I have been operating and collecting Marklin trains since 1958. My current layout is 12' by 8' and consists of four interconnected loops of K track over cork roadbed with 27 switches, a turntable and two rail yards. Everything I own is analog and is run by six 6627 transformers. I have over 27 engines spanning the 65-year time period from a venerable model BR89 3000 to a more modern digital BR39 39392. Staying analog worked fine during the era of the universal electronic circuit in the Delta and Digital systems. Although the more modern engines were really made for Digital, they were reverse compatible and worked just fine on analog, you just couldn't access the sound functions, which meant little to me anyway.

Two things have happened recently which have given me concerns about staying analog and continuing to purchase new locomotives. First, my son, who runs digital, purchased a new locomotive and we test ran it on my layout. It ran fine on my setup, but when he took it home he found that the digital card was fried, and it would not run at all on his. Apparently switching into reverse fried the card given the electronic jolt is about 24 volts from my analog transformers. Second, I just purchased a very nice digital BR39 passenger locomotive (39392). The instructions indicate it can run on analog BUT there are warnings to "use only switch mode power supply units", "never supply with power from more than one transformer" and a note on the outside of the box that warns it runs between 16 and 22 volts. As I indicated before, the old analog transformers throw 24 volts thru the engine for switching purposes.

So that brings me to my question for any electronic wizards out there. Is there a way I can stay analog but run my new digital locomotive without destroying it? Also, what the heck is a switch mode power supply?

You should replace your older transformer which puts out too great a voltage with a newer one which respects the new limits. The older transformers can put out reverse impulses greater than 23VAC. The newer ones (6647) are designed to work with voltage norms set out in the last decade and will not create a surge that can damage electronics. If you are still using the old metal transformers from the 50s/60s, the risk of shock from wire deterioration makes the change worthwhile in any case.

Regards

Mike C

Thanks to all who replied. Very useful explanations and suggestions. I think I will swap out my 6627s for the more modern 6646 transformers to protect the engines. Shifting to digital is not an approach which seems useful since it would leave many of my vintage engines unusable and, frankly, the cost and complexity of all the new power equipment, swapping and modification of engines, adding digital cards to switches etc. leaves me cold. Will 6646 transformers protect the digital locos better or are they not switched mode power supplies and therefore of no more use than the 6627s I already have?

As hxmleisa says, a few diodes can work wonders. I run analogue now and mostly have the white transformers but I also have a couple of blue ones - the later ones with a plastic case.
I have wired two antiparallel pairs of diodes on the output of these (so four diodes in all) which give a voltage drop of around 1.4volts regardless of the load. This brings the output to the track for both the driving voltage and the reverse pulse voltage in line with the white transformers.

I don't have many digital locos but the ones that I do have not experienced any issues. It also makes adjustment of the springs on the old electro mechanical reversers easier as the reverse pulse voltages are the same whichever controller is used.

The newer ones with plastic case have a lower reversing voltage than the older ones with a metal risk, so the risk for decoders is much lower.

You live in an area where the nominal mains voltage was reduced from 240 V to 230 V, thus also reducing the reversing voltage.
Eddie lives in an area where the nominal mains voltage was raised from 110 V to 120 V, thus also increasing the reversing voltage.

Long story short: Solutions that work for one person may not work for other persons with completely different configurations.