DC Throttle Recommendations

[mmagliaro]

Okay, I see now.  Well, that could be the speed pot, the direction switch, or both, sounds like to me.
I think those "Allen" screws on that thing have the security nub in the center, so you might have to do what I've done with those -
You can drill out the center pin and then try an Allen wrench in there, but it's easier to just use a Dremel cut-off
disk to slice a slot right across the head of the thing and unscrew it with a flat-blade screwdriver, and then replace them all with regular screws when you put it back together.

[peteski]

Okay, I see now.  Well, that could be the speet pot, the direction switch, or both, sounds like to me.
I think those "Allen" screws on that thing have the security nub in the center, so you might have to do what I've done with those -
You can drill out the center pin and then try an Allen wrench in there, but it's easier to just use a Dremel cut-off
disk to slice a slot right across the head of the thing and unscrew it with a flat-blade screwdriver, and then replace them all with regular screws when you put it back together.

Those could also be Torx screws, but Allen wrenches can be used on them.  Cutting a slot is a good idea, but if they are recessed then that will not work.
I picked up an inexpensive ($10) set of security screw bits at Harbor Freight. That way I can deal with those pesky screws without mutilating them.  I used another trick on Torx security screws before picking those bits up:  I have a very narrow flat head screwdriver.  I was able to wedge it between the nub in the center of the screw, and 2 lobes in the "star" shape opening and loosen the screw that way.

https://www.harborfreight.com/security-bit-set-with-case-100-pc-68457.html
These work in most instances, except when the screws are deeply recessed.

[mmagliaro]

Peteski is right.  His remarks reminded me of two things:  1 they are Torx screws, and  2: they are recessed, so you can't slice them with a Dremel like I've done on other cases.  I drilled the center pin out of these buggers and then used a Torx screwdriver.
And once they are out, I still heartily recommend tossing them in the trash and using regular screws to replace them!

[mmagliaro]

If this helps...

A friend of mine is sending me his MRC 760 because it has suddenly gone "hog wild" with full output voltage at all throttle settings.
Either the speed pot is broken or disconnected, or possibly the main  output transistor is blown.  Either way, the diagnostics and repairs will be very similar to what's going on with yours.  So I will photograph it as I go and post in here, since the photos might be of use to you as
you fix your 260.

AND...
I found some posts online with photos of the insides of an MRC 260.
I slapped them together into this humongous photo:





1. Speed pot is a 2k ohm linear taper
2. Main power transistor (if you have to replace it) is a Sanyo D1230, NPN Darlington.  An NTE 2541 is listed as "equivalent", and honestly, a TIP 142 ought to work in there.  It has plenty of current and power dissipation for a lil' ol' N Scale throttle, and I seriously doubt that throttle needs an actual gain of 4000 (which the Sanyo part is rated at).   
3. The reversing switch is a standard 6-lug slide switch soldered to the board, and although it has metal ears for screw mounting,
I don't believe they are use on the MRC packs.  So that switch can flex back and forth a wee bit every time it is thrown.  If you do manage to get it open, double check and resolder the 6 lugs on the reversing switch to make sure a hairline crack hasn't developed in the solder joints on the board.

My second thing would be to unsolder the main speed pot, the use an ohmmeter between the center lug and one of the outside lugs, and make sure the resistance sweeps smoothly between 0 and 2000 as you turn the dial.  Then measure between the center lug and other outside lug for the same thing.  Make sure there are no spots where it drops out to infinite ohms (or goes to zero).

[mmagliaro]

Finished fixing up my friend's Tech 7 pack.  It was the pot, as I suspected.  I did do video of the whole thing, so I am going to slap it together into a quick YouTube movie so folks can see how I did it.  The Tech 7 has security screws that are the plain slotted screw with the nub in the center of the slot.  So I used my handy dandy flat-blade with a notch filed out of it to get them out.

I disassembled the bad pot and interestingly, it was not the wiper or the wiper surface that was bad.  It was the 3 terminals.  They are crimped onto the pot circuit board and that crimp is the only thing making contact (you cannot solder to the "painted on" conductive coating on the pot, so the crimp has to do it).  So it wasn't wear and tear on the shaft, wiggling back and forth deforming the wiper, or corrosion or dirt on the wiper surface... nothing like that.    I tried squeezing the terminal crimps to see if that would make them work, but to no avail.  I ended up cleaning the 3 areas and then coating them with silver-based conductive epoxy.  That DID work, and the old pot has a smooth scale from 0 - 2k ohm now.  But I don't trust that over time that epoxy wouldn't develop a crack or separate from the board, so I just replaced the pot anyway.

Anyway... in a day or two I hope to get this video done and then I'll post in here.  I really think this is the same problem in your 260.

[peteski]

Good job Max!
Some better-quality pots which use phenolic resistor board have their terminals riveted to the board (those are more robust than just crimped ones).
Relatively low resistance pots (like 2k) can also come as wirewound pots.  Those are very reliable, but also expensive.

[mmagliaro]

Alrighty, the tutorial video is up:
https://www.youtube.com/watch?v=k1L8LPqffe8

I talk too fast... LOL, but I think people can still follow along. 

If you are already versed in this type of basic repair, you'll be bored silly.  But I think it might save a few power packs out there
for people who can handle a soldering iron and basic hand tools, but are a little intimidated about opening one of these up.

[reinhardtjh]

Very well done, Max.  I don't think you talked too fast.

[videobruce]

I've been a big fan of the Varipulse throttles.
http://www3.sympatico.ca/kstapleton3/Index.html

I have two of his supplies built into one unit. A dual cab version, one with a wired remote hand held controller. It needs a good source of 12vdc of which I bought two supplies and removed the upper half of the enclosure to save space, but mostly aid in cooling.

The entire enclosure does have a exhaust fan and it also has a volt & amp meter for the non-tethered throttle.

The case use to hold two old school very heavy xformers which turned it into a huge paper weight and power hog. It used as much electricity at idle as it did running trains so I looked to upgrade. His PCB's at mounted at the rear each side of the fan. The (formerly) enclosed PS's are stacked and cable tied at the bottom. The intake vent holes are on the bottom.

[mmagliaro]

I have two of his supplies built into one unit. A dual cab version, one with a wired remote hand held controller. It needs a good source of 12vdc of which I bought two supplies and removed the upper half of the enclosure to save space, but mostly aid in cooling.

The entire enclosure does have a exhaust fan and it also has a volt & amp meter for the non-tethered throttle.

The case use to hold two old school very heavy xformers which turned it into a huge paper weight and power hog. It used as much electricity at idle as it did running trains so I looked to upgrade. His PCB's at mounted at the rear each side of the fan. The (formerly) enclosed PS's are stacked and cable tied at the bottom. The intake vent holes are on the bottom.

I have a question about your box (nice work, by the way).
Is that fan really necessary?  Were the throttle boards making heat to the extent that you needed a fan?
I ask because being PWM circuits, I would have thought you would get very little heat off them.  I could see
having some vent holes, but a fan?

[peteski]

Good point Max. PWM type of voltage regulation/control is very efficient, with very little heat dissipating on the output transistors. That is why DCC decoders use PWM for motor control.

Might be one of those "belt and suspenders" things where a bit of over-engineering won't hurt.

[mmagliaro]

Good point Max. PWM type of voltage regulation/control is very efficient, with very little heat dissipating on the output transistors. That is why DCC decoders use PWM for motor control.

Might be one of those "belt and suspenders" things where a bit of over-engineering won't hurt.

The ironic thing is that that board has a full-wave bridge rectifier and an LM-type voltage regulator on it.
Depending on what your input voltage is, and your current draw, you will get some serious heat off those, and they are not
attached to the heat sink!

[peteski]

The ironic thing is that that board has a full-wave bridge rectifier and an LM-type voltage regulator on it.
Depending on what your input voltage is, and your current draw, you will get some serious heat off those, and they are not
attached to the heat sink!

Yes, the analog voltage regulator can generate a fair amount of heat. Old school power supply section meets new school variable voltage throttle section.

[videobruce]

mmagliaro;
As to my version of this dual supply, the fan was a just in case. I was more concerned about the 12v supply packs piggybacked on top of one another then his PWM's.

peteski;
Those 12v supplies piggybacked are not "old school" if that is what you were referring to. Both are switching supplies. 

[videobruce]

Come to think of it, that fan was a left over from the old school analog xformers based dual supply I updated to this version.