It sounds like you really did your homework there and I think it makes great sense with respect to the monitor and the video lines. Granted, I've not taken the time yet to absorb all of that yet like I plan to, but my first thought is with regards to the AVR and the Vsense leads and how your mods might effect the functionality of those. You said that you did not add another ground to the AVR. You could if you wanted to as long as it remains on the input side and not on the output side. There are 2 leads each for +5V and ground return running from the AVR to the game board. They are NOT shorted together but are separated by a 10 ohm resistor on each side. The idea with that is to provide the bulk power to the game board through the fat leads so that the AVR can read the board voltage via the thinner leads and then compensate for Vdrop over the fat leads. When the connection between the fat leads and the game board is degraded or burnt out, all the game board current tries to go through the little 10-ohm sense resistors and burns them up. Replacing the gameboard power leads with fatter wire wouldn't help. Improving their connection to the gameboard itself would. If you were to short the circuit ground leads together at the AVR or bypass them with a separate ground to the game board, the Vsense function on the negative side of the supply would be defeated and the AVR would not be able to regulate, or even properly measure, that side.
You said that prior to the mods that you could read tenths of a volt between the gameboard ground and the power brick ground... You're supposed to. That voltage drop occurs over the wires running from the brick to the AVR and then more again over the fat leads from the AVR to the gameboard. The Vsense leads between the AVR and the gameboard are then used to compensate the supply so that the gameboard chips see an accurate +5V supply relative to themselves locally. So long as the ground mods that you made do not interfere with the AVR's ability to sense voltage drop over the leads connecting its output to the gameboard, then it should continue to regulate properly.
William Boucher
----- Original Message -----
From: js@cimmeri.com
To: vectorlist@vectorlist.org
Sent: Sunday, September 14, 2008 3:25 PM
Subject: VECTOR: 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 17:30:06 2008
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