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RCA1802

Vertical Bands/Lines on Left Side of 130XE Screen

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Hello,

I recently acquired a 130XE that works great, the only problem is that there are faint bands/lines on the left side of the screen. I originally used an S-Video cable from eBay and thought the problem was with my LCD monitor, but they also show up when I use my Commodore 1084S Amiga monitor and a Chroma/Luma cable. I can adjust the color/contrast/brightness and that helps until the background color changes and they are visible again. After reading through the forums I found a few posts regarding 9 lines on the left side of the screen on a 5200 S-Video mod being "ANTIC noise during a DRAM refresh" and I am guessing I have the same problem. Anybody have any ideas on a fix? I have the later revision with 64x4 RAM. I've attached pics showing the lines, I turned up the contrast to make them more visible to the camera.

 

Other Topic:

http://atariage.com/forums/topic/218246-s-video-mod-9-vertical-lines-on-left-side/

 

Thanks!

post-38669-0-84604800-1394204308_thumb.jpg

post-38669-0-35623600-1394204312_thumb.jpg

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Usually you get these over RF if the channel is poorly tuned.

Modern LCDs seem susceptable regardless, it's like they're much more sensitive to small fluctuations in the signal that older TVs barely register.

 

The lines seem to coincide with where Refresh cycles occur - the other thread mentions interference among system components and changing caps. Tend to agree, the refresh cycles mean the CPU goes idle and the RAM goes through a Cas-only cycle for refresh, which seems enough to change the overall blend of electrical crosstalk going on.

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The first thing I would try is modifying the power path to the video circuits so there's less influence from nearby stuff. I don't have a 130XE nearby to look at, but I've cleaned up XL circuits by running power directly from the 5V jack area and bypassing the path on the circuit board.

 

EDIT: After doing a little more research, it looks like L5 is where power comes in. I would try lifting it and running a wire back to the output side of the power switch.

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Wow... COSMAC - RCA 1802. My very first computer. Had to make my own keypad controller for input. Still have it - somewhere.

 

As for your display noise, it's almost voodoo when you are messing around in the video. The lines are probably ground bounce. Does that help? You need to lower the inductance in the video and memory power lines and add bypass capacitors as power supply filters. I don't recall seeing this on a normal 130XE (with 16 memory chips), so the newer board (with 4 chips) may need some 'magic'.

 

Bob

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Atari's engineering on the analog side of things leaves something to be desired.

 

How/why they left chroma out of the 800XL and fudged up the source impedance will always be a mystery. I hate having to tell people I prefer the 800XL, but only after I tear into it to fix the video.

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The first thing I would try is modifying the power path to the video circuits so there's less influence from nearby stuff. I don't have a 130XE nearby to look at, but I've cleaned up XL circuits by running power directly from the 5V jack area and bypassing the path on the circuit board.

 

EDIT: After doing a little more research, it looks like L5 is where power comes in. I would try lifting it and running a wire back to the output side of the power switch.

 

Sorry, I'm kind of confused, could you describe this fix in greater detail? I've found L5 on the schematic, when you say lift it do you mean remove it from the board, and then directly connect the video circuit to +5 from the power switch?

 

Thanks for the help.

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Sorry, I'm kind of confused, could you describe this fix in greater detail? I've found L5 on the schematic, when you say lift it do you mean remove it from the board, and then directly connect the video circuit to +5 from the power switch?

 

Thanks for the help.

Yes, the 5v to the video circuitry comes in through inductor L5. If you lift one end of L5, you can feed 5V in directly from the power supply. The end of L5 you want to connect to is the one that goes to C50 and C51. Solder the other end of the wire to the switched power pin on the switch (goes to positive side of C1).

 

I believe the same power trace that feeds the video circuits also powers the RAM which explains the noise. However, Bob mentioned ground bounce which is also a possibility. You can also try soldering a wire from the other (negative) side of C50 & C51 to the negative side of C1.

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Yes, the 5v to the video circuitry comes in through inductor L5. If you lift one end of L5, you can feed 5V in directly from the power supply. The end of L5 you want to connect to is the one that goes to C50 and C51. Solder the other end of the wire to the switched power pin on the switch (goes to positive side of C1).

 

I believe the same power trace that feeds the video circuits also powers the RAM which explains the noise. However, Bob mentioned ground bounce which is also a possibility. You can also try soldering a wire from the other (negative) side of C50 & C51 to the negative side of C1.

 

I connected L5 directly to the switch power pin and connected C1's ground pin to C50/51's ground with no change, the lines are still showing the same as before. :(

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I connected L5 directly to the switch power pin and connected C1's ground pin to C50/51's ground with no change, the lines are still showing the same as before. :(

Hmm... can you post a picture of the left side of the motherboard? Also, did you lift one end of L5 out of the board?

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Here are pics of the top and bottom wiring jobs. Please forgive my crappy soldering skills, I've built kits (Cosmac ELF2K, Micro-KIM, Replica 1) and they all worked I promise!

 

I cut the L5 pin connected to 5+ and bent it up, I then soldered a wire to the lead and covered it with shrink tube.

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Edited by RCA1802

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Well, it looks like the power routing skirts around L5 quite a bit on that board (yuck, it doesn't look like much of anything is using the filtered power)! It would take some trace cutting to really isolate the video components. I'll look at it some more...

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Well, it looks like the power routing skirts around L5 quite a bit on that board (yuck)! It would take some trace cutting to really isolate the video components. I'll look at it some more...

 

Thanks!

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I see no evidence that the positive side of C50/51 is connected to anything except L5. This is an error. I wonder if this is a known issue.

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Cut this trace. It directly connects the video circuits to the RAM. Plus, it's an easy spot to put a jumper if you want to restore the connection (in the vias). You could also cut it closer to L5 on the underside.

 

Next, install a piece of wire across the 2 pads for L5. This will put the capacitors back in the circuit.

 

I doubt the ground wire is needed, as there's some pretty heavy copper areas there.

post-3606-0-26023000-1394243278_thumb.jpg

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Ok I cut the trace and connected the 2 L5 pads and it seems to be worse, the banding is more distinct and now I can see faint banding on the other side of the screen but its barely visible in the attachment.

I also left the ground wire on, tested, and then removed it with no effect.

 

Thanks for the ideas though.

post-38669-0-56934100-1394249683_thumb.jpg

Edited by RCA1802

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Hmmm... why is there nothing on the screen? Is the computer working?

 

Is the power wire still connected to L5?

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This is puzzling because I've reduced the noise in all my other 8-bits by basically doing the same thing- running a separate power lead to the video circuits and breaking the original complex power path. The fact that that revision of the board appears to have power filtering errors on it would certainly explain why the refresh shows up on the screen.

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The power is still connected via the original jumper, I just bridged the 2 L5 pads with a piece of wire. I also cut the trace in 2 places and pulled up a piece to make sure it was a clean cut.

 

I have a defender cartridge plugged in and I took the picture before the screen came up to get the clearest picture of the lines.

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The RAM chips consume a lot of power during refresh cycles and their timing is consistent with the screen so if there's any filtering issues it'll be one of the first types of interference to show up.

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The RAM chips consume a lot of power during refresh cycles and their timing is consistent with the screen so if there's any filtering issues it'll be one of the first types of interference to show up.

 

I attempted this mod as an experiment to attempt to at least isolate one source of noise from my video circuitry. I was uncertain if it gave results or not. However in retrospect I am unsure if I connected up the correct side of the L5 Inductor and I certainly did not find the correct trace to break.

 

My motherboard is a different revision than RCA1802 displayed above and has a somewhat different layout - perhaps it is just the difference between the NTSC and PAL variations. However, could you tell me which side of the L5 inductor I am supposed to lift and patch directly to the power switch?

 

Is it the end with the blue arrow or the end with the red arrow? I initially lifted the 'top' side nearest the cartridge and ECI slot:

 

post-31546-0-07655600-1457651301_thumb.jpg

 

Also, would you mind identifying which track to break on my motherboard? As you can tell from this image previously I chose...poorly...:

 

post-31546-0-21323200-1457651313_thumb.jpg

 

In either case the negative rail of the power switch is connected to the negative leg of C50 - even I cannot get that mixed up. Yet.

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