Using Fixed Input 2 with FI1 dead

There are no FETs in the fixed channels AFAIK.  The fixed voltage goes right through to the outputs.  The only thing in the path is the relay to cut off power if you hit the red button.  The FETs are in the variable channels, that's the line-up in the center of the board.

 

There isn't a whole lot in the the path of the power in the fixed channel, are you SURE it's inside the TIU that you have the short?

I think John is right on with his info.

To answer your question however, you can supply aux power to the TIU through it's aux power input and then run your layout using Fixed #2.

 

27 VAC seems unbelievably high for a brick output.

Are you sure your meter is accurate?

Do you have another AC power source you can try?

 

Rod

27 volts is high, my Z750 output is only 21 volts, which I believe is what the manual states it should be.

When you measure these unloaded with many meters, you get wild readings.  Try a load on the transformer and see what you get.

Okay, I guess it makes sense that there's not a FET in the fixed path since it doesn't need to be varied. The only reason I think it's in the TIU is that with the TIU disconnected from absolutely everything, FI1 measures 0.5 Ohms or so, FI2 is (IIRC) 5 MOhms, and the VIs are around 15 MOhms. 

 

I'm pretty sure my meter is accurate. It's an Agilent digital, and pretty new. I have previously checked it against Flukes at my old job.

 

Bob, when you measured 21V, was that plugged into anything, or just measuring at the connector? I agree, 27V seems high for a unit that says its output is 24V!

 

I guess I'm worried now about blowing out the other FI, but I can find an appropriate brick for the Aux in, power the thing back up again and quickly measure the loaded voltage at FI2.

 

Thanks!

 

Fred

There is a TVS diode on the output of the TIU channels, sounds like one of those may have shorted.  It's about the only thing I can think of that could be dead across the fixed outputs.  Even if the relay shorted, it would just inhibit the shutdown function.

As an additional data point, I plugged the brick into FI2 (6MOhm load), and still measures 26.77VAC. :-/

 

Fred

6 megs isn't a "load", it's a feather!   I'm talking about a LOAD, like an amp or more.

 

I'm wondering if for some odd reason the brick is really putting out too much voltage, that would tend to kill the TVS diode.

Have you tried plugging the brick into Fixed 1 with nothin connected to the outputs?

If there is a short in the TIU it will still trip the brick.

 

Also if the 27 volts is too high for the TVS, it should just short it out and immediately trip the brick's breaker, but it should not stay shorted out with no input.

 

Rod

Okay, that's true. :-) 

 

I even cracked open the brick to see if there were alternate taps that might have been hooked up accidentally.  

 

I'll rummage around. I'm pretty sure I have some monster-sized 50 Ohm resistors around that I could measure with. That would at least be half an amp. I don't want to wreck another channel!

 

Fred

 

I did try plugging in to FI1 with nothing else attached, and it does trip the breaker right away. 

 

I also just checked the bricks output across a 20 Ohm resistor (just north of an amp, correct?) and the voltage only drops to 26.33VAC.

 

Measuring resistance across the inputs with nothing attached to the TIU:

 

FI1 0.3 Ohms

FI2 12 MOhms (keeps going up, so probably a cap in there somewhere?)

VI1, VI2 3 MOhms and climbing

 

Similar order-of-magnitude measurment on the outputs, including the shorted FO1.

 

I would love it if I was just being dumb and forgetting to do something very basic. Thanks for everybody's help and patience on this.

I made it through the DCC transition okay, so I was hoping this would go smoothly too!

 

Fred

Okay, I found an appropriate Aux power brick, and fired up FI2. Seems to work, so no big harm done. However, it also happens that a Z750 brick just went up in the sale forum as well, and the seller measured the voltage at 21VAC, so I think I'll buy that to be on the safe side. Another crisis averted!

 

Fred

" I just  received my first DCS system in the mail"

Was this a new or used unit? If new, I would get it repaired under warranty.

If used, I think you solved your problem by getting one channel to work.

Just a suggestion re; the warranty. Greg

I'd replace the TVS as a start, because that seems to be the most likely suspect.

Fred,

   It really sounds like you are going to have to bite the bullet and return it.

PCRR/Dave

When you send back the TIU include the brick. Maybe they can do something to turn down the voltage.

 

You could turn down the voltage on the brick by using diodes in a string. I know it cuts down voltage. Not sure what it does to the wave.

I'm still having trouble with the brick putting out 27 volts.  The brick is just a transformer, so I don't see how it deviates much from the standard 21 volts they normally put out.

There is an above post where there are 2 generations of 750 bricks. 1 put out 24 volts and the 2nd generation 21 volts with a polarized plug. With that said I have trouble with this being so far out of spec.

Here is a link to a Diode voltage dropping thread. I believe there are more threads on the subject.

 

https://ogrforum.com/d...ent/2415514336733692

If you use 6A bridge rectifiers, you could wire three of them in as voltage droppers to get you back to sanity with that transformer.

 

Of course, you could also cut your losses and invest $75-80 in a Lionel PowerHouse 180 and have plenty of power and a true 18 volts output.

 

Excellent! My thinking about the tied diodes seems to be correct. Thanks for the link.

 

 

Quite right. As noted above, I've purchased a 21V Z750 from a forum member, so just waiting for that to arrive.

 

I'm new to O scale, and have been having trouble keeping the power supplies straight. This whole thing started when my Dad gave me his pre-war Lionel and Marx tinplate, and I was struggling to make it run with the included transformers. I started with a used Z750 (with controller of course), and that really worked well, so I assumed I'd be home free when I added an MTH GP38/PS1 to the mix. And it did work nicely.

 

Along the way, I also procured a Lionel Polar Express set, which includes the CW-80. I thought that would be a nice backup. 

 

So, when I decided to upgrade the GP38 to PS2, imagine my surprise to find out that neither transformer seems well-equipped to work with the TIU!

 

I'll keep the Powerhouse 180 in mind. That indeed looks like a good choice. Thanks for the input.

 

Fred

I have three of the PH180 supplies, they are rock solid and on my power of 124 volts they deliver around 18.4 volts with a 1A load.  With an 8A load they drop to about 16.8, still pretty acceptable.

Bear the bummer & get it fixed under warranty.  Well down the road, you'll kick yourself for not doing so when you want to use that channel.

I'd certainly get it fixed, no sense in having a warranty item that doesn't work properly.

If memory serves me correct there is service bulletin on a over voltage condition where you have open it up and replace some components. I don't have a service manual in front of me so somebody else might chime in and verify this.

I'd be surprised if it's not the TVS diode, as he's hit it's voltage limit.  A continuous voltage applied at the protection limit of the TVS will eventually cook it.

Fred,

 

I measured 21 volts on the track with an engine running round the layout.

 

Check out an earlier post I made (man, times flies) and see if this doesn't sound like your problem with the TIU:

 

https://ogrforum.com/d.../lastReply#lastReply

 

I'm currently using VAR2 OUT, both FIXED sides are good, only VAR1 OUT is dead.

I recently replaced my Z750 brick with a 180 Powerhouse...all is well as far as voltage concerns (still no VAR1 OUT ).

Send it back to MTH to get repaired.

 

Interesting thread. I'm pretty sure that's not it, as I have a short, not an open (if I read your post correctly).

 

Yes, I will, although I just got the second Z750 in the mail (21V) and my loco is now humming happily on FO2. I also picked up some 24V TVSs at the local electronics supply house. I'm almost tempted to lift just one lead of the possibly-failed TVS and see if it's the cause of the short.

 

Fred

Since the TVS diodes are on the output, I'd use at least 27 volt ones, no reason to have it that close to the possible input voltages.

 

I'll bet money that it's the TVS, sounds like the exact component that would cause this.  I looked at one of my TIU's when I had the case open, and there is very little in the fixed channels other than the relay and that TVS.

They didn't have a huge selection, mostly in the 5V range, so I just picked up the 24V just in case. I have yet to scrabble through my junk box to find some hi-capacity diodes for bring the other 750 into a reasonable range, but no hurry there. 

That's the reason I order them on-line, tons of selection and exactly the ones I want.