Anyone know where to find a busbar?

[Mtlatc]

I know this is a longshot but I have a 99 4/A with a badly damaged busbar. It is the longest busbar on the board, and is the piece that is front of the ROM and RAM. Does anyone have any idea of a source for these?  I'm going to attempt to repair the one I have, but I have a feeling that may not work out for me. Does anyone know another name they might be called so I can google, I've tried everything I can think of?  Photo attached for reference:

 

 

[+TheBF]

You might be better off making one from copper sheet, tracing the original and cutting and filing until it fits.

Or perhaps cleaning the existing one and soldering a copper strip along both sides it to hold it together.

I see they have copper sheet at Home Depot.

 

?

[Stuart]

Be aware that at least one of the busbars - maybe all of them - have more than one copper strip inside them and distribute more than one voltage. They're bespoke to the machine - you won't find another unless it has been removed from another machine. You can remove it and wire the connection points up individually - but need to be careful to establish which pins are connected to which.

[ti99iuc]

as @Stuart already said I know the same. 
The white strips are called bus bars, they distribute +5, -5, 12 vdc and ground
@CC Clarke I am sure could help tu understand better on this

Edited January 30, 2023 by ti99iuc

[+TheBF]

4 hours ago, Stuart said:

Be aware that at least one of the busbars - maybe all of them - have more than one copper strip inside them and distribute more than one voltage. They're bespoke to the machine - you won't find another unless it has been removed from another machine. You can remove it and wire the connection points up individually - but need to be careful to establish which pins are connected to which.

Thanks for the that Stuart.

I had no idea it was a multi-conductor buss.

[Stuart]

8 hours ago, TheBF said:

Thanks for the that Stuart.

I had no idea it was a multi-conductor buss.

Neither did I until I tried to repair a pin that broken on one of them!

[+arcadeshopper]

I can send you a broke motherboard for $30 shipped in the usa and you can scavenge parts

[Mtlatc]

@Stuart and everyone else. 
 

I carefully deconstructed the busbar and determined that this particular piece was two strips separated by the “masking tape”

 

Logic tells me this particular piece either has one voltage and a ground or two different voltages.

 

What I assume (and I know that’s dangerous) is if I replace the pins on the proper bars, insure it is insulated, and put it back in the main board everything should be good.

 

What says you folks?

 

Thanks!

[Mtlatc]

Just in case anyone else like me comes searching this topic I wanted to close it out so they will know how it turned out.  

 

After deconstructing the busbar I soldered eight new pins total on the two rails.  There were four on each side.  I insulated the "touching" side of the bars with electrical tape on each side and for good measure put a piece of the masking tape between them.  I then insulated the entire bar in masking tape so there was no conductive material exposed.  

 

Put it in the board, checked to make sure it worked, and then soldered the bar back into the main board. A successful repair of an unavailable part.

 

EDIT: Additionally, the pictured busbar above provides 5V on the "front" rail - the rail I consider the front is closes to the user (with the RAM/ROM) is on the far side of the bar, when the main board is in front of you. The back bar was a Ground. (See orientation of board pictured above) 

 

@TheBF Thanks for your suggestion, thankfully my bars were in good enough condition that I did not have to do a complete refabrication.  Now that I have deconstructed one of the bars I believe you suggestion is very viable and would easily work if needed.

 

@Stuart Thank you again for your information.  For some reason I had it in my head that the busbar was "JUST" a grounding rail.

 

Thanks to all. 

Edited January 31, 2023 by Mtlatc
Was not done

[Tursi]

That's good to know, I've always been somewhat curious about a repair to those bars myself

 

[+FarmerPotato]

Thanks for this thread. I did not know anything about  bus-bars. I didn't know it was power AND ground. 
 

Now I notice them elsewhere.
 

The 990 uses a bus bar near the backplane , between all the bus driver chips (244s, 245s, and such) and the rest.  

Previously I thought the busbar was just a solution to a routing problem. I think there is a distinct advantage to it though!

 

TLDR; My guess is that the busbar provides ground return paths of least impedance for all the constantly-switching bus drivers. The noisiest part of the system. 

See the book "High-Speed Circuit Design: a Handbook of Black Magic" (published when high speed meant 20 MHz.)

 

Maybe it's a chance to bring back the busbar!

 

Theory (I am learning this stuff) 

 

Every signal driver and receiver pair will have a corresponding return path, through a ground trace connecting the two chips.
 

This will be through the shortest path (technically, the path of least impedance)

 

If the ground winds about the board  crazily, or is thin and parallel to another trace, there is coupling and noise in the system. Solid ground path directly underneath every trace is ideal. That's why modern boards have solid copper planes sandwiched inside. A winding or fragmented ground return is totally unacceptable at 20 MHz (not such a big deal at 3MHz.)
 

A busbar  even takes the return path *outside the board*. This immediately cuts the impedance by a factor of two!  I think. not sure.. Reason why: any conductor on the outer plane radiates half the electric field into space.  Whereas a buried copper plane is coupled on both sides, which creates a capacitor. (Extra impedance.) 
 

So I guess the busbar would reduce signal ringing and crosstalk, by making ground return paths short and away from other stuff.  
 

The busbar could give you less noise in the  whole  system. 

I'm still a novice at this sort of thing: studying backplanes and what I call "the analog side of digital." 

 

[+FarmerPotato]

I notice that TI boards of this era often use a ground path around the perimeter of one side, power in the other side. Fingers reaching out to each of the chips. 
 

Another pattern I see are finger-trees: power and ground fingers going down each row of a grid of chips. 

 

This book says that you got away with that back then   because clock speeds were low. 
 

The next better approach (still in a grid) is for ground fingers to be perpendicular to power. Worst approach is a single meandering path. S shaped = very bad. Best approach is solid power and ground planes sandwiched inside. 

I assume TI knew what they were doing, and that they valued lower noise, no glitches, and above all, reducing radio frequency emissions! Not sure, maybe the conducting ring around the perimeter  will reduce emissions?

Anyway, someone told me to heck with all this. On a two layer board, just make sure to do ground fill on one side, power plane fill on the other. Good enough.  
 

I may do an experiment measuring noise, then add  a bus bar.

 

See "High-Speed Circuit Design: a Handbook of Black Magic" (published when high speed meant 20 MHz.)

[+OLD CS1]

3 hours ago, FarmerPotato said:

I notice that TI boards of this era often use a ground path around the perimeter of one side, power in the other side. Fingers reaching out to each of the chips. 

If I understand your description, I have seen a few arcade boards which do the same.

[Stuart]

3 hours ago, OLD CS1 said:

If I understand your description, I have seen a few arcade boards which do the same.

https://electronics.stackexchange.com/questions/326189/pcb-design-of-the-80s-arcade-pcbs-ground-power-tracks

[GDMike]

Ahhh..yes. I've seen this too. Noise filtering doodad

[madmonkeystyle]

On 1/31/2023 at 12:20 PM, Mtlatc said:

Just in case anyone else like me comes searching this topic I wanted to close it out so they will know how it turned out.  

 

After deconstructing the busbar I soldered eight new pins total on the two rails.  There were four on each side.  I insulated the "touching" side of the bars with electrical tape on each side and for good measure put a piece of the masking tape between them.  I then insulated the entire bar in masking tape so there was no conductive material exposed.  

 

Put it in the board, checked to make sure it worked, and then soldered the bar back into the main board. A successful repair of an unavailable part.

 

EDIT: Additionally, the pictured busbar above provides 5V on the "front" rail - the rail I consider the front is closes to the user (with the RAM/ROM) is on the far side of the bar, when the main board is in front of you. The back bar was a Ground. (See orientation of board pictured above) 

 

@TheBF Thanks for your suggestion, thankfully my bars were in good enough condition that I did not have to do a complete refabrication.  Now that I have deconstructed one of the bars I believe you suggestion is very viable and would easily work if needed.

 

@Stuart Thank you again for your information.  For some reason I had it in my head that the busbar was "JUST" a grounding rail.

 

Thanks to all. 

This helped me a lot!  I picked up a few TI-99/4A's and the busbar between the 9900 and the GROMs was broken off on all but one pin.  While searching for where to buy something to use as a bar, I stumbled across this blog post.

 

My bar rebuild is now much thicker than the original but it seems to be doing the job!  

[matthew180]

If anyone wants to put in the effort to reverse engineer the bus bars, accurately measure their physical dimensions and pin locations, I would be happy to make a schematic and PCB design in KiCAD.  This way anyone who needs a new one could just grab the design and have a few made at OSHpark, JLCPCB, PCBWAY, etc.

 

Alternatively, a single board that replaces all the bus bars by having point-to-point connection wires that go to all the bus bar locations.  So you would remove the bus bars, put a wire at each point, and run it back to the distribution board.  Oh course the board would have to fit somewhere inside the console.  Anyway, just a half-baked thought.

 

Let me know.

[Geoff Oltmans]

On 2/3/2023 at 10:53 AM, FarmerPotato said:

Thanks for this thread. I did not know anything about  bus-bars. I didn't know it was power AND ground. 
 

Now I notice them elsewhere.
 

The 990 uses a bus bar near the backplane , between all the bus driver chips (244s, 245s, and such) and the rest.  

Previously I thought the busbar was just a solution to a routing problem. I think there is a distinct advantage to it though!

 

TLDR; My guess is that the busbar provides ground return paths of least impedance for all the constantly-switching bus drivers. The noisiest part of the system. 

See the book "High-Speed Circuit Design: a Handbook of Black Magic" (published when high speed meant 20 MHz.)

 

Maybe it's a chance to bring back the busbar!

 

Theory (I am learning this stuff) 

 

Every signal driver and receiver pair will have a corresponding return path, through a ground trace connecting the two chips.
 

This will be through the shortest path (technically, the path of least impedance)

 

If the ground winds about the board  crazily, or is thin and parallel to another trace, there is coupling and noise in the system. Solid ground path directly underneath every trace is ideal. That's why modern boards have solid copper planes sandwiched inside. A winding or fragmented ground return is totally unacceptable at 20 MHz (not such a big deal at 3MHz.)
 

A busbar  even takes the return path *outside the board*. This immediately cuts the impedance by a factor of two!  I think. not sure.. Reason why: any conductor on the outer plane radiates half the electric field into space.  Whereas a buried copper plane is coupled on both sides, which creates a capacitor. (Extra impedance.) 
 

So I guess the busbar would reduce signal ringing and crosstalk, by making ground return paths short and away from other stuff.  
 

The busbar could give you less noise in the  whole  system. 

I'm still a novice at this sort of thing: studying backplanes and what I call "the analog side of digital." 

 

I think in general they were probably used to reduce the complexity of the PCB. Makes for a cheaper PCB if you don't have dedicated ground/power planes.