Problems with TIU VAR2

Your post is confusing. Please explain what power supplies and/or transformers you have connected to what tracks. You connected the grounds together of ???

Sounds like out of phase transformers, but not enough info yet.

This can't actually be the TIU, it's impossible for it to increase the voltage like that.  John H has the answer I suspect, transformer phasing.

When you apply power to VAR 2 input only do you get power out?  Can you control VAR 2 output?  Ie turn it off?  If not you have a blown Power Transistor on VAR2.  G 

Check 2 things:

• As suggested above, ensure that the power sources feeding variable 1 and variable 2 are in phase with each other,
• Make sure that the center rails of the variable 1 tracks are center rail isolated from those of the variable 2 tracks.

My setup is two transformers a Z-4000 and a ZW. The transformers are in phase if my meter is right. I get .07 volts when I check the two hot terminals. The ZW is supplying power to VAR1 and VAR2.  The Z-4000 is supplying power to Fixed 1 and 2.  The center rails of VAR 1 and 2 are isolated. I get no continuity when I check with my continuity tester.  I believe GGG has the answer.  Power can be applied to VAR2 but I cannot cut power to VAR2 using the DCS remote. Set to 0.0 the power will not turn off when the wheel is pushed. I have to turn the handle down on the ZW to accomplish that.  Is there any way I could replace the blown Power Transistor on VAR2 myself if some supplies the parts or do I need to send it in?

I don't know what is your skill level.  I can do this repair if you need.  E-mail in profile, G

I am suspicious of your phasing check.  Reading .07 volts is just too low.  Did you have the Common terminals of both transformers tied together when you tested?  Describe your test procedure.

If you had a meter that was reading 40 volts, something is dreadfully wrong.  If you don't sort this out, you will probably have similar problems after you get the TIU working again.

Power can be applied to VAR2 but I cannot cut power to VAR2 using the DCS remote. Set to 0.0 the power will not turn off when the wheel is pushed. I have to turn the handle down on the ZW to accomplish that.

Delete the TIU Track for variable #2 and recreate it.

I have the commons connected to the one another to support Legacy. I then set one handle to on each transformer to approximately 18 Volts. Then took a wire and connected it to the ZW hot terminal and then connected one lead from the meter to the wire and the other to the hot terminal on the Z-4000.  I took two readings one said 1.1 Volts and the other 1.0 Volts. I may have misread the original reading it could have been .7 Volts.

What voltmeter are you using?  Or if you measure, say, a 9V battery with your meter in AC voltage mode do you read 0 or 18? 

If your VAR2 failed in a way that is only passes DC to its output, this might explain your 40V AC reading.  That is, some AC voltmeters only perform a DC voltage reading measuring only one half of the AC voltage.  It doubles the measurement and shows the 2x value on the readout....hence it show 18V AC when measuring a 9V DC battery!

As G says, a TIU VAR channel (the ones I've seen) use a set of four transistors rather than a single-component "triac" to accomplish voltage control.  You have to be quite savvy to troubleshoot this design with just a voltmeter so I think most guys wishing to roll-the-dice would just swap all 4.  They are not expensive - maybe $1-2 each or even less on eBay (for inquiring minds, the part numbers are clearly marked) but then again it might not be the problem either.  I recommend sending it to G.

I tried the reset of VAR2 but it didn't work.  When I check the voltage in it says 18 Volts but the out put is in the 30 to 40 or more Volts.

I'm using a 9 Volt Meter and it does double the output if you measure with DC with the AC setting.  I guess the transistor is blown. 

maxmn_98 posted:

I tried the reset of VAR2 but it didn't work.  When I check the voltage in it says 18 Volts but the out put is in the 30 to 40 or more Volts.

Is that with nothing else connected to Var 2 out?     Try it again with nothing connected (track or commons)

Gregg,

I don't understand. If nothing is connected to the common then the circuit is not complete, you would get no reading at all.  I did delete VAR2 and renamed it A. I also reset the TIU.  The same result.

Ok you have 18 volts on the input side of Var 2. (good)    You also should have  roughly 18 volts in var mode with the voltage scrolled   on the output  side.   There also should be 18  volts on the output side  if the var 2 is set to fixed mode.   You don't need any track connected to the output  to measure the voltage.  I'm just trying to determine whether the var channel is working as it should or is something else jacking up the voltage.

Menu/System/Dcs set up/   The FXD changes the var channel  from var mode to fixed mode with each press.

Gregg,

I tried what you said to convert VAR2 to fixed mode and I still got the high voltage readings. Again the input side was set to 18Volts.

With everything else shut down, can you vary the voltage output of VAR 2 using the remote?  If not, but you can vary VAR 1, repair is needed.  I just had a TIU repaired by GGG:  he did it quickly and charged a fair price.

I'm right up the road from you, just across the Occoquan.  Be glad to help you check it out.

RJR,

If you want to discuss this further contact me in my email in my profile.

A lot of folks have jumped in and are mudding the water.  Worrying about the accuracy of the meter, and such.  Or re doing test already done.   If you ONLY apply power to VAR channel and you get an output, one or more of the transistors have shorted.  Period. 

This is not the first time folks have reported an abnormally high output on a VAR channel when it has a transistor short.  G

GGG posted:

A lot of folks have jumped in and are mudding the water.  Worrying about the accuracy of the meter, and such.  Or re doing test already done.   If you ONLY apply power to VAR channel and you get an output, one or more of the transistors have shorted.  Period. 

This is not the first time folks have reported an abnormally high output on a VAR channel when it has a transistor short.  G

Well maybe so but  most of don't have the Electronic "no how"  you have. Should we quit posting?  I don't think so because it's not always hardware failure,  sometimes operator error.  All here are experienced with the remote's capabilities.

In this case send the TIU to GGG.  

Gregg,  Once the test is done and results posted, why should he repeat it if the result indicates a hardware failure?  I agree not all are hardware failures, but this one sure looks like it.  G

GGG is spot on  We had the same problem on many Rev L TIU's  Its a bad power FET on the variable channel  It will read 40 volts out   If you put a load on it it will drop below 10 volts 

It's very interesting you get such a high voltage reading, I'd like to figure what is going on there.  I suspect something must be acting like a voltage doubler with the shorted FET, an interesting failure mode.

I had a failure in a Rev L that precluded any   control of output voltage via the remote, but didn't observe any high voltage.

If GGG gets this for repair, hopefully he'll report his reading of output voltage.

gunrunnerjohn posted:

It's very interesting you get such a high voltage reading, I'd like to figure what is going on there.  I suspect something must be acting like a voltage doubler with the shorted FET, an interesting failure mode.

Yes, the voltmeter is acting as a voltage doubler!  I'm pretty sure the actual voltage at the VAR2 output is about 20V DC.  His voltmeter is apparently of the type that makes AC voltage readings by measuring just the DC voltage and doubling it.

stan2004 posted:

gunrunnerjohn posted:

It's very interesting you get such a high voltage reading, I'd like to figure what is going on there.  I suspect something must be acting like a voltage doubler with the shorted FET, an interesting failure mode.

Yes, the voltmeter is acting as a voltage doubler!  I'm pretty sure the actual voltage at the VAR2 output is about 20V DC.  His voltmeter is apparently of the type that makes AC voltage readings by measuring just the DC voltage and doubling it.

Yup-

As I have said before, unless you really, truly understand the test equipment you are using and the circuit you are measuring, meters just cause a lot of grief. Even if you understand them there is plenty of opportunity for trouble.

Hooking a 14 or 18 volt incandescent bulb to the output and watching it respond is a perfectly valid test for problems of this nature, IMO. AND, an incandescent bulb reads out in true RMS! 

BUT.....the actual output voltage doesn't matter.  After he explained how he phased transformer (correctly), and the fact that VAR2 pass voltage when powered alone and with no control, points right at the FETs 95% of the time.  The other is when the TVS fails shorted.  And not at the output.  Rather across the FETs.  G

 George is absolutely correct   We had 8 TIU's with the same problem   They went back to MTH Development and they couldn't explain it   If he put a bulb across the output he wouldn't read more than 10 volts 

GGG posted:

BUT.....the actual output voltage doesn't matter.  After he explained how he phased transformer (correctly), and the fact that VAR2 pass voltage when powered alone and with no control, points right at the FETs 95% of the time.  The other is when the TVS fails shorted.  And not at the output.  Rather across the FETs.  G

Yes - I am agreeing with you - If a bulb on the output stays lit, even dimly, when the control is off, there is a problem. The 40 volt reading on a particular meter is a measurement anomaly and probably not useful for troubleshooting.

Stan, why would voltage at the output be DC?  If it were, wouldn't all whistles on that channel be blowing?

When my FET in a Rev L shorted, everything ran normally (only command mode used).  Since my Variable channels are set to fixed, no telling how long then situation was there until I noticed I couldn't kill power to that channel with the remote.

Interestingly, I have 2 15+/- year old Rev G TIUs, and none have had this problem.  I guess that there is a supplier-related problem????????

Luckily we have GGG available to do this kind of repair, and who has the components in stock.

RJR posted:

Stan, why would voltage at the output be DC?  If it were, wouldn't all whistles on that channel be blowing?

When my FET in a Rev L shorted, everything ran normally (only command mode used).  Since my Variable channels are set to fixed, no telling how long then situation was there until I noticed I couldn't kill power to that channel with the remote.

...

The voltage at the output might be DC because it's busted!  As to whether whistles would blow depends.  That is, as pointed out earlier, one failure mode is where you read some large voltage with nothing connected but as soon as you load it, the large voltage drops to something relatively small...perhaps so small you can't even power an engine in which case you'd obviously hear no audio much less a whistle.

There are 4 power transistors (FETs) that are used to vary the output voltage in a TIU VAR channel.  Transistors exhibit different failure modes such as shorted or open.  And when you have 4 of them turning on and off to chop an AC waveform into another AC waveform, you don't have to imagine very hard that if one (or more) transistors fail, the AC output waveform may no longer "alternate" (i.e., AC = Alternating Current) and you end up with what is effectively a DC waveform.

As to whether a voltmeter or light bulb or whatever are useful troubleshooting tools is a matter of opinion.   I used to test 9V batteries by putting it on my tongue and feeling the tingle; your mileage may vary.  Is a tongue a useful electrical troubleshooting tool? 

Note that the OP in his initial post asked if there was a way to fix this without having to send it in.  The way the conversation has evolved, I'd say he should just send it in to G.