What could this resistor be used for?

[raymondjiii]

I was hoping to install a video/audio upgrade on this 7800 and I open it and find this oddly placed resister.

 

 

[+-^CrossBow^-]

I answered this for you in your PM to me.

 

But that resistor on the video ram is really common. It is factory installed and I usually desolder it, place some shrinktubing over it and then solder it back on to make it look a little cleaner. I think on some later made 7800s they actually incorporate that resistor into the board design.

 

[RevEng]

It looks to be a pull-up or pull-down on /WE, which is interesting. It's not in the AA 7800 schematic. @-^CrossBow^- is the other end of that resistor connected to +5v or ground?

[+-^CrossBow^-]

1 hour ago, RevEng said:

It looks to be a pull-up or pull-down on /WE, which is interesting. It's not in the AA 7800 schematic. @-^CrossBow^- is the other end of that resistor connected to +5v or ground?

Not sure right off hand. I've never looked into it but I see it in probably like 1 out of every 3 consoles I've serviced. I think at least one of my consoles has this so I could check that. My guess is that it is attached to ground.

 

[+selgus]

2 hours ago, -^CrossBow^- said:

Not sure right off hand. I've never looked into it but I see it in probably like 1 out of every 3 consoles I've serviced. I think at least one of my consoles has this so I could check that. My guess is that it is attached to ground.

 

On my motherboards where I've seen this 1K resistor, it is put in to replace a solid trace. If you look on the underside of the PCB, between the via and the /WE pin, the trace will be cut.

[Dissy614]

14 hours ago, selgus said:

On my motherboards where I've seen this 1K resistor, it is put in to replace a solid trace. If you look on the underside of the PCB, between the via and the /WE pin, the trace will be cut.

Mine is an "A1 77" serial#

It has this resistor installed, but the bottom side trace is completely intact.  The meter probing each leg of this resistor confirms no resistance and tones out with continuity.

Also of note, you can see another trace on the top side of the board from /WE going upwards to connect to the other chips /WE pin.

 

Until this thread, I too assumed it was a weak pull-up added after the design was finalized (/WE is active low so a pull-down would serve no purpose, but a pull-up would assist keeping the signal inactive)

But nope!

 

@selgus Do you happen to remember or know where the trace was broken on the units you saw this?

If I had to guess, was it right at the 90 degree bend?  

 

[+selgus]

38 minutes ago, Dissy614 said:

 

@selgus Do you happen to remember or know where the trace was broken on the units you saw this?

If I had to guess, was it right at the 90 degree bend?  

 

I checked a second motherboard, and it has this same trace cut..

 

[Dissy614]

14 minutes ago, selgus said:

I checked a second motherboard, and it has this same trace cut..

Interesting.  Mine doesn't have that... via? hole?  

(Attempted to crop the same for comparison)

 

[Edit] and the component side:

 

Edited March 9, 2023 by Dissy614

[Dissy614]

Also I see some other differences.

[Selgus] mentions 1k, as is mine. 

[raymonjiii] appears to be a different value.  Blue ; ?? ; black/brown.  So 6x or 6x0 (Probably 680 by E6 standards)

Also the via doesn't appear to be present from the component side there either, although I admit it's hard to tell at that angle.

 

Were the top-layer and bottom-layer people at atari not on speaking terms when that via was added?

[+selgus]

It's not a via, it looks like they used a drill bit, at the factory, to break that trace. The hole doesn't penetrate through the PCB.

 

I'm not sure why they would put that resistor in place, without breaking that trace.

[+-^CrossBow^-]

10 minutes ago, Dissy614 said:

Also I see some other differences.

[Selgus] mentions 1k, as is mine. 

[raymonjiii] appears to be a different value.  Blue ; ?? ; black/brown.  So 6x or 6x0 (Probably 680 by E6 standards)

Also the via doesn't appear to be present from the component side there either, although I admit it's hard to tell at that angle.

 

Were the top-layer and bottom-layer people at atari not on speaking terms when that via was added?

I believe different values were used depending on the speed of the RAM used. Notice in the picture of the component side you are showing 120ns Sony while the OP has Samsung 150ns speed. I've seen 100ns used as well. I thought it was only the Sony branded ones that had them so the Samsung RAM having the resistor might be new to me as I don't recall seeing that before. Then again, I'm not sure I've seen Samsung used before?

 

There is another thread where @CPUWIZ wanted pics to start tracking the different RAM and speed used in the 7800s and I provided with all the different ones I had pics of from service work at that time.

 

[+selgus]

On 3/9/2023 at 9:58 AM, -^CrossBow^- said:

I believe different values were used depending on the speed of the RAM used. Notice in the picture of the component side you are showing 120ns Sony while the OP has Samsung 150ns speed. I've seen 100ns used as well. I thought it was only the Sony branded ones that had them so the Samsung RAM having the resistor might be new to me as I don't recall seeing that before. Then again, I'm not sure I've seen Samsung used before?

I was just desoldering some custom and memory chips from a 7800 motherboard, and the revision-b PCB does have this resistor included in the layout..

 

 REVISION A PCB

 

 REVISION B PCB

This is not a pull-up or pull-down resistor, but seems to be for impedance reasons. It is in series with the /WE signal.