I don't have the original 77, but I have a 75 series II. The service manual for it says that (page 2-4)
http://transmille.net/ProCal/Procedure%20Library/Fluke%20Digital%20Multimeter%2023%20MKII%20%5B1.10%5D/Technical%20Data/Fluke%2070%20Series%20MKII%20(PN%20896204%20July%201991%20Rev%201,%2009-92)%20Service%20Manual.pdf"AC CONVERTER
The ratio of R17 and R18 determines ac voltage and current accuracy, and C11 is the averaging capacitor for the half wave ac coverter portion of U1."
If you look at page 4-9, you will see the above 3 components are near the bottom of the multimeter just above F1. If you look at the backside, you will see they are all connected to U1. I suggest cleaning the backside of these 3 components and all the pins to U1 using a stiff toothbrush with IPA.
A tip I learned from modemhead, another member here, is to use a dental pick or very sharp probe to clean the pins in between U1. A 10x jeweler's loupe helps to make sure there are no contaminents on U1. I have "fixed" a number of used/abused multimeters that had abnormal offset readings with nothing more than a generous amount of IPA.
If cleaning doesn't work, you might want to resolder these 3 components as well just in case they have a bad solder joint?
PS. I measured all 3 components "in circuit" with my 87V and compared them to the BOM on page 4-8 and here is what I got.
R17 - BOM says 20.5k - I got 20.51k ohm
R18 - BOM says 9.20k - I got 9.199k ohm
C11 - BOM says 2.2uF - I got 5.50uF (no relative mode).
The reading for C11 is high, but I did it in circuit and other components may be affecting it?