OK, so installing decoders in Z-scale locos has become a much easier task with many drop-in decoders available today. That said, some are easier than others and I discovered that I definitely prefer one brand over another for my fleet conversion.
The brand and decoder model choices for converting my MTL Geeps over to DCC specifically came down to two: the Digitrax DZ123M0 and the TCS MZA4.
Seen to the right is one of my MTL GP35 units with a TCS decoder installed. Below is the Digitrax after removing it from the Geep.
Both units install essentially the same, although I found it easier to get the TCS board wedged in between the posts as opposed to slipping the Digitrax board into the existing slots of the posts. In order to get the Digitrax board in and still clear the locking post (seen to the left of the board), I had to file open the slots a bit or the board wouldn't wedge in far enough ... apparently the decoder board was a bit thicker than the lighting board that came with the loco.
Attaching the springs that carry the current to the motor is quite easy using tweezers as long as to start by looping the spring onto the board first and then extend to the motor pick-up loop.
Both decoders worked with the NCE system that I'm evaluating with both being fully responsive both in the programming and operational modes. As part of this exercise, I certainly had to familiarize myself with the whole CV model and options for each decoder. Not much to it for Z-scale locos since most of us will never get beyond running the motor and controlling the front and back lights.
So, which to pick for my MTL Geep fleet? The decision actually came down to one simple thing ... the choice of lighting implementations. As seen above, the TCS uses traditional LEDs extending from the board whereas the Digitrax now uses surface-mount LEDs. When I had the Digitrax board installed, the bright LEDs clearly bled through the plastic shell on both sides and the top of the loco making it quite an unsatisfactory situation. My MTL loco came with traditional LEDs on the original lighting board and they used a plastic tube to direct the light toward the intended areas of the shell ... this method still works with the TCS decoder but isn't possible with the Digitrax. I suspect that I could try to coat the inside of the shell to reduce light bleeding out but that might not be effective enough so I've decided to go with the TCS decoders. BTW, while I was surveying the forums regarding decoder selection, I noted that both brands are well regarded and have been reliable for many modelers so I didn't see that as a major decision factor.
Off to go order a set of TCS decoders for my collection of five Geeps and one MTL SD40-2. I do have an AZL SD70 to deal with but I'll hold off on that one a bit longer - a drop-in decoder is available :o). Also, I don't intend on converting any of my older MTL F7 locos as I'm not really interested in trying to deal with non-drop-in decoders.