage at the zero pot setting was about 3 volts (as measured with a cheap multimeter). I also opened up my MRC1500 which has no residual voltage (see photo below). From what I read it seems as if the culprit is probably the TIP121 Darlington transistor (the three leaded device with the attached heat sink hanging loose on the right side of the photos). If that Darlington is shorted full voltage (20V) will be put across the output terminals no matter the pot setting…I imagine if it is just “leaky” the residual voltage will be something less than full voltage. However, I am also suspicious of that pot. BTW…how is the “pulse” created…I don’t see any oscillator module. Thanks for any help.
it uses the 60 Hz from the unrectified transformer output.
the question is how does it time the width of the pulse?
an RC circuit could be used. It needs to be sync’d (reset) with each 60 Hz zero crossing (???) and while it charges or discharges using the pot, the output transistor is on. But there needs to some sort of comparator.
at least one of the large electrolytic caps is a filter cap connected across the rectifier (20V?). you might see a saw tooth on the other electrolytic cap.
the speed pot could divide the sawtooth voltage that might drive a smaller transistor. when the divided down voltage exceeds ~0.7V the small transistor conducts, turning off the darlington by pulling it’s base voltage close enough to zero (the two transistors in the darlington require ~1.4V at its base to conduct).
what do you mean shorted? the base connected to the collector?
is this at the output or base? if output, with the pot at either extreme, is the base zero (< 1.4) V? Tip 121 datasheet
what are those two other pots for?
is that large square high wattage 0.33 Ohm resistor in the output path and used for current limiting?
This might not apply but over time an electrolytic can develop high equivalent series resistance and look normal, Happened to my Tek 485 scope many years ago. It was getting flaky. Just a thought.
I took apart on old MRC throttle just to see how it worked. Probably was as old as what you have maybe older. One I took apart had Selenium rectifiers so that might date its age. Hard to tell tho when manufactures started using Silicon diodes to replace Selenium.
I’ve found when the voltage don’t go to Zero you can drop the voltage by adding a diode in the circuit before the reversing switch. Each diode you add will drop the voltage 0.7 volts. 2 in series will drop the voltage 1.4 volts. Make sure the diode had the capacity to handle the highest current the throttle will put out.
The MRC throttle I have is a model 801 and probably a lot older than yours. The 801 had no circuit boards, and used Sleenium rectifiers, everything was hard wired to terminal strips.
The pulse throttle puts out half wave unfiltered (no capacitors in that curcuit) rectifier current while the full throttle setting puts out full wave rectifier (two diode) current which has one capacitor. The 801 has 3 throttles all separate but all are tapped off the single transforer for throttle. There is a separate transformer for the 18 VAC output and a third transforer for a 12 VAC output. Both AC outputs are for accessories.
I built a pulse throttle for my HO scale trains. Its a different kind of pulse from the MRC half wave rectifier. I used a circuit out of Model Railroader magazine January 1981 in column Symposium on Electronics. Its a Pulse Width Modulated using a NE555 timer to vary the with of the pulse output to drive a LM350 IC for the output. Used pretty much the same circuit only added a few more filter capacitors in the power supply voltages, and changed the fixed output voltage from 12 volts to 15 volts. The 15 VDC remains on the track at all times, it just the duration of how long the pulse is that controls the motor in the engines.
