Hi.
I have a V659 and the full Murphy manual covering all models and the UHF add on.
Re the LOPT, IMHO they never fully recover from years of being damp. I've changed the oil in umpteen Murphy LOPTs with varying degrees of success. My 659 is now on its third LOPT a Thorn 1400 Jellypot. This mod works very well indeed and it can be hidden in the can. Use the stabilised Thorn circuit. The only issue is the Murphy derives a negative supply for the frame from the Line TB. (one reason why after an hours running with a damp Murphy LOPT the linearity and height increases)
I got a negative supply by feeding the heaters from a negative going silicon rectifier and tapping off the heater chain for approx - 35v suitably smoothed and stabilised with a Zener.
Great sets when right.
https://youtu.be/jMKGGdd8ezo