Relay-Version: version B 2.10 5/3/83; site utzoo.UUCP
Path: utzoo!mnetor!uunet!husc6!bloom-beacon!oberon!cit-vax!amdahl!kim
From: kim@amdahl.amdahl.com (Kim DeVaughn)
Newsgroups: comp.sys.amiga
Subject: Re: The time (my watch has troubles) has come ...
Message-ID: <16531@amdahl.amdahl.com>
Date: Mon, 19-Oct-87 04:34:06 EDT
Article-I.D.: amdahl.16531
Posted: Mon Oct 19 04:34:06 1987
Date-Received: Tue, 20-Oct-87 01:56:12 EDT
Organization: Amdahl Corporation,  Sunnyvale, CA 94086
Lines: 56
Keywords: I should learn to RTFM


[ Some days you eat the line ... some days the line eat's you ... ]

In an earlier posting, I mentioned that both the timer and RTC in my B2000
were overly inaccurate.  After making the posting (naturally), I remembered
that there were schematics in the back of the manual, and although they are
for the A2000 (?), I decided to take a look.

Sure enough, they showed a jumper going to one of the 8520 timer chips, and
a small variable cap in the oscillator feeding the Oki 6242B clock/calendar
chip.  Also, the Manual Update sheets said that the J300 jumper was used to
select the time-base for the 8520 between the line-driven TICK signal (50/60
Hz), or the vertical sync pulse for the video section.  The default position
of the jumper is *supposed* to be the TICK signal.  Since this description
agreed with the schematic, I decided to open the box up for a look.

Naturally, the J300 jumper is underneath the power supply, but the sub-frame
holding it and the drives comes out easily ... just 6 screws, and a couple of
connectors (thanks!)

Yep ... the jumper was between pins 2 and 3 rather than the pictured default
between pins 1 and 2!  So the timer chip had been getting popped at the
vertical sync rate, rather than being locked to the line frequency.  No big
deal, just swap the jumper.  Now the question is ... why would one want to
drive the timer from the vertical sync, rather than from the line?  Maybe
for use in areas with ill-controlled power?


And there was indeed a small trimmer cap near the Oki chip (just in front of
the 86-pin coprocessor slot).  No markings to tell which way to go, but it's
a 50-50 shot ... I tweaked it a hair CCW (is that the correct way to turn it
to slow it down, Dave?)  We'll see ...

BTW, it would be nice if the RTC were locked to the line freq. when the machine
is plugged in (or at least is powered on), and switched to using the xtal
oscillator only for battery backup, but I guess that'd be too expensive.

Anyway, if your clock(s) aren't keeping the right time, and particularly
if the timer driven system clock is running about 0.3% slow, you now have
a couple of things to check out.

/kim


P.S.  FYI ... the Fat Agnus chip is called "FAT LADY" on the silk-screening,
      and for some reason, Buster isn't named, though Gary, Paula, and Denise
      are (this is on a rev 4.2 board).



-- 
UUCP:  kim@amdahl.amdahl.com
  or:  {sun,decwrl,hplabs,pyramid,ihnp4,uunet,oliveb,cbosgd,ames}!amdahl!kim
DDD:   408-746-8462
USPS:  Amdahl Corp.  M/S 249,  1250 E. Arques Av,  Sunnyvale, CA 94086
CIS:   76535,25