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.
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.
--Randy
Modeling the Reading Railroad in the 1950's
Visit my web site at www.readingeastpenn.com for construction updates, DCC Info, and more.
My MRC Tech 2 2500 does the same thing. It is really annoying when running locos that start up a lower voltage.
I hope someone knows the fix!
Regards, Isaac
I model my railroad and you model yours! I model my way and you model yours!
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 ..
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".
Redvdub1 BTW..we are of the same generation. I was born in the last year of "peace".
BTW..we are of the same generation. I was born in the last year of "peace".
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.
Redvdub1My Tech II 2500 has about a volt or two at a zero potentiometer setting.
From Question about currently manufactured DC Power Packs from April 2018 (http://cs.trains.com/mrr/f/744/p/269380/3053917.aspx)
gregc gregc i happened to see a product review for the MRC Tech II models 1400/1500/2400/2500 in RMC, i believe march april 1980. It showed scope traces. Those traces showed rectified AC. A switch on some of the models controlled whether it was half or full wave rectified. the may 1980 RMC completed the review of the 1500/2500. It had described the 1500/2500 as having momentum/brake and the 2400/2500 as higher powered versions of 1400/1500. but the 2500 uses PWM unlike the other 3. The review showed scope traces for the 1500/2500 at 4 throttle settings. PWM pulses are ~16 msec apart, which is the period of 60 Hz. I could see some 60Hz ripple riding on top of the wider PWM pulses. the article discusses motor heating as a disadvantage of PWM, which it probably is at 60 Hz. I believe modern day decoders use processors that can generate PWM pulse at 200 kHz.
gregc i happened to see a product review for the MRC Tech II models 1400/1500/2400/2500 in RMC, i believe march april 1980. It showed scope traces. Those traces showed rectified AC. A switch on some of the models controlled whether it was half or full wave rectified.
the may 1980 RMC completed the review of the 1500/2500. It had described the 1500/2500 as having momentum/brake and the 2400/2500 as higher powered versions of 1400/1500.
but the 2500 uses PWM unlike the other 3. The review showed scope traces for the 1500/2500 at 4 throttle settings. PWM pulses are ~16 msec apart, which is the period of 60 Hz. I could see some 60Hz ripple riding on top of the wider PWM pulses.
the article discusses motor heating as a disadvantage of PWM, which it probably is at 60 Hz. I believe modern day decoders use processors that can generate PWM pulse at 200 kHz.
greg - Philadelphia & Reading / Reading
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.
MRC2500 circuit board underside
MRC 1500 circuit board underside
Redvdub1BTW...how is the "pulse" created...I don't see any oscillator module.
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).
Redvdub1If that Darlington is shorted full voltage (20V) will be put across the output terminals no matter the pot setting
what do you mean shorted? the base connected to the collector?
Redvdub1The residual voltage at the zero pot setting was about 3 volts
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.
Rich
If you ever fall over in public, pick yourself up and say “sorry it’s been a while since I inhabited a body.” And just walk away.
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.
I would suspect the cap or other component before I'd suspect the main power transistor, especially if the voltage varies normally except that it won't go totally to zero at the stop position. If it were full voltage no matter what, or no voltage no matter what, then I'd consider that transistor.
gregcts base to conduct). Redvdub1If that Darlington is shorted full voltage (20V) will be put across the output terminals no matter the pot setting what do you mean shorted? the base connected to the collector? From "34 Electronic Projects for Model Railroaders"-1982-Peter J. Thorne-pg11 Sympton-Throttle runs flat out all the time" Cure-.....check for short circuited transistor so I would assume that "shorted Tx means collector to emitter short? And I would think that if the emitter is shorted to the collector so is the base. BTW..I have an MRC 6200 with a residual voltage of 15 volts at the zero pot setting (as measured at the DC output terminals) and full voltage (18.5V) at the max pot setting. Redvdub1The residual voltage at the zero pot setting was about 3 volts is this at the output or base? if output, with the pot at either extreme, is the base zero (< 1.4) V? I measured the residual voltage at the DC output terminals. So far I have not tried to probe inside the "box" with power on...am awaiting expert advice on that one. 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?
From "34 Electronic Projects for Model Railroaders"-1982-Peter J. Thorne-pg11
Sympton-Throttle runs flat out all the time"
Cure-.....check for short circuited transistor
so I would assume that "shorted Tx means collector to emitter short? And I would think that if the emitter is shorted to the collector so is the base. BTW..I have an MRC 6200 with a residual voltage of 15 volts at the zero pot setting (as measured at the DC output terminals) and full voltage (18.5V) at the max pot setting.
Redvdub1The residual voltage at the zero pot setting was about 3 volts is this at the output or base? if output, with the pot at either extreme, is the base zero (< 1.4) V?
I measured the residual voltage at the DC output terminals. So far I have not tried to probe inside the "box" with power on...am awaiting expert advice on that one.
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?
I don't know what those two trim pots on the 2500 are for..nor do I know what that large resistor is for. I don't see either on the 1500..
the following schematic shows an transistor used as a voltage regulator, commonly used in transistor throttles. The emitter (E) is the ouput of the throttle. For a Darlington transistor, the emitter voltage will be ~1.4V less than whatever votlage is at the base (B). The transistor will supply as much current as possible to maintain that voltage.
i misunderstood your previous comment about shorted transistor. If there is a residual voltage at the output (E), i'm curious to know what the voltage is at the base of the transistor. If residual output is 3V and the transistor is not damaged (i.e. "leaky"), then I would expect to see ~4.4V (3V + 1.4) at the base, that the transistor is OK and something else has failed.
since you opened the unit up and asked about how the pulse is generated, i assume you're interested in better understanding how the circuit works. With the negative lead of the voltmeter connected to one of the output terminals, can you measure the voltages on components? (you may need to toggle the reversing switch).
while writing this, i got a response from MRC asking for a schematic.
We do not give out schematics for our products. The Tech II line has been discontinued for a very long time now. Could you describe the problem?
the output voltage is ~3V when the throttle is set to zero.
If you can turn up the throttle and have the 2500 operate normally, with the exception of the 3 volts at zero, then that isn't a matter of a repair. There are two trimmer pots on the circuit board which need to be adjusted. To make the adjustments you need to use an Oscilloscope and make the adjustments to three different spots on the speed dial. Unfortunately anyone who worked on those units in the past are no longer working here today. We will not be able to offer any further assistance with this unit.
I can try to make a few voltage measurements on the Darlington with my multimeter. I don't have a scope. "Three different points on the speed dial (Pot?)" ....zero, midpoint, max? I'm still suspicious of that pot...can't find it's specs...
I don't think you will be able to get it totally adjusted without a scope, since you are trying to adjust a PWM wave, and a multimeter isn't going to help much there. The procedure they mentioned in resposne to Greg's inquiry (amazing how much info they DO have on products they actually designed and engineered - they can't tell me what horn type I get for a given CV value in their sound decoders, but even though no one involved with the design of the Tech II line works there any more, they do have information on calibrating it) is a pretty straightforward typical sort of thing, although without specific details as to the waveform you shoudl get, it's goign to be a lot of guesswork. You're goign to have a specific amplitude and diuration output with the throttle on stop, a much longer duration at full throttle, and ideally exactly half that at 50% throttle. So you tune the circuit at each extreme and then adjust the center to be as close to center as possible without messing up the low end or the high end.
Unless someone messed with it, those pots won't turn themselves. So something through it out of "tune" - most likely a bad cap. If you have no way to test the leakage and ESR, jsut repalce the electrolytics. A test of value isn't enough, although if the value tests way off, it's definitely bad - it's just that the value may come up within tolerance but it still has high leakage. If your meter has a capacitor mode, unsolder one leg and give it a test. If the value is way off, definitely bad. If the value reads good, within tolerance to the value printed on the cap itself, it may or may not be good, but it's already halfway out so eay to swap in a new one of the same value.
I mean - if you really want to fix it - I bought a Tech II 1500 some 40 years ago, and they were already out for a few years when I got mine. Is it worth fixing? A few dolalrs in capacitors and some time isn;t much to spend, but much beyond that, it's probably not worth the time and effort, and replacing other parts willy-nilly in a shotgun approach is rather pointless, you could just use those parts and build your own equivalent. Electrolytic capacitors are known to fail over time, so they are an easy target. And a drifting value can easily explain the behavior you are seeing.
to all..I removed the circuit board from the heat sink and let the Darlington TIP 121 and the Pot "hang free". I then used my cheap little multimeter to probe both the pot and the Darlington at "full off" and "full on". I also probed the variable and fixed DC output terminals at the same time. I found the results reasonable and unexpected.
Unexpected: In the open and "hanging loose" scenario as described above there
was NO residual voltage at the variable DC output terminals at the
"full off" condition". I will next reassemble the supply and see if
the "residual voltage at full off" returns.
Note: The white tab on the pot is the middle tab of the three tabs...yellow, white, and "resistor" tabs.
At the "full off" pot setting.
1. There was 10 volts between the yellow wire tab and the
and resistor tab and 10 volts volts between the white wire tab
and the resistor tab. There was zero volts between the yellow
wire tab and the white wire tab. The ~10V reading matched
the ~10 volt reading on the constant DC voltage outputs.
2. At the "full off" pot setting there were no voltage readings
on any of the Darlington leads (i.e. colector, base, and emitter)
At the "full on" pot setting:
1. There was 20 volts across the variable DC outputs
2. There was 10 volts across the yellow-white pot tabs
3. There was 10 volts across the yellow to resistor tabs
4. There was zero volts across the white to resistor tab
5. There were small 1.5 to 3.5 volt readings across the Darlington c-b-e leads). I think this means that at the "full on" settingthe Darlington was conducting which seems to make sense because when the Darlington is full on there should be full voltage across the output variable DC output terminals (and there is).
I'll be away for the next 4 days and when I return I will repeat the above measurements and then start "reassembly". Thanks to all for your comments and suggestions. I appreciate the help.
I reassembled the MRC 2500 and all is well..no residual voltage at the zero pot setting...go figure. Upon disassembly the Darlington Tx was literally "submerged" in heat sink grease but I would be surprised if that was causing a problem. Something was causing "leakage" at the zero pot setting. There's a lot of flux residue visible on the circuit board...if that was "no clean" flux that shouldn't be a problem ...if that was an acidic flux that might result in dendrite formation between closely spaced lines...but I don't have the equipment at home to even look into that. I did scrub down the PC board with isopropyl before reassembling "just in case".
Thanks to everyone for your comments. It would be good to get a circuit diagram for these TECH II throttles but that's a big above my pay grade. On to my defective MRC 6200.
Redvdub1Upon disassembly the Darlington Tx was literally "submerged" in heat sink grease but I would be surprised if that was causing a problem. Something was causing "leakage" at the zero pot setting.
when i took an auto shop class, the instructor said that dirt on top of the battery is conductive. He put a voltmeter on ground and the other on the top of the battery and it showed a not insignificant voltage.
maybe that grease is semi conductive or contaminated with enough dirt and stuff that it was shorting to the base resulting in a slight bias.
Redvdub1Our club now has three MRC throttles which don't "shut off" when the throttle is set to "zero".
It all depends on the motor and the circuit and the motor involved. I had one locomotive that never stopped moving once it started. Welll, never is a long tim, but wat was explained to me was "Back EMF"
ROAR
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