Reparing a vintage JVC KY-310 ENG Saticon tube camera

[SiO2]

Hi nerds,

recently I acquired a wonderful JVC KY-310 camera.

It's a heavy beast from the early '80s, almost entirely made from aluminium and was sitting in a basement for many many years before I got hold of it. The camera is from the pre-CCD era and instead uses 3 Saticon CRT tube and a dichroic prism to capture the picture in its RGB components. The tube/prism assembly looks almost like the flux capacitor .
Lots of analog electronics, neatly organized in 10 plug-in cards with tons of labeled (!) trimmers, test points etc. An absolute dream for maintenance!

So, what's the problem? First of all, this camera relic is primarily a showpiece for me. But, I'd also like to understand what it was like to use such a thing. Comet tails, not pointing the lens at bright objects to prevent burn-in etc. seems interesting  for me to experience first hand .

The camera mostly works. At least for the red and blue channel that is. Green, not so much. This channel flickers like crazy at 5-10 Hz with a vertical gradient: https://youtu.be/BVXw1Lz7ngg.
I can set the camera into registration mode (switches behind the door at the lower right side), which is meant to electrically align the three tubes in terms of horizontal and vertical shift. You can activate one at a time and overlay them as you which. Red and blue look pretty good, the problem is only the green channel.

Now what I tried so far is to start at the source and swap the signal output cables from the green and red tubes. With the tube output swapped, still the green output channel flickers. So I assume, the problem is not the tube itself, but the electronics.

And the issues must be located somewhere between the tube and the composite video encoder, because the camera can output a clean color bar signal.

But where to start debugging? I have the comprehensive service manual, but I don't want to mess with the trimmers and hope that anyone of you might be able to push me in the right direction. Maybe even someone has experience with tube cameras .

Cheers,
Marco

P.S.: Here is a nice photo gallery I found, featuring this model as well: https://www.engcameracollection.com/jvc-1ky-310e

[Renate]

I remember that JVC red/orange.
Visually, it's hard to tell what's going on.
A scope on the green channel at V rate will tell you more.
Blanking/clamping problem and a DC offset? Who knows?
With the lens cap on does it still wobble?

[james_s]

That's neat, I remember my dad had an early consumer video camera, it used a single vidicon tube and the recorder part was a separate section in a bag with a shoulder strap. Lights would leave green trails on that camera too.

I don't know enough about these to suggest how to fix it but I do think your assessment of the green tube being ok is correct. Can you find a service manual? Given the era and the fact that looks like a semi-professional camera I would expect a real service manual to exist.

[SiO2]

Thanks for breaking the ice

A scope on the green channel at V rate will tell you more.
Blanking/clamping problem and a DC offset? Who knows?
With the lens cap on does it still wobble?

Indeed, the scope reveals some fluctuating amplitude/gain on the green signal at the PR2 (video process board) and EN (composite encoder) stages. So at least now we can assume the issue originates from the first 6 of 10 boards .

It still wobbles with the lens cap on, yes. Then the control monitor screen flashes only in green. Could this mean, it's not a gain but kind of an offset issue?

I don't know enough about these to suggest how to fix it but I do think your assessment of the green tube being ok is correct. Can you find a service manual? Given the era and the fact that looks like a semi-professional camera I would expect a real service manual to exist.

We are lucky: https://freeservicemanuals.info/en/servicemanuals/viewmanual/JVC/KY320/ This camera is very similar. The manual is really detailed, but looks a bit intimidating to a video newbie.

The instructions make heavy use of test patterns. I hope they can be found online and I should print them to be able to see something similar on the scope.

[Renate]

I get seasick looking at that 2nd capture!
I don't know how many clamps are in the signal path, but at least one has a big problem.
Are there channel test points on the card edges?
Can you give us a shot of the board chassis?
Are there any switches like "gamma off" or calibrate signals (besides color bars)?
I couldn't download the manual.

[SiO2]

Thanks for the input

I get seasick looking at that 2nd capture!
I don't know how many clamps are in the signal path, but at least one has a big problem.
Are there channel test points on the card edges?
Can you give us a shot of the board chassis?
Are there any switches like "gamma off" or calibrate signals (besides color bars)?

That's a good advice. I'll try to check the clamp transistors in the signal path on the PR1 and PR2 cards. Test points are either on the card edge where they can be accessed while the card is in place, more test pins are located on the card PCBs. Those could only be observed with thin test leads, but it's possible.
The camera's electronics only lives on theses cards and everything is accessible from one side after you take off the side panel. That's really neat design! The backplane behind the cards is a passive routing board.

I couldn't find many additional switches. There are gamma and matrix switches on one of the boards (PR2), but this doesn't make a noticeable difference at this point. Could however exclude issues in theses circuits?

I couldn't download the manual.

The download is hidden behind the model name... why? 

[Renate]

Normally, you'd have a card extender so that you have full access to a card while in operation.

[SiO2]

Normally, you'd have a card extender so that you have full access to a card while in operation.

Yes that'd be ideal! Maybe it'll be enough to probe at least the connector pins from the backside of the backplane. I'll try to poke around a bit. An extension card would be easy to make if the connectors are somewhat  standard. But given the market segment, it might be quite possible.

[Renate]

Sorry. I'm in the poor internet neighborhood (20kb/sec) I'll try that later.

[SiO2]

Hooray! After figuring out that the green ("G INPUT, pin 37 in the schematics) video signal distortion appears only after the input coupling capacitor C201 of the second processor board (PR2), I guessed and removed the JFET Q201 from the signal path, that looks like it's involved in horizontal blanking. Now, I have to measure the transistor and figure out if itself is the issue or the rest of the circuit, connected to its source.

Either way,  the picture now is perfectly stable! There's still a lot of adjustments to make, but that's a very encouraging step  .

[SiO2]

No comment

[SiO2]

Well, after replacing 12 bipolar capacitors across 3 boards, the image is stable (with the transistor back in place).
But now the green channel is so dominant, that the other channels have maybe 10% intensity in comparison, resulting in a ... green picture

Next step would be to check the auto-gain stages etc. but how to tell if the green is too hot or the other ones are too dim?

[james_s]

Does the service manual have setup and alignment procedures? If you found several bad capacitors on that board, I would first look for any more similar capacitors, there's a good chance they are either bad or will be soon.

[Renate]

It's possible that somebody twiddled with it when the green was out.

[SiO2]

Does the service manual have setup and alignment procedures? If you found several bad capacitors on that board, I would first look for any more similar capacitors, there's a good chance they are either bad or will be soon.

Yes it does, but indeed I found another round of bad bipolar caps. This time in the tube preamp section . They are out now and once again it smells like fish. I need fresh air. To be continued

[james_s]

The prudent thing to do if you haven't already is take a close look at all of the boards in there and replace any more electrolytic capacitors you find, especially ones of the type you have already replaced. You've found multiple bad ones and the rest can be assumed to be the same age and have lived in the same conditions, if they aren't bad already they are bound to fail soon.

[SiO2]

You are certainly right! All those electrolytic caps should be way past their best days. I measured a few of the polarized ones and they were ok, even in terms of ESR. And most important, the polarized caps have not leaked yet (half of them are Tantalum types btw.). But all the NP caps grew blue crystals on their legs.

When the input stage is equipped with new caps and if this fixes the color imbalance, I'll feel motivated to do a complete overhaul. The glimpse at the surprisingly bright and clear image on the green channel was promising .

[james_s]

Tantalum caps are pretty reliable, I would not advise messing with those unless they are found to be faulty. The only capacitors I sometimes replace preemptively are wet electrolytic.