The following was prepared as an answer to your question. I am not the author. The source documents are on the internet (hopefully still available). These are excerpts from my Amiga database content. It is from this database that I research many answers that I give this forum, if I do not readily know or remember the correct answer. At my age, I sometimes forget the answers, so I refer to my database.
Motherboard revisions according to Dave Haynie (Commodore Engineer)
REV 6.x Series:
The Rev 6 A2000 was never called the A2000C, except maybe by geeks on usenet :-).
The Rev 6 A2000 changed the DRAM array from 256K x 1 parts to 256K x 4 parts. Some layout and FCC changes were made, I don't know just what, I was kind of out of the picture at the time (Rev 5 was just Rev 4.5 with the DRAM changes, nothing else. Rev 6 did more, I wasn't really involved). The original Rev 6 board has some noise problems on the expansion bus, possibly due to a new production of 68000s from Motorola that happed to be used at the same time. Rev 6 also had some extra noise on its time-of-day clock. Rev 6.1 corrects the clock noise, Rev 6.2 addresses the expansion bus noise.
Model C (US Design. The term "A2000-C" was invented by the community)
Rev 6.0 (Has ROM tower, buggy with some cards)
Rev 6.2 (Removed ROM Tower)
Rev 6.3 (Changed to ECS and KS V2.04) IC 8373, ECS Denise 390433-02
Rev 6.4
Rev 6.5
This is information only, I can not verified accuracy
UUCP-Project/Commodore 12/21/87
From: grr (George Robbins) Tue Aug 15 03:59:58 1989
To: phil, sterling
Subject: A2000 Rev 6 rework instructions (tentative)
Please try these and get back to me with your perceptions of efficacy...
Field Upgrade / WIP rework:
Remove RP901 and RP902 by flexing until they break off
Solder 2.7K 10 pin "pullup" resistor packs along side of
U603 and U604, such that the common pin is soldered to
pin 10 of the IC's.
Solder a 1K resistor between pins 11 and 20 of U605.
Production Changes:
Delete RP901 and RP902.
Insert resistor packs along side of U603 and U604 before wave solder.
Change value of RP604 to 470 Ohm
Change value of RP903 and RP908 to 1K Ohm
Change value of R1000, R917 and R111 to 680 Ohm.
Change value of RP101 to 2.7 K ohm.
