Asteroids proper grounding and monitor DC reg bug.

>>William Boucher wrote:
/>> /
/>> While none of that could really hurt anything, I have never had to do /
/>> this sort of thing on any Atari vector games and I own several. I /
/>> think you guys are confusing these 6100 monitors/games with Sega GO8 /
/>> which is a whole other story. /
/>> /
/>> The worst common supply problem with Atari games that have an AVR /
/>> board is when the either the +5V or ground edge fingers burn due to /
/>> bad edge connector terminals. Once the main positive or negative /
/>> supply paths are compromised, the 10 ohm resistors on the AVR board /
/>> will burn up. It's not the wires that aren't heavy enough, it's the /
/>> terminals inside the connectors that need replacing and the edge /
/>> fingers that need fixing (simple self-adhesive solderable copper tape /
/>> works wonders). /
/>> /
/>> William Boucher /
/>> /
>Indeed, this is not nearly as needed as for the GO-8s! However when the
>card edge connections start to fail this will give the common/ground
>shift that can take out the transistors on the monitor...
>Consider it preventative maintenance.
>John :-#)#

    I'd been reading John and William's discussions re grounds, and
decided to investigate an Asteroids I'm restoring since I was right at
the point of redoing the harness anyway. I found some interesting
things along the way that lend support for what John says about grounds.

    My Asteroids uses the G05-801 monitor. While examing the grounding
scheme, I found an error in both the schematics and factor monitor
wiring harness which I'll discuss below.

    In this machine, you have a power brick where the AC comes in ties
earth ground to the chassis. The brick puts out an unreg 10VDC and
72VCT, both with a common ground (but floating -- not earth grounded
here), as well as a 65VCT and 6.3VAC for the monitor.

    The 10VDC goes to the Atari regulator board, gets regulated down to
5VDC for the PCB. The 10VDC grounds, now named the +5 Return lines,
pass right through to the PCB. This 10vdc/5VDC power is still floating
(not connected to EARTH) at this point, and if you don't have the
monitor video cable plugged in, you can measure from 5VDC (or GND) to
power brick chassis and get both an offset VAC of around 9VAC and a much
smaller VDC offset.

     The two wires of the 72VCT (hi and lo) pass right to the PCB, and
since that supply is common GND with the 10VDC, the center tap wire is
not needed.

      The monitor is a different story. All 3 wires of the 65VCT supply
go to it, as well as an EARTH wire, in the P100 connector on the DC
regulator board. CT and EARTH come in at P100, and are immediately
tied together there. This ground then passes the whole length of the
regulator board, goes out at P101, and gets tied to the monitor's metal
frame.

      While tracing out P101, I found an error in the factory manual.
The schematics throughout the G05 service manual consistently show that
the L100 inductor filtered power comes out of pin 1 of P101, and that
unfiltered power is at pin 2. Yet, if you turn the DC regulator board
over, you'll see that the exact opposite is true -- the filtered power
is on pin 2. Further, if you check the P101 wiring harness, you'll see
the fat +25VDC wire going to the deflection board is wired correctly
according schematic, thus wrong because the board is mixed up. The thin
+25vdc wire going to the EHT unit is receiving the unfiltered power; the
fat wire is getting the filtered. I have two of these G05-801
monitors here, and both have the same flaw. Easy to fix by reversing
the two wires in the P101 harness. Now, back to grounds.

       The metal frame grounding point has a few other things tied to it
-- the aquadag strap, and the main ground for the deflection board.
Tracing the def board ground, I saw that it enters the board, heads
south, and provides the ground for the video in connector. From the
PCB, each of the 3 video signals (X-Y-Z) has its own ground wire, and
all 3 are tied to essentially earth/chassis ground at P703. 3 nice
huge ground loops.

     Now, if you've ever done audio work, you'll look at this grounding
scheme as really a mess... although functional. More grounds are not
always better -- it's how you do them that matters. And what matters
most is proper grounding to begin with.

    It's about impossible to go through the entire asteroids and convert
it to a properly done star grounding arrangement, but I did make a few
changes that support John's comments.

    First, I lifted the flimsy earth ground wire from the monitor's P100
connector, and ran a fat wire directly from the power brick chassis to
the monitor's frame ground point. The advantage here is that chassis
ground doesn't have to occur through the entire DC regulator board
(which is very fragile and has thin copper traces). This reduces the
chassis-2-chassis resistance by a few tenths of an ohm.

     Second, I lifted all 3 video grounds from the PCB end... leaving
them in place and still connected to the def board P703 (grounded at one
end and twisted, they'll still provide shielding). This lifting
removes the EARTH grounding of the 10/5/72 volt power supplies from
occuring through the video lines.

    Third, I provided an EARTH ground directly to the PCB (front, lower
corner has a nice hole for it near the X-Y video outs) using a fat wire
from the brick chassis to the PCB.

     The end result here is that all major ground loops have been
removed. Note that I did not run another ground to the Atari
regulator. Why? Because this would have re-established a major ground
loop. The regulator has no need of a chassis ground because it has
no separate chassis or other 2nd ground plane. The stock 2 ground
wires from it to the PCB should be sufficient, but if you wanted to beef
it up, you'd simply replace those 2 as well as the power lines with
fatter copper.

     Prior to these mods (but still with clean edge contacts and edge
connector), if I measured VDC and VAC between the various grounds with
all cabling in place, eg. PCB ground and the brick chassis or the
monitor frame, I'd get readings in the tenths of a volt... 0.30 VAC for
instance. After these mods, all grounds are solidly even.

     With far less stray AC and DC currents flowing around where they
aren't needed or wanted, this has got to improve reliability of any game
+ monitor. But I caution, it's not simply a matter of stringing
grounds around. You've got to examine the schematics and actual wiring,
and see what makes the most sense to do if anything.

  JS

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Received on Sun Sep 14 15:25:58 2008