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@TheSteve posted:

the output voltage on my tiu variable channels  is always half what i input.   any body have a fix for this?

Sorry, I kind of need more information to give you better detailed technical advice.

If you mean that you cannot turn your variable channel fully 100% off, that even when a remote or app says the variable channel is at 0, you are getting voltage out- that points to 1 or 2 failures. The variable channels use MOSFET transistors to switch on and off and vary the AC waveform, but also used a TVS diode (Transient Voltage Suppressor) to help protect the FETs. With derailments and other accidents that backfeed high voltage spikes into the TIU, it's possible one or more of the FETs have failed or the TVS diode specific to that variable channel in the FET section has failed and is conducting the AC waveform or some part of it out- resulting in a stuck on output condition.

Again, a couple of known possible component failures:

If the TVS diodes for each channel at the output fail (circled in red, left side of this picture) they short the output typically in failure. In other words, they short the incoming transformer and do not allow any power out of that channel.

If one of the two yellow circled TVS diodes fail for VAR1 or VAR2 channel, then that channel would have the "stuck on" output condition until that TVS burns completely open circuit under load.

It's also possible for one of the FETs (the row of vertical transistors with heatsink fins) in a channel to fail, either alone or in conjunction with the TVS of that channel also failing.

TIU TVS logic

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  • TIU TVS logic
Last edited by Vernon Barry

Again, yes, most likely, a failure in a variable channel is hopefully a repairable fault in a TIU.

If electronics is not your thing (understanding that people have different skill sets) it's recommended to send your TIU to a technician for repair rather than end up with more damage from a failed self repair. This forum has all the info for self repair, but again, recently, there was a situation where mistakes were made, and nearly cost that user the entire TIU. Given the insane used market prices of these things, and the fact the new WTIU doesn't support remote, it's probably best again to let a professional do the work if you just are not 100% sure and have the right soldering and desoldering equipment.

Also note that timing wise it is dead in the middle of the York show. A lot of the folks that do repairs and offer technical support answers are likely tied up with show activities at the moment. Just saying, if your next question is who to send it to, wait a little bit, maybe Saturday, Sunday, maybe even Monday as folks would be traveling and trying to relax.

Thanks for the reply I’m using a z4000 when I hit it with 20 V it puts out 10 V to my track through the TIU this only is happening on the two outer connectors variable one in and variable 2 in I’m thinking that it could be the mosfet transistors those are voltage regulating I ordered eight new ones to change them all and I’m gonna see if that helps. if you think it could be something else let me know so I could check its not the software because I’ve gone through it many times and reset it reset and reset. i’m thinking it’s a hardware issue, but if anybody out there has a known fix for this, I would appreciate it. it sure would help if there was a schematic and a parts list for the dcs TIU

@TheSteve posted:

Do you have a part number for the diodes I can just swap them out for new ones along with the mosfets which I already ordered

First, with no power to the TIU do the obvious thing and check the TVS with your meter set to Ohms or diode test.

I also would replace the components in that order first- the TVS diodes associated with the variable channel FIRST, before doing the FETs.

https://www.digikey.com/en/pro...inc/1-5KE51CA/688036

https://ogrforum.ogaugerr.com/...l-variable-2-failure

There are different TVS values.

For all 4 outputs, the ones circled in Red, those factory are 33CA.

While you can change those to higher values- that number represents the peak voltage rating the TVS is protecting against- lower= more protection. The tradeoff is, if you keep blowing them, then you can raise the value, but now they protect slightly less at a higher voltage reaching internal TIU components. So sure, 36CA, even 39CA you have seen referenced. Again, higher values means higher voltage before the TVS kicks in and shorts the max voltage.

The yellow circled ones in the variable circuit are 51CA and I would not use different values in that location.

More important, DO NOT touch the ones in the TIU logic power regulating section. Unlike the others which all end in CA- meaning bi-directional, they are very specifically one direction TVS. The diagram was just showing what part of the circuit they are in.

i ordered the ke 33ca this is the same number thats in there.

also i ordered irfz44nl mosfet for this mth dcs tiu.

also i have a second mth dcs tiu i going to replace the same two components because i have issues with this one also.

i have a question about the mosfet transistors one of the tiu uses the irfz 44n  but other tiu uses the           psmn 8ro.  are thease 2 mosfets  equal or is there a difference.   which one should i buy?

@TheSteve posted:

i ordered the ke 33ca this is the same number thats in there.

also i ordered irfz44nl mosfet for this mth dcs tiu.

also i have a second mth dcs tiu i going to replace the same two components because i have issues with this one also.

i have a question about the mosfet transistors one of the tiu uses the irfz 44n  but other tiu uses the           psmn 8ro.  are thease 2 mosfets  equal or is there a difference.   which one should i buy?

IRFZ44N

N-Channel 55 V 49A (Tc) 94W (Tc) Through Hole TO-220AB

PSMN8R0

N-Channel 40 V 77A (Tc) 86W (Tc) Through Hole TO-220AB

Bottom line, pretty sure they picked whatever one was cheaper in mass quantity that production run.

Specification comparison, just trading peak voltage for peak amperage at a lower voltage rating.

#1 they do the same purpose.

#2 they see the same exact source voltages well below the max rating

#3 They both are passing under 20A, hence the 20A fuse in the TIU circuit.

https://www.digikey.com.au/en/...il/nexperia-usa-inc/PSMN8R0-40PS-127/2122714 is $2.60 each

https://www.digikey.com.au/en/...fineon-technologies/IRFZ44NPBF/811772 is $1.59 each



FYI, the Z4000 uses insulated tab versions of the IRFZ44N and controls near 200 Watts per channel. Yes, a larger heatsink and fan, but again that's the transistor used in that system.







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