PS2 5V uses a 9V rechargeable style battery and was the first implementation of DCS. These boards were made roughly 2000-2004 models.
The reason why it is considered more problematic :
#1 Many were built using known to fail capacitors from that early 2000 era.
#2 Boards are doublesided and double stacked and soldered together and then extra components added after the boards were soldered together. This makes them extremely difficult to repair at the component level.
#3 again, between just age of the board (now approaching 20+ years old, and often original 9V batteries that have failed, many of these die on first power up by a new user. Sure, there are examples of people still running more or less untouched engines with new batteries but the ratio of failed vs still working is kind of luck.
#4 the PS2 5V all used 16 Ohm speakers which are not as common now for replacements as 4 and 8 Ohm versions.
#5 The connectors and wiring harness are specific to the PS2 5V, and are slightly different than the later PS2 3V boards.
Bottom line, once a PS2 5V fails, it's usually not repairable and requires newer either PS3 upgrade kit, or in some cases one can maybe get a used PS2 3V boat set and retrofit.
Conversely, PS2 3V was found in some early MTH G gauge before it began to get put into O scale engines roughly tha 2004-2005 timeframe.
These are newer boards by roughly 5 years and a redesign based on making it easier to repair. The boards are a plug in stack allowing the power board to be replaced separately or repaired if needed.
Since they used a 2.4V rechargeable battery pack, the replacement batteries are more common (cordless phones, and many household items used 2 battery packs that are similar).
Many of the known failures of outputs (example audio amp, comparator for speed control, individual transistors for lights or smoke) are chips that are still available and can be repaired at the component or chip rather than all new boards.
In general, these are believed to be extremely robust by comparison to the 5V boards.
It's this easy to identify:
PS2 5V has the 9V style battery. Either has no changing port or has the round charging port.
PS2 3V has the 2 cell AA or AAA battery pack and a 2 pin charging connector. Also mostly built 2005 or later when speaking about 0 scale.
My personal rule is, I always find the product number of an engine, look up the product page, and note the year built and then pull down the manual. In the manual, I go to the battery replacement section and find out what kind of battery is listed or shown. I also then look att detailed pictures of the engine and try to confirm the charging port style (round or none, VS 2 pin). By doing these steps, I have a good idea if it is a 3V PS2 which is far are desirable IMO.
Another way I know is by product number but this is not all inclusive and there are exceptions. Again this is just my observation and not official MTH guidance.
Example in Premier, for steam engines, 20-31xx-x is the beginning of the PS2 3V. Example, 20-3048-1 is a 5V board with known 9V battery, where a 20-3148-1 engine is most definitely a 2005 made PS2 3V as confirmed via the manual. 20-32xx-x and higher is more definitely 3V, but even then I still follow the process, go to the product page, note the year and read the manual to validate.
For Diesels in premier, 20-22xx-x all the way up to 20-24xx-x are typically PS2 5V, where around roughly 20-25xx-x and higher are PS2 3V and thus a more desirable series.
In Railking, obviously it is 30 series numbers, but example 30-12XX-1 is PS2 5V (using 30-1248-1 as a sample), VS 30-1348-1 is a 2005 produced 3V version.
Again, do not go on number series alone, check other details (manual, timeframe, battery type, and charging port).
