Valhalla Scientific tells me that the LNF option was "discontinued years ago", even though it shows up on the 2701C's webpage; perhaps there is another "LC" variant that provides that low noise floor in current models.
This 2701C-LNF I have diverges from the schematics posted by a reasonable amount...including capacitors where resistors should be, and extra capacitors on the bottom of the board piggy backed on some of the high precision resistors. The high voltage caps, by comparison, have dotted line silkscreening on the front of the board to identify where the resistors on the bottom of the board would go.
So.. at my skill level I' having a hard time telling if someone tried to repair this, tried to modify it's functionality, or if this is a variation due to the Low Noise Floor option coupled with various repairs (there are very obvious signs of repairs that I would consider poor; flux that wasn't cleaned up, pieces of paper towel caught on the pins on the bottom of the board as if someone tried to wipe the board off, etc).
I've been scouring the internet for information on the LNF option, hopefully with associated pictures, but haven't had any luck yet.
Calibration keys arrive tomorrow, and while I don't think all of the wonkiness on this can be explained by calibration alone, I'm going to give that a shot as the next step. I've got a Hakko 808 on order so make it easier to pull and test these some of these thruhole components, and I've ordered a spare set of NoS ICs for everything except the microprocessor from fleabay... The next bit to order are some of the relays; I've got a couple that are questionable on their latching, and a couple of the latches seem to allow a small bit of conductance on what should be open circuits.
(.... and with the baby now crying, this thought process escapes my head... so hitting post and hoping it's cogent.)
*EDIT*: IC9 is unpopulated from sheet one, lower right. IC10 is populated. IC9 is a JFET just like IC8 and IC11; IC9->IC10 feeds into line 15 on IC108 which controls the relays. I'm unsure of the control flow here, but I can't grok why IC would be unpopulated. Currently trying to scavenge a LF356N (unlikely) before ordering a few. Either the -LNF option is primarily "remove a lot of parts" or someone decided to use this one as a donor unit or some-such. Meh.
-j