Noisy video signal on Apple II clone

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Noisy video signal on Apple II clone

Hello,

I hope this exact problem was not discussed before. I tried the search function, but I am not able to limit the results to the Apple II forum.

 

I have a relatively good working Apple II: boots, loads software and games. However the video signal is bad, there is a lot of noise in the signal.

I am able to get picture on my monitor, but it is noisy (obviously) and has a bad contrast ratio between black an white.

 

I tried to diagnose the problem and attached an oscilloscope to the internal aux video connector:

 

On the left the sync level is visible, followed by a lot of  black and the APPLE II text.

As you can see, the noise level is significant high.  And there is a short glitch down to almost sync level between real sync pluse and the text.

I recorded the video-signal and the sync-signal as well (the color burst signal is inactive and almost noise free, therefore I skipped it).

 

 

I think most of the noise is introduced by this signal. The high level seems a bit low (3.6v).

 

 

The sync signal shows less noise and has a better high level of 4.1v. However the sync glitch is clearly visible.

Does somebody have an idea what might cause the noisy video signal. I traced the sync glitch a few years ago and I temporarily  fixed it by adding a delay. However, this did not improve the picture quality.

 

During my invesitgation I probed the ground level at different places of the board (top-bottom, near to far away):

The more distance between the probe ground and the probe, the more noise is visible.

I thought that it might be a problem of measuring high frequency signals over long distances.

However, the noise pattern resembles the pattern in the other signals and the video output. 

 

Does somebody have an idea how I should proceed from here? Has somebody reference mesurements to compare to?

With regards 

              berni79

 

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Addendum

I forgot to mention, I am using a ATX power supply and the voltages lock good (measured close to the conntector).

 

I was wondering, if the ceramic  caps on the mainboard could be faulty?

Replacing these would be difficult, because of the weak traces of this old mainboard. Is there any way to know without replacing them all.

 

I am grateful for every hint or opinion.

 

With regards

           berni79

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Probably something you've already tried...

Silly question: Have you tweaked the video signal with the onboard video trimpots? (assuming it's a clone of the II+, they're usually up the left hand side above the video out port)

 

Also, just to eliminate it, what's the picture like with no cards installed? (Mine is much worse with the FDC card in)

 

From where I'm looking, those signal lines are no more noisy than I'd be expecting.

 

Cheers!

 

Chesh

 

 

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It is worse than you think

Hi Chesh,

thank you for your reply.

I tweaked the video signal before, like you already guessed.  However, this affects mainly the brightness of the picture.

The contrast won't execeed a certain level (e.g. I may choose between white cursor on gray ground or gray cursor on dark gray background).

The oscilloskope was attached to the auxilary video port. The video gain trimpot does not influence this port, as far as I undestand.

I thought this simplifies the comparision to other Apple II systems.

 

Also, just to eliminate it, what's the picture like with no cards installed? (Mine is much worse with the FDC card in)

 

Sadly, I did all the tests without any additional cards...  Maybe I should remove the optional ram chips as well.  

 

Wtih regards

                berni

 

 

 

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As you observed in your last

As you observed in your last picture, the noise level will definitely depend upon how short your probe ground lead is and where it is attached. But since you can see the noise in the picture I would guess that there really is excessive noise in the composite signal. Just to be sure however, use a high quality video cable between the computer and monitor (not just some ordinary audio cable). And check the signal at the RCA jack, since there is a LP filter on there that is not present on the AUX video connector.

 

I would then start tracing the video output back through Q3, IC B2, IC A3, etc. I wouldn't worry about the glitch at this point and depending on your ground connection in the original pictures this does not look all that bad.

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Before you go overboard: What

Before you go overboard: What are you using as a display, and do you know the specifics on the clone itself? 

 

If you are using a CRT display and cannot adjust it to clear this, then it may be a transistor + cap on the mainboard failing. Some designs use caps to filtrate out noise and smooth the signal. You'd usually see a small cap with a 75 ohm resistor and a transistor to boost the signal in these sorts of video amp designs. Further, if the traces are too close to the earth plane, or the voltage rails, you can get noise from any of those. (This can also occur if the PSU itself is noisy and is sending that noise down its eatch path.)

 

 

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Hi Jeffmazur,thank you for

Hi Jeffmazur,

thank you for your response. I use a very good cable and did the measurement at the RCA jack. However, signal quality at jack and aux pin is approximately equal.

I checked the video signal, but could not find anything but signal degredation caused by long traces.

 

The video signal looks fine if I measure the signal at B2 directly. However, the same signal looks very noisy, if I measure at R7 (using ground close to R7).

The noise is visible after Q3 as well, therefore I consider it to be "real".  From there the noise  is propagated to the RCA jack.

 

I will try to regenerate VIDEO-DATA and SYNC signals using a TTL-IC close to the resistors and Q3 (a short test looked promising).

However, this is only a workaround and I really want to find the cause of the problem. 

 

With regards

                  berni

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Hi Timelord,thank you for

Hi Timelord,

thank you for your reply. 

I tried several signal converters which translate composite to vga or HDMI (one Extron which normally does an excelent job). Aditionally I tried with a TFT -TV with composite input. 

I know that a normal CRT would give better results, but I think the current picture quality is far worse than what is actually possible.

I will look for the filter cap and take some pictures of the board, which might help identify the clone.

 

With regards

            berni

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berni79 wrote:the same signal
berni79 wrote:

the same signal looks very noisy, if I measure at R7

  Which side of R7? The end connected to B2 pin 11? If so, then perhaps a bad trace? If not, then the noise must be coming from the sync or color burst gates. Also check the +5 at the collector of Q3.

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Would you be able to share a

Would you be able to share a picture of the the actual video output  on your monitor?

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jeffmazur wrote:berni79 wrote
jeffmazur wrote:
berni79 wrote:

the same signal looks very noisy, if I measure at R7

  Which side of R7? The end connected to B2 pin 11? If so, then perhaps a bad trace? If not, then the noise must be coming from the sync or color burst gates. Also check the +5 at the collector of Q3.

   

Hi Jeffmazur,

I measured R7 at the end connected to B2, sync is approxmately as noisy as the video signal (measured against ground close to the video connector). The +5V at Q3 seems to be noisy as well. I forgot to write down the value, I think it was +- 100 mV, but I will check again to be safe.

With regards

         berni

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berni79 wrote:Hi Timelord
berni79 wrote:

Hi Timelord,

thank you for your reply. 

I tried several signal converters which translate composite to vga or HDMI (one Extron which normally does an excelent job). Aditionally I tried with a TFT -TV with composite input. 

I know that a normal CRT would give better results, but I think the current picture quality is far worse than w

Please, please, try any CRT first. It can easily be an issue with your conversion box. 

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I would try...

Hi Timelord,

I would try a CRT, but I don't have one.

The reason for the investigation is not the picture quality itself. I experimented successfully with a simple circuit, which uses TTL sync and  TTL video to display a nice BW picture.

I am worried that there might be a fault that someday destroys the Apple for good. 

In other words, I am happy, if somebody tells me that these measured video signals are normal for a Apple II.

 

With regards

         berni 79

 

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berni79 wrote:Hi Timelord,I
berni79 wrote:

Hi Timelord,

I would try a CRT, but I don't have one.

The reason for the investigation is not the picture quality itself. I experimented successfully with a simple circuit, which uses TTL sync and  TTL video to display a nice BW picture.

I am worried that there might be a fault that someday destroys the Apple for good. 

In other words, I am happy, if somebody t

 

Are you in a PAL or an NTSC region? The Apple II Pseduo-PAL spec is something that a lot of decoders or upscalers will have trouble reading, in my experience. If it is trying to output a signal compatible with PAL CRTs, it is going to cause a lot of issues as the signal is not 100% PAL compliant. 

 

 

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How the video looks on my TV

 

Something went wrong, please see post below...

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How the video looks on my TV
tony359 wrote:

Would you be able to share a picture of the the actual video output  on your monitor?

Hi Tony359,

this is how the video signal  looks on my TV.

The image actually looks worse in person.

The sync glitch is clearly visible. It is the vertical black line, the only part of the image which reaches actual black levels.

This is the reason why I temporaily fixed this glitch a while ago. However, this removed only the vertical line and did not affect the overall black level of the picture. Therefore, I did not make it a permanent fix.

 

Here is a short video, which shows the noisyness better than the picture:

 

With regards

              berni79

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Timelord wrote:berni79 wrote
Timelord wrote:
berni79 wrote:

Hi Timelord,

I would try a CRT, but I don't have one.

The reason for the investigation is not the picture quality itself. I experimented successfully with a simple circuit, which uses TTL sync and  TTL video to display a nice BW picture.

I am worried that there might be a fault that someday destroys the Apple for good. 

<

 

Hi Timelord,

PAL or NTSC both give the same result. First the computer was configured as NTSC, then I changed it to PAL (replaced oscillator and changed jumpers).

I wanted to try PAL and a PAL color card, because I throught my video converters might have less problems with PAL (since I live in Europe).

However, the PAL color card does not work at all and the black and white image (without color card), just looks like the NTSC image.

Therfore, I will change the computer back to NTSC.

 

With regards

               berni79

 

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The line seems to be where

The line seems to be where the picture should end, right? The cursor should be at the leftmost end of the screen. The picture is a bit wobbly but to be honest I do not think the one generated by my Apple //e on an LCD monitor is any better? Particularly when the output is set to colour.

 

I'm not sure why your black level is incorrect. My Apple //e PAL version has two potentiometers which change the red/blue content of black. Not sure if yours have such control?

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That line is normal. On a CRT

That line is normal. On a CRT is is outside the visible image on the overscan area.

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Glitch

Not outside the visible area. But negative going from black so it would normally not be seen if the monitor black level was wet correctly.

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Just FYI, this is what my

Just FYI, this is what my Apple II/II+ machines look like when attacked to an LCD- it just doesn't handle it and looks like crap. The pre-RFI boards may look worse than the RFI, but I think none look "good".  It's just not anything close to a standard signal they are expecting to decode.

 

They are fine using old CRT displays.

 

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