Warning: I do not accept any responsibilities over any damage to any of your equipment or loss of lives/ injury should you service your synth using my steps.
I am just documenting what I have done, it does not represent any professional standpoint or advice.
Please take all precautions while servicing your synth, especially when it comes to live electricity.
I make sure I do not touch any electronics with my bare hands. All tuning is done using a test-pen and I made sure I wore rubber slippers while servicing the synth with live current.
I took apart the top panel of the DV800 and located the pots for VCO on the top left hand corner. I am a newbie so by trial and error and advise from friends from the vintage synth forums, I adjusted the 2nd, 3rd and 4th pots on the left VCO circuit panel. I tested and noticed that the right VCO panel, which is identical is for the lower deck.
I tuned the pots and played the notes against my MiniKorg to tune, I also tested the tuning against my Ukelele tuning app on my Iphone.
The previous owner has marked with a sharpie pen the position of the pots. I tuned it all the way to the extreme end just to get the synth in tune.
I noticed that this helps to alleviate my out-of tune upper VCO deck, but does not solve the problem 100%, it still takes time to warm up to get in tune and some extreme low notes (low G and C) are still out of tune by a cent.
I have made an appointment with a local vintage sound gear technician to troubleshoot and service it next week.