Does anyone know what would cause a TIU to produce a constant DC offset when using the variable channels? On both variable channels I get the following positive DC offsets as the voltage increases. At full throttle, the offset disappears. Horns & whistles on all equipment (PW and modern) blow constantly through the full range of voltages unless I am at full throttle. If I press the whistle or bell buttons, about 2 volts will be added or subtracted from these values respectively, as expected. Just wondering what would cause this offset from equilibrium? 

Does the TIU control variable voltages in a similar way to the Lionel Powermaster? If this were a Powermaster I could envision that the partial failure of a negative phase FET might be involved in this unbalance. If something similar is possible on the TIU, I wonder which components in the TIU should be replaced?

AC Voltage      DC Offset Voltage

5.0                 +2.2 
10.0               +3.2
15.0               +3.4
19.0               +3.0 
20.0               +0.1 
20.5               +0.4
21.0               +0.7
21.5               +0.65
22.0               -0.03 (negligible offset)
           

Original Post

I can't imagine a TVS doing this, it's wired directly across the power.  If it were acting like a diode, it would quickly overload the FET voltage control and likely cook them. 

If one FET fails, you can get a DC offset, I just fixed a pair of TIU's with that exact problem.  I replace all four as you can't really do a test in circuit of the FET, and since I have to remove them anyway, it's easier to chop them out and remove the leads individually.  The parts are only a buck, so saving them isn't worth the labor.

If a FET has failed, I am not sure how you can have full voltage control over the VAR channel unless it is some partial failure/short.  You can diode check the fets, when one is shorted you will see it.  Usually (almost always) only one fet fails.  G

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All I can say is I had one FET fail and replacing all of them resolved the oddball DC output indication.  I agree it's usually one, but it's much easier to remove them one leg at a time, so I just chop them and pull the legs individually.  You can't really test them in circuit, so it's pretty hard to determine which one, hence my technique of replacing them all.

Actually you can test them in pairs, and I know the one that typically fails 80% of the time.  So I remove it first.  For me, I bridge the legs with solder on the back of the board, leave heat sink on, and pull the fet out as  unit.  Works great.  Then retest.  Confirm the Fet pulled has shorted.  Replace.   G

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Interesting leaving the heatsink on, that saves a little time.  I've only done maybe a dozen of these so far, so I haven't "polished" the technique.  I did note that they seemed to be in pairs, that helps.  Which one fails most of the time, useful information to have.   If I knew that, I could pull the pair and replace, could save time.

gunrunnerjohn posted:

I can't imagine a TVS doing this...

But it seems reasonable to assume that the purpose of the TVS is to protect the TIU electronics.  So if a TVS fails (for example from repeated spikes from the track), then the previously protected FETs would be exposed to those spikes and possibly fail.

It appears the 10-cent FETs (free shipping from Asia) are now 16-cents.  

irfz44n ebay

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That's certainly true Stan, if the TVS fails open, you'll never know.  Perhaps it's a good idea to replace the TVS whenever you replace the variable channel FETs.

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