Hi all, I have a 6646 and a 6627 and I'd like to use the old blue trafo with the new small scale layout for sentimental reasons. I just purchased a 60760 kit to upgrade my Delta 3000 so that it may fare better with the inclines on my layout. I tried finding an older 60901 etc kit and they don't seem to be reliably available.

I've read that blue trafos and digital decoders don't get along. I've also read you have to watch out for the reverse pulse output voltage, and to keep in mind the voltage rating of the trafo. I checked mine out and it reads 120v on the face plate, and outputs about 18v at maximum speed and 27.5v with the reverse pulse.

I've tried researching the manufacture suggested voltage limits but they don't seem to be available. I've read a bunch of posts on here from some knowledgable people, some of whom suggested a pair of antiparallel diodes in series with the red wire going to the track.

Is it necessary to wire up the diodes given the voltage readings of my trafo? If so which should I use? I have a few on hand, mostly silicon diodes from my other hobby - guitar effects pedal building.

Thank you all

Hi Brian
I am a big fan of old Märklin and trafos of that period go along with those.
Old transformers (except those pre-war units) are OK BUT:
- the metal casing is not on par with safety regulations and they should have the frame grounded
- the power cord can be dangerous (e.g.: a rubber-based insulated cable from the early 50s desintegrates) and need replacing.
- the old transformers are huge, have a very large iron core but are very poor with regards to efficiency. The result is when running with no-load, the voltage can be quite high. This is quite fine with electromechanical Märklin but very bad with recent electronics (Delta or digital decoders).

What is the limit to use or not to use a blue transformer?
Simple: measure the voltage between B and O studs when the direction change is activated permanently at no load. It should read 25 VAC or below.
A higher value means that the peak voltage is above 40 V peak which is above the "absolute maximum ratings" of those recent decoders. The risk is to fry them.
Here is a blue transformer from Märklin in the 60s

As you may see, nice production
Cheers
Jean

My view on model rail is to start with a new up to date powerpack and control system. Other items are usually safe but not all as said above. Older locos and coaches are great to run but other items I feel should be new.