Broken Video Mixer Problem

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Broken Video Mixer Problem

Postby Lawnboy » Tue Jan 01, 2013 11:55 am

I am having an interesting problem with a broken video mixer that I bought myself for Christmas this year. Maybe someone here can offer me some clues as to what is wrong. So here is the story: I bought a Panasonic WJ-AVE5 (NTSC) digital video mixer with internal frame synchronizers on Ebay. The listing said that it was producing distorted video, and after some diagnostics I found that the sync trigger on the NTSC decoders are firing during the line. After talking with a friend with some equipment I got my hands on a proc amp, and by extending the sync on the video input signal I can get a stable picture. The output signal from the mixer is clean. All three of the inputs have the same problem, and the unit still had its warranty sticker intact, so no one had opened the case prior to me.

Here is what I am confused about:
Why is it set up this way? I have tried multiple video sources including a Panasonic camera with no luck. It seems to be a consumer-oriented device (i.e. RCA connectors, not BNC, no impedance switches on the back.) So I would assume it should accept a regular composite / S video signal. I can’t think of any special circumstances in a production chain that would require a different sync level set up, and I assume someone would have had it fixed if it was like this when new.

How can I fix it?
I have the service manual for the mixer, and out of all the adjustments that could be made to the analog signal section there was nothing to adjust the sync slice. I don’t know if anything else is out of adjustment (Y gain, etc) that would cause this behavior since I don’t have a true waveform monitor, only an ordinary oscilloscope.

That should be enough info. Any thoughts?
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Postby Steve Anderson » Tue Jan 01, 2013 1:05 pm

Without actually seeing exactly how the unit behaves, it could be triggering when the sub-carrier extends below black level, e.g. on colour bars. If you can somehow produce a mono 525 source (with or without the burst) that may give you a clue. Use a colour camera and a printed page of text or a monochrome picture as the subject, the sub-carrier level should be as good as zero. If the camera has auto white balance, use it when the subject is in place and focused

A first thought...

Steve A.

I downloaded the "Operating Instructions" for this (52 pages) and I note an error on page 51 where it mentions PAL colour standards (point 1)...it's clearly meant to be a NTSC device...so I assume this document is a cut-and-paste of the PAL version. The Service/Technical manual doesn't appear to be available for download.
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Postby Lawnboy » Wed Jan 02, 2013 4:45 am

Thanks for the quick response Steve. My proc amp has a color filter that also suppresses the burst and that doesn't seem to make a difference. Also I noticed that the distortion really depends on the content of the image. Not so much the overall brightness, but the content. A piece of white paper in the left 1/8th of the screen seems to cause the most disturbance in the color. It can range from a stable color picture with a few lines skewed to a black and white blur with complete loss of vertical sync.
I found the service manual somewhere on the internet. The scan of the diagrams aren’t that great, but its better than nothing. I can PM it to you or post it here, whichever you prefer.
Yes, I noticed the mention of PAL in the user manual as well, but this is definitely outputting an NTSC signal. That was one of my first fears.
I also included the waveform from the color bar generator (actually a camera with a “color bar” option) for reference, along with a few shots of the screen.

P.S. the title on the second image automatically pops up when the camera turns on. How festive.
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Color Bars.png
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Most stable image.JPG
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Postby Steve Anderson » Wed Jan 02, 2013 3:49 pm

Now I've seen the screen-shots it's not the sub-carrier. I would guess something is very wrong in the back-porch clamping area which is all part of the sync processing/separation circuit(s).

If you could PM the circuit of that part I'll have a look at it...no guarantees though!

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Postby Lawnboy » Wed Jan 02, 2013 5:28 pm

PM sent.
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Postby AncientBrit » Wed Jan 02, 2013 7:02 pm

Sounds like the 'sync separator' is responding to picture content rather than to true syncs.

So the Average Picture Level (APL) is upsetting the process.

Usually the sync sep is preceded by a clamp/diode DCR to ensure the bottom of syncs is at a known level, then the syncs are sliced at half amp point.

Check with a 'scope at the sync sep. that the DC level is not dependent on picture content.

Cheers,

Graham
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Postby Steve Anderson » Thu Jan 03, 2013 4:37 pm

OK, erm, it's a shame that the scan of the technical manual is so poor, not your fault I know, but in many places the circuits are illegible.

But first off I would see how the pulse (sync) train looks at pin 6 of IC209 and IC218. I assume these will be at some logic level and may be inverted, but there shouldn't be any spurious pulses during the active video period. And of course they should be stable.

If you have a dual-trace scope compare this point with the raw input video, there may be a delay but it should be consistent.

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Postby Lawnboy » Thu Jan 03, 2013 10:41 pm

Steve: I've already taken a look at the sync output (pin 28) of the M51271FP and it is firing randomly during the line, so I am assuming the problem occurs around that chip or before it. Unfortunately I only have a single trace scope. Also, which diagram were you referring to?

AncientBrit: Yes, I do believe APL has something to do with it, but its not totally dependent on the brightness of the video. I played with the exposure on the camera and the output image doesnt react in step with the changes in brightness.

I think my next step will be to try to find a waveform monitor, since my scope can't (to my knowledge) clamp to the sync pulse or blanking level, then I will try to adjust to the levels called for in the manual. I noticed that when the video is running through the proc amp and is stable, the colors are highly saturated, even when reducing the chroma level. I initially put this down to my late 80s to early 90s proc amp, but now I am not so sure. I'm starting to think that the mixer may not be adjusted correctly. I will play around with it again later today.
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Postby Steve Anderson » Fri Jan 04, 2013 12:33 am

Do you mean IC5 & IC6? They're the only chips that seem to correlate to your part number. (M51271FP). If so pin2 (from the fuzzy diagram) appears to be a DC bias input, rather than an output. Sync out appears to be on pin 28.

Diagram I'm referring to is page 31.

Steve A.

Ah, you do mean pin 28...this board for some reason interprets '(pin 28)' as a smiley...disable the damn things...
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Postby Lawnboy » Thu Jan 10, 2013 9:06 am

Guys, after some searching I found a better copy of the service manual here: http://elektrotanya.com/panasonic_wj-av ... nload.html I have also uploaded a few captures from it showing the circuits that are most likely the problem, specifically the sync section, which I circled. Also uploaded is a portion of the rear panel board, where the video comes in. Outputs 12 and 13 on the rear panel coincide to inputs 19 and 17 on the main board. Would changing the value of the divider on pin 27 of the IC help with the problem? Unfortunately all the components are SMD so it would be a tough fix but I would like to avoid having an external box to "correct" the video signal before it enters the mixer. The "Sep C" on pin 26 is also interesting. The datasheet for the M51271FP calls it a "Feedback capacity of sync separation", but other than that it is not much help.
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Postby Steve Anderson » Thu Jan 10, 2013 6:20 pm

I've downloaded the second version of the Service Manual which is somewhat better than the first.

I haven't been able to find a copy of the M51271FP datasheet, could you post it or PM it to me...even if it is a bit useless, there may be a clue in there somewhere.

R2 probably sets an internal current source (via R3) on pin 27, I don't see any problem with altering R2 to 'see what happens', but no less than say 22k...an increase should be no problem.

The R125 & C8 on pin 26 would seem to be some form of associated time-constant of about 18ms which is close to a vertical field period (for both NTSC and PAL variants)...I would leave them alone unless they are way off-value. The same may be true of R2 & C7...do all these components fitted match the schematic?

Steve A.

There also appears to be a discrepancy between the block diagram (page 30) and the detailed schematic (page 31). IC5/6 are shown as sync out on pins 23 & 24 and blanking in (also horizontal drive) on pin 28. Within the detailed schematic these are reversed. The M51271FP datasheet hopefully will clear that up. But it doesn't bode well for whoever put this thing together.
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Postby Lawnboy » Fri Jan 11, 2013 9:37 pm

So I was looking at the board this morning, trying to verify the values of the electrolytic cap C7, (1uf by the way) but couldn't tell because of its size and location, nor could I get a good reading on it from my multimeter. I then used a magnifying glass to try again to read the value, and noticed a very small dot of crystallized capacitor shmutz beneath it on the board. The identical cap on source 2 had the same issue, but all of the other caps around them looked fine. I think I may have found the problem. This weekend I will try to replace these caps and report back my findings.
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Postby Steve Anderson » Fri Jan 11, 2013 11:08 pm

Any 'magic mushrooms' growing amongst that fungi?

They don't usually put a desiccant in the original packaging for nothing...

SMD devices, especially small electrolytic capacitors, have far less protection from moisture than 'standard' variants...but if it lasts the warranty period, who cares?

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Postby Lawnboy » Tue Jan 15, 2013 9:09 am

Success! I replaced those caps and now it works! Not perfectly, but I'm hoping that a few adjustments will fix the minor sync variations. Thanks Steve very much for your help. Couldn't have done it without you.
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Postby Steve Anderson » Tue Jan 15, 2013 11:55 am

It may be worth replacing all the electrolytics in that area of the circuit, for both inputs. If they're of similar capacity/voltage there's a good chance they'll fail in the not too distant future...and you'd have to go through the same hassle again.

So I suggest you do it now while you have the thing in bits...it may also clear up the remaining sync errors.

Steve A.

Some years ago my S-VHS camcorder developed a fault, there was no way I was going to dive into the guts of this device so I sent it off to Panasonic for repair. Here, when you get something repaired they send back the defective parts to to you in a little plastic bag. Guess what? All (about 10) were SMD electrolytics.
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