Many contributors have pointed out that the unreliable aspect of much individual equipment/circuitry/"Studio lash ups" etc at the time [with the Mellotron being a particular culprit perhaps] and for example the chaining together of two massive studio Tape Decks producing a wobbly 8 Track resource for the Beatles, may well have been [paradoxically] the source of some really evocative and affecting music. The "Soundscape" of Strawberry Fields is indeed a classic example [among many] but I always felt that "We Can Work It Out" [with Mcartney on keyboard] had the same "feel".
I wondered if the Mellotron had been in use for that and consulted the Geff Emerick biography "Here, There and Everywhere" which gives the very best technical, social and personal insight into a world [half a century ago now] that I have ever come across. It's also an EMI version of the Peter Sellers film "I'm All Right Jack!
" Using that in conjunction with the Hunter Davies book "The Beatles Lyrics" [p122] I see that, in 1965, it took 12 hours in production to get the sort of swirling Fairground Harmonium sound so reminiscent of a Mellotron "adding more textures and effects than they had ever done in the past". It's described as a precursor to what came later with "A Day In The Life". No Mellotron was involved as far as I can tell but it's not hard to understand why Paul M was so fascinated when he first came across one.
Dave W