[AMRadio] Need a Collins 75A-2 expert

[CL in NC]

I know, go to the CCA, but I dropped out of the CCA reflector after I sold all my Collins stuff.  But, now working on a 75A-w for a friend.

This 13XX SN rcvr has a 6DC6 V1 RF amp per service bulletin 3, removing the 6BA6, which actually applied to the 75A-3.  But, it does not do everything in SB-3, such as changing the S-Meter resistors and moving one lead.  It has a 6BA7 first mixer, V2, and this was changed at some point from the original 6BE6, maybe by Collins since you can see where the 6BE6 chassis ID was removed and 6BA7 professionally imprinted in its spot that is much cleaner than the surrounding chassis.  A clean repunch of the hole for the new 9 pin 6BA7 socket is evident, no rough file hole.  But, V4, the LF mixer is still a 6BE6.  I have found no schematic of a 75A2 with this line up.  Anybody know what was going on here, a mid 1951-52 improvement for a few models?

Charlie, W4MEC in NC