Confused by multimeter measurements of Tech II DC power packs

[Maletrain]

My club gets donations of and either uses of sells many MRC Tech II DC (and similar) power pack throttles.  I usually check them out a bit before deciding what to do with them.  (Some are not working at all and go to trash.)

A while back, we noticed that one of the ones used on our N-Trak modules was leaving lights in some passenger coaches still visibly lighted when the throttle was turned down to zero.  That prompted me to check that one with my multimeter, and I found that the DC voltage did go to zero, but that there was still a 3 volt AC voltage on the rails with the throttle at zero.  That seems to have (unintentionally) provided "constant lighting" for the LEDs in those coaches.  We set that Tech II aside.

Fast forward to yesterday afternoon, I had 5 donated throttles to test - 3 MRC Tech II and 2 Bachmann Spectrum.  And, I had not been able to bring my usual multimeter, so used another member's different meter.  That is where I got some readings that confuse me.

Other than the one (now trashed) Tech II that put out zero DC and zero AC volts on the variable output, while still supplying 17.5 volts on the Accessory AC output - the other 4 acted the same, as follows:

Their variable DC outputs worked smoothly from zero to full throttle with their direction switches set in both directions.  Max voltages ranged from about 17 to about 19 volts open circuit.  So far, so good.

When I checked them for AC voltage with the throttle turned down to zero, no AC voltage in either direction.  (Well, maybe a couple of millivolts.) Good again.

Now the confusing part:  When I turned the throttles up from zero to full, I did get AC voltage (open circuit) that went up smoothly to max values of 30-to-40 volts AC, with the throttle's direction switch in one direction, but zero AC volts over the whole throttle range when the direction switch was in the other position.  Switching the leads to the multimeter did not change which direction of the throttle switch resulted in AC voltage readings.

So, it looks to me like the throttles are all behaving differently from one direction to the other direction.

But, I am not familiar with that member's multimeter.  After coming home, I used my customary multimeter to take similar measurements on my MRC Tech 7 DC throttle, and did not see the same effect.  Both the DC and AC measurements on my Tech & showed the same behavior in both directions.  The AC readings on the Tech 7 with my own meter varied from 0 to about 8 volts.  (DC readings run smoothly from zero to 15.5 volts.)

So, my question for the folks here on TRW is what is likely causing the other member's meter to show different AC behavior on the variable DC output when the direction switch is moved between the two directions?

I  am thinking that the measure AC on the variable DC output is actually some sort of ripple effect that may even be intentional to boost low speed motor performance.  But, I don't understand why, or how, that would occur in one direction but not the other. 

[peteski]

That is weird couple of ways.  First of all the direction with is a simple DPDT switch (in all the throttles I have ever dealt with) which wired to flip the polarity of the DC output.  Very simple. No electronics involved  Like reversing the wires on the DC output terminals.

The AC output viable with throttle turned down can be some leakage of the "pulse" circuit in the throttle.  3VAC is fairly high. If you put a DC engine on the track you should hear the motor buzzing.

[Maletrain]

Yes, in the previous case with the 3 VAC at zero throttle, you could hear the motor buzzing - if you made the effort to listen for it.  The background din of the sound locos on 3 adjacent layouts pretty much drowns out subtle sounds.

I agree that the throttle reversing slide switches should not do anything more than swap which wire on the output gets + and which one gets - .  That is why I tried switching the leads between the outputs and the multimeter.  I was thinking the multimeter was having a problem - and maybe it is.  But, the issue did not follow the multimeter input - it was definitely following the throttle direction switch.  And it did the the same way for 4 different DC power packs - 2 Tech IIs and 2 Spectrums.

Remember, these are open circuit measurements, so there may be some sort of leakage with very little current capability.  Also, the 2 meters may have very different frequency responses, and I have no idea what frequencies these AC readings are coming from.

I should also mention that the AC plugs going to the wall outlets are position controlled with a wider prong for one side.  So, no swapping of the input hot and neutral was possible.

I may find out more in a week when I can get my usual meter to the Club and repeat some of these measurements.

[peteski]

Open up the case and trace the reversing switch wiring (or traces if it is soldered to the PC board). IIRC, MRC throttles use some sort of security screws.

[Point353]

My club gets donations of and either uses of sells many MRC Tech II DC (and similar) power pack throttles.  I usually check them out a bit before deciding what to do with them.  (Some are not working at all and go to trash.)

There aren't so many electronic components - mainly the semiconductors and capacitors - inside those Tech II power packs that you couldn't simply replace all of them and likely restore the pack's function, at a cost far below that of a new pack. Shame to see any of them disposed of if they're otherwise in good condition. 

[Maletrain]

Peteski, I don't have any Tech IIs at home, but will dissect the the dead one next week at the Club.

Point323, I don't have the time to waste fixing other people's junk.  If I am going to build a DC throttle, it won't be like a Tech II.  (One on the workbench now, but under several other half-finished projects).  Pulse-width control is the way to go.  Another member is working on a walk-around version for the Club's N-Trak layouts.

[mmagliaro]

In answer to Peteski's question, I have some photos of the insides of a Tech II 2500, and I can see that the direction switch is nothing but a DPDT that comes from the main throttle board, goes through the usual cross-wired DPDT switch, and then goes to the DC track terminals on the case.

Maybe this is a silly question, since you would probably have already done this if you had a scope, but...
did you put these on an oscilloscope?  That's really the only way to know what's coming out of there.
DVM's, unless you have a high end one, are rigged to measure AC voltage "RMS" at 60 Hz.  Pulse
patterns in the DC can fool the meter's AC circuit into showing something that is meaningless.  When you flip
the direction switch, that might change how the meter's AC circuit is measuring an AC voltage, just by flipping the
polarity of the pulses, but it still doesn't mean anything.
I have had a few Tech II's here to fix for other people, but I don't have any of my own.  As I recall, the pulses are
low frequency, so they might be 60 Hz.

If you don't care about paying for postage, feel free to mail me one of these puzzlers, and I can scope it and  see if anything is really misbehaving.

------
Okay, this piqued my curiosity.  A cursory web search revealed that this symptom is not uncommon in the 2500 (leaking the pulse voltage at zero throttle).  You could try two things.  1. There are some trimmers on the circuit board.  Try slightly turning the one nearest the 4 slide switches, just a bit one way or the other,  and see if the leak goes away.  I have no idea what it is, because I don't have a schematic.  But it could be a pulse amplitude adjustment.  It's plausible that given its age, that is out of adjustment.  2. Alternatively, you could try replacing the main power transistor in case it has gone leaky in its old age.

[$2 Bill]

MRC Tech 2 & others the Model(s) Matter when talking about output to trains because 1 model can be way different volts etc to another @ same % of Throttle. Even when any Brand say "DC" to Trains, can't trust that as they may have same "Features" as MRC products.

Why?
Any Power Packs with "Pulse Power" like Tech2 1400 for the trains will easily make meter on DC ranges see "weird" volts @ low to 0 % Throttle. This "Feature" is Not only MRC but from other Brands. I forget what German maker also use "Pulse Power."
Tech 2 2500 & other PWM output "Hates" meters on DC ranges because output is a "Square wave" 0-max that Change the Duty Cycle as % Throttle changes. If have a "Smart" Meter, look for Duty Cycle often labeled as % next to Hz/Freq & watch output with a train or SMALL Tungsten light bulb to load the pack. Other Brands or DIY may use PWM too. 194 Bayonet Bulb is safe for most HO & smaller & has wires to connect to whatever.

More read all What's inside those Power "Bricks?" https://www.therailwire.net/forum/index.php?topic=60146.0

LED...
Use LED In Trains, more so when have "Keep alive" caps, the LEDs will Light @ Very Low or 0 % Throttle on either Type because easily pushes power to light them. But even the "Dumbest" DC packs with Nothing but Transformer, Rectifiers & "High watt" Potentiometer can Turn On LED @ 100% @ very low % Throttle depend just how the LED are wired. If, likely, have 1 to many LED in Parallel & resistor to set Current, the LED can be On @ 100% when % Throttle just pushes > ~ 3V.

LED are Not Like any type Tungsten Bulbs even when the the LED are in "Dimable" setups that uses way more parts.
"Normal" & Halogen Bulbs react from 0V to whatever Max Volts often in a Linear way & often you see happen fast for low volt bulbs.
LED is 0% @ 0 to < V Turn On OR 100% @ Turn On or any Higher Volts. "Dim" by mainly Controlling the Current to the LED or by PWM that Turn full On & Off fast to Fake Dim.

[Doug G.]

There are two different versions of the 2500 but I have forgotten what the exact differences are but one was touchier about the AC voltage for pulse zeroing out at minimum throttle.

As Max said, there is a trimmer to adjust that and also to adjust the maximum output.

Doug

[Maletrain]

I got the chance to use my own multimeter on those same DC power packs last evening, and the results were not the same.

With my meter, there was no difference between the two positions of the direction switch on the power packs (nether the MRC Tech IIs nor the Bachman Spectrums) for the AC readings on the variable DC outputs.

Measuring the AC on the variable DC outputs did not show the steadily increasing AC as the DC was increased.  Changing the throttle knob position would show a small AC voltage that dies away quickly when the knob is still.  The other guy's meter showed a steady AC reading that was proportional to the DC voltage and maxed out at 30 to 40 VAC at full-open throttle (but only with the direction slider in one of its two positions).

Because they all did the same with my meter, I am calling them all "good enough"  (except the one that has zero voltage on the variable DC output no matter what the throttle setting).

So, mystery not solved, but not worth the effort to investigate further.

[Doug G.]

On the Tech IIs, the pulse fades away as the DC voltage (throttle) is increased. This is to aid slow speed performance while avoiding excessive motor heating as locomotive speed increases and pulse is unnecessary.

Doug

[$2 Bill]

Measuring the AC on the variable DC outputs did not show the steadily increasing AC as the DC was increased.  Changing the throttle knob position would show a small AC voltage that dies away quickly when the knob is still.  The other guy's meter showed a steady AC reading that was proportional to the DC voltage and maxed out at 30 to 40 VAC at full-open throttle (but only with the direction slider in one of its two positions).

Because they all did the same with my meter, I am calling them all "good enough"  (except the one that has zero voltage on the variable DC output no matter what the throttle setting).

So, mystery not solved, but not worth the effort to investigate further.

Should Never see > 15-18 VDC or VAC on DC Train Outputs on Tech2 & other HO packs.
 
"30 to 40 VAC" may mean the unit has Shorted out somewhere inside but have enough resistance to Not trip the Breaker but the Trains will Hate you.

[$2 Bill]

On the Tech IIs, the pulse fades away as the DC voltage (throttle) is increased. This is to aid slow speed performance while avoiding excessive motor heating as locomotive speed increases and pulse is unnecessary.

Generally True but also depends on exact model.

Many models not just MRC units the Pulses are @ max volts all the time just > 0% Throttle. Some like T2 1400 have proportional pulse that go slowly higher then "DC" volts. Others like T2 1500 have better electronics with "pure" DC. All change the shape on "DC" to the trains @ < 60 to 90% Throttle.

Non Filtered DC & Pulses do "Weird" things on Meters on DC ranges & AC ranges because meters see "RMS" volts. Even with "True RMS" on new "Smart" meters can look "weird" depending on exact setup. "RMS" "True RMS" is quoted because many things are done to get those values. Example: Analog meters "RMS" often assumes Sine Wave < 90-100 Hz for common 50/60Hz power because of mechanical, electric or both. Just changing the Shape &/or Hz can make the value change a little or way more. Old Digit Meters often have same issues. New "True RMS" meters still Digitize the input then try whatever new formula to display a Volts for any Shape input but still have issues on odd shaped signal/volts.

Note that Later versions of Tech power packs often have little or no differences on Tech 2 in design but often just switch what parts are made from. Examples: At some point might change from Thru Hole parts to Surface Mount, Change from Glass to PTC(PPTC) Auto Breakers, Or Nothing change electrically but new case design & all MRC just slap a new name.