Jump to content
Muddyfunster

800XE vertical lines composite

Recommended Posts

Hi All,

 

I picked up an 800XE today with a good GTIA (it passed the test here: http://atariage.com/forums/topic/233858-testing-gtia/)

 

What I have found however is that the composite signal out seems to be quite noisey. Comparing to the RF signal, the RF signal surprisingly seems to have less interference.

 

I expected to see some vertical bars, pretty much all of my other un-modded retro machines have them to some degree when running over composite (C64, ZX Spectrum, VIC). However my 800XE seem to have really strong bars.It's more pronounced on the left side of the picture which made me think it might be a fault or chip on the way out. The picture seem to tone down the bars a bit.

 

I don't have much experience with A8 machines and was wondering if this Is this normal or whether I have picked up a machine with some issues?

 

I was hoping the more experienced heads can advise, I appreciate any help or guidance.

 

Pictures are not the best but they do illustrate what I was trying to explain.

 

 

This is via composite

post-61789-0-30313400-1559251808_thumb.jpg

 

This is via RF

post-61789-0-94902300-1559251835_thumb.jpg

 

This is via composite

post-61789-0-49632300-1559251876_thumb.jpg

 

This is via RF

post-61789-0-86467900-1559251902_thumb.jpg

 

GTIA Test

post-61789-0-34460400-1559251946_thumb.jpg

 

Share this post


Link to post
Share on other sites

It's bus noise. Mine get worse or move around during high memory or CPU usage.

 

I have a couple 800's and a few 600's and I see these on some more than others with composite mods. Many things can introduce it but I notice it most on certain machines as soon as I replace the OS ROM with an EPROM. It can be toned down by adding filter capacitors from 5v to ground on it, or swapping with another EPROM. The same can be accomplished by filtering other parts of the board (especially around the video section) the same way.

  • Like 2

Share this post


Link to post
Share on other sites
Posted (edited)

Thanks R.Cade, was just concerned something was wrong with it. I think I'll look to get a video mod of some sort done in the future to hopefully clean it up some.

 

Thanks,

Edited by Muddyfunster

Share this post


Link to post
Share on other sites

Bus noise is a bit generic - the problem exists with all the Atari 8-bits and will often look worse on modern LCDs which seem to be able to amplify defects such as these.

 

I'd call it more like crossover noise from the clock generation and memory (esp Ram) access circuitry into the video generation. And shared power probably makes it worse, it's almost as if certain things (like memory refresh) cause a voltage dip which is reflected in the video output.

 

Memory refresh "bars" are the vertical interference that coincide with the refresh cycles and populate the left to middle of screen at 9 regular intervals. You'll often also get less prominent bars that coincide with general video memory accesses and can vary depending on what mode is in use.

 

The black and grey mismatched luma thin vertical stripes as you see in the last pic is an entire different issue - the D to A circuitry consists of a resistor ladder which converts the 4 bits of luma output from GTIA into the analog voltage quantity that mixes into the video signal. Certain transitions like b'0111' to b'1000' mean that for a short period the outputs that have switched to 1 haven't charged to full voltage which means the video output is the wrong value until that time which translates to inconsistent luma values appearing as can be seen.

  • Like 3

Share this post


Link to post
Share on other sites

Rybags, Thanks for that explanation, at least I know its not about to explode and what i'm seeing sounds pretty normal :) Appreciated.

 

If I installed something like a Sophia mod or VBXE, would that eliminate the bars completely?

 

I've decided I can live with it, but I'm always looking for my next "project" with my retro boxes.

Share this post


Link to post
Share on other sites

If I installed something like a Sophia mod or VBXE, would that eliminate the bars completely?

Yes Sophia or VBXE would eliminate them, and output DVI or RGB signals. A simpler mod like UAV does a good job eleminating most of the artifacts you highlighted for composite and s-video output.

  • Like 3

Share this post


Link to post
Share on other sites

Um... it depends. I've seen clocking lines show up just fine on VBXE machines, although - interestingly - this is sometimes fixed by using a higher-quality, well-shielded video cable. For this reason I'm not entirely sure lines on RGB output and Y/C output are caused by the same thing.

 

I don't think 'high CPU usage' is going to be the cause of the issue, since the 6502 runs flat-out 100 per cent of the time, even when executing 'JMP *'.

  • Like 2

Share this post


Link to post
Share on other sites

I can confirm that increased system activity (memory/data) can intensify the refresh lines on my 1200XL (UAV rev C & U1MB both installed). Moving the U1MB ribbon cables and providing a separate cleaner +5V and Ground to the UAV can help limit the obviousness of them on that machine but they’re still faintly visible.

 

Now, Michael’s XEL and XLD designs have such a lovely, clean and completely noise-free video through the integrated UAV’s that it can be painful to go back to my classic machines. The giant ground planes inside the 4-layer boards really help out there.

  • Like 1

Share this post


Link to post
Share on other sites

better caps, grabbing power from a different location, ferrite beads, clearpic mod, and proper shielding as a quick run down. Be advised also the video cable as Jon mentioned, the original circuit is tuned to crappy center conductor wire and good shielding with the wire wrapped around a toroid core ring (ferrite) for rf... bet the keystone kops did their thing earlier in the circuit to make a sort of balanced mess that becomes more apparent with age and newer displays.

Share this post


Link to post
Share on other sites

You have to change the capacitor C50 (10uf 16V) and the vertical lines will disappear. Replace it with 10uf 25V. The capacitor must be placed with care, do not push it hard or it will be damaged again.

 

fullsizeoutput_16a5.thumb.jpg.30f26af3f1f88c5ddeddb79d9bb890ce.jpg

 

  • Like 1

Share this post


Link to post
Share on other sites
4 minutes ago, tane said:

You have to change the capacitor C50 (10uf 16V) and the vertical lines will disappear. Replace it with 10uf 25V. The capacitor must be placed with care, do not push it hard or it will be damaged again.

That doesn't make any sense.

 

The spec you're proposing to change is the maximum voltage the capacitor is rated to handle, not the actual part that makes it, you know, a capacitor (which is the capacitance). The voltage at C52 is only 5V DC. The original 16V rating cap is fine, presuming the cap itself is still good (hint: on an Atari, unless you damage it yourself, most of the caps are fine). 

 

  • Like 1

Share this post


Link to post
Share on other sites
Posted (edited)
37 minutes ago, tane said:

You have to change the capacitor C50 (10uf 16V) and the vertical lines will disappear. Replace it with 10uf 25V. The capacitor must be placed with care, do not push it hard or it will be damaged again.

 

fullsizeoutput_16a5.thumb.jpg.30f26af3f1f88c5ddeddb79d9bb890ce.jpg

 

I wouldn't be to quick to discount his suggestion

the voltage rating might be higher but this does help and works on some machines. It's a fresh cap, and yes the old ones can go out of spec or be damaged when folks poke about, prod and add mods... there is a whole grocery list of fixes to do though. I have still found the choice of cable/wire and shielding with any number of clear pic mods have worked wonders, bad caps happen for a number of reasons and not all the caps used through out the manufacturing time line were the best.

Edited by _The Doctor__

Share this post


Link to post
Share on other sites
34 minutes ago, _The Doctor__ said:

I wouldn't be to quick to discount his suggestion

 

The cap itself may or may not make a difference - depends on if the electrolyte in the old cap has lost its capacitance.The part he specified (the maximum voltage rating) most assuredly will not. 

  • Like 1

Share this post


Link to post
Share on other sites
9 minutes ago, _The Doctor__ said:

The voltage rating being higher is just fine and won't hurt anything ;).

I never said it did. I said it won't make any difference. And it won't. If you can find a .10 uF cap with a 6V rating, that would work too. ;) 

  • Like 2

Share this post


Link to post
Share on other sites
56 minutes ago, _The Doctor__ said:

The voltage rating being higher is just fine and won't hurt anything ;).

Won't hurt anything, but it won't fix anything either.  Who cares if a person can lift 50 pounds or 500 pounds, when they are lifting 10 pounds.

  • Like 1

Share this post


Link to post
Share on other sites
2 hours ago, tane said:

You have to change the capacitor C50 (10uf 16V) and the vertical lines will disappear. Replace it with 10uf 25V. The capacitor must be placed with care, do not push it hard or it will be damaged again.

 

fullsizeoutput_16a5.thumb.jpg.30f26af3f1f88c5ddeddb79d9bb890ce.jpg

 

Capacitors, especially electrolytics, are complex components. They have many characteristics besides capacitance, particularly at high frequency. You will often see an electrolytic shunted by a ceramic, just for this reason. Things go wrong inside and you may filter OK at 60hz, but fall down flat at 1.79mhz. Bad electrolyte, high inductance, phase of the moon - who knows what may be happening. The higher voltage rating may help with your noise because of improved geometry. Worth a try...

 

This is what we are doing here, right? Bunch of folks try this and it works, we learned something. If it doesn't work most of the time, we learned something. Our brains will just be jammed with smarts!

 

Bob

 

  • Like 3

Share this post


Link to post
Share on other sites
Posted (edited)
4 hours ago, DrVenkman said:

That doesn't make any sense.

The spec you're proposing to change is the maximum voltage the capacitor is rated to handle, not the actual part that makes it, ....

I don't know how you conclude that. I'm not telling the higher voltage will make a difference in terms of electrical design. I said to change the C50 capacitor, and I propose a safer voltage for it.

I have all my devices with capacitors with one step higher, instead of 16V -> 25V, and the non-polar instead of 35V -> 50V. So far so good. Why I did it?, just because I can, and it's a safer design at the same price.

 

1 hour ago, Stephen said:

Won't hurt anything, but it won't fix anything either.

You are wrong. I had the same problem because I replaced the same capacitor, but I pushed it very hard damaging it, just because the new capacitors are smaller and the separation is not the same. Then I had to replace it for 2nd time, and the problem of vertical lines was solved.

 

The problem of the vertical lines lies in a defective capacitor C50 that needs to be replaced by a new one. Of course it could be other reason, but it is the first step to be discarded.

 

 

Edited by tane

Share this post


Link to post
Share on other sites

“Safe voltage” is silly. It’s 5V DC on that rail, and if the voltage ever even gets close to the rating of the original cap (16V), you’ve already fried every IC in the system.

 

Now if you want to replace a 35 year old electrolytic with a brand new Nichicon or Panasonic, sure, that might make a difference *if the old one wasn’t working properly.* But if the old one isn’t working correctly, you could just as easily replace it with anything that matches the capacitance rating and voltage spec greater than the 5V on that power rail and you’d be fine.

 

Otherwise you’re wasting your time and money and peddling snake oil with no basis in engineering or objective reality. 

  • Like 2

Share this post


Link to post
Share on other sites

Just to add a little to this. Electrolytic capacitors are generally used with a safety margin, and their lifespan is affected by this. Typically, 2-3x the normal operating voltage is used - plenty of headroom and greatly extends the life. But going higher with the rating doesn't make it "safer" - as was already explained, they're not in any danger of being unsafe running on 5V. Given that the 16V parts involved have lasted 35-40 years at this point, how much safer do you really think they'll get at 25V? You're into the sort of territory where shelf life is a more significant factor than use, and 40 years is a perfectly adequate shelf life for such parts.

 

But another factor that you haven't considered is that traditional electrolytics also need a reforming voltage in order to keep their dielectric in good order. Without it, the capacitor will slowly deteriorate, losing capacitance and gaining ESR. Ideally, this voltage should be as close to the rated voltage as possible, but when in-circuit, this obviously isn't the case. The effect diminishes the further you go from the rating, which means that going for much higher ratings effectively shortens their life spans.

 

So, any perceived "safety" benefits or lifespan increases you get from changing from 16V to 25V parts is likely offset by depriving the capacitor of more of the reforming voltage it needs for a long life.

  • Like 1

Share this post


Link to post
Share on other sites
Posted (edited)
On 6/30/2019 at 8:59 PM, tane said:

 

 ...but I pushed it very hard damaging it...

Can you explain this a bit more? Why would you be "pushing" at all? I've replaced many a cap and I've never had to "push" it. I make sure the "stems" match the trace holes in their separation and they slip in very easily, most of the time, unless they come bent, they naturally slip in with room to spare. It sounds like your talking about inserting a plug into a port that has too tight a fit or something...🤨

Edited by Gunstar

Share this post


Link to post
Share on other sites

It sounds to me like the pitch of the legs on the replacement did not match the pitch of the holes on the motherboard ('new capacitors are smaller and the separation is not the same'), causing the cap to perhaps split on insertion since the legs were being forced away from one another. Of course there's no need to push down so the body of the cap is flush with the PCB if the leg pitch is problematic.

  • Like 1

Share this post


Link to post
Share on other sites
Posted (edited)
15 minutes ago, flashjazzcat said:

It sounds to me like the pitch of the legs on the replacement did not match the pitch of the holes on the motherboard ('new capacitors are smaller and the separation is not the same'), causing the cap to perhaps split on insertion since the legs were being forced away from one another. Of course there's no need to push down so the body of the cap is flush with the PCB if the leg pitch is problematic.

I see, I just naturally bend the legs at about a half-centimeter from the bottom of the cap, if they are not at the same pitch, to match the pitch and make them parallel, before I ever try and insert them. Of course I have never tried to insert a cap so it is flush with the mobo, I always leave at least a half-centimeter between the bottom of the cap and the mobo, if there isn't enough head-room, I carefully bend the cap over, but that has rarely happened, and in fact, when I am working on a circuit board, I make sure all caps and anything else on legs are all standing as vertically straight as possible, especially if they are close to other components, so nothing is touching. More to make it all look nice and neat than anything else, but I like to make sure every component has as much breathing room as possible.

Edited by Gunstar

Share this post


Link to post
Share on other sites

Join the conversation

You can post now and register later. If you have an account, sign in now to post with your account.

Guest
Reply to this topic...

×   Pasted as rich text.   Paste as plain text instead

  Only 75 emoji are allowed.

×   Your link has been automatically embedded.   Display as a link instead

×   Your previous content has been restored.   Clear editor

×   You cannot paste images directly. Upload or insert images from URL.

Loading...

  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...