[Solved] JVC KY-1900 Triple Saticon Camera: blue channel is feeling blue

[akasaka]

It's been only a few days since I've finished torturing you guys with my previous riddle, and now I've picked up a bunch of old Saticon based cameras from a friend. Guess that's what funemployment does to you keeping your hands open!

The set consisted of a more-or-less working JVC GX-S700, a portable VTR+tuner dock combo, and a JVC KY-1900. The VTR wouldn't turn on — I suspected capacitors and undervoltage, turns out one of the power transistors in the tuner's stabilizer went, and the VTR received 38V where it expected 12... Swapped that out with some junk power transistors I've had lying around and it's all now funky. Granted I have no use for it and will make an adapter to record video directly into my cell phone later down the line.


Now, onto the main star of the show... The JVC/Victor KY-1900.



On day 1, it powered up just fine, however, there was no blue channel whatsoever. Tube reconnections ensued and a few anode zaps to my finger later, I confirm the blue tube is well alive. In the end I come down to the fact the fault is located in the Process Board. This little dense thing here:



So, my first thing was, of course, to check the inputs and outputs of the HA11732 (https://datasheetspdf.com/mobile/499542/Renesas/HA11732/1). Sure, the blue one has input but no output, so I promptly swapped around the blue and green ones. Surprise, the fault remained in the blue channel.

Now, I don't exactly remember the diagnostics steps I took — first I swapped all electrolytics in vicinity, even though they looked good, and measured fine (as much as my DMM allows — no ESR or leakage, only capacitance). No dice.

I've measured around the chips, and found out that pin 1 of the blue HA11732 is at around 1V, which is too low. The others are at around 4V. Probing around for shorts and things, I've noticed that between pin 1 and ground my DMM goes "bip..bip..bip.." as if there is "almost continuity". On the other color circuits, it doesn't do that. So I swapped the "blue droplet" capacitor (what are those called?), 1uF, for a 6.5uF one, as that was the closest of this type I've had on hand.

Start up again, seemingly no dice — but now the blue channel is constantly filled with color.

Then I've noticed that turning the black level trimmer on the blue channel (the closest to the stripe on the HA11732) to the minimum position makes a faint, noisy image appear. Respectively measuring the pin 1 when turning the black level trimmer confirms — in the minimum position it's at the "normal" voltage of 4.2V, but any position beyond that makes it skyrocket to 8V. On others, the trimmer moves the voltage in small increments from around 4 to around 6V. Additionally, the Knee and Clamp adjustments don't make a difference on the blue circuit.

I thought, maybe an opamp is wacky — one of them was getting pretty toasty when the blue channel is "filled", and the pins on it look rusted. Swapped them around, no dice.

At this point I've also taken out and checked most transistors in vicinity (see white dots on the photo) — all are "fine" (not shorted, not open, but can't test further). Ceramics and the green (what are those called too?) caps are seemingly OK as well. I didn't bother with the resistors for they are too tiny. Diodes are OK too. And of course I washed and cleaned all the trimpots, so they output super smooth with no crackle (tested on a scope).

The TA chips are the input amp, so that stage is fine (and swapping them around makes no difference). The TC chips are logic, and are for the color bar generator, not the primary video circuit.

I feel like I'm missing something very simple here. Does anyone have experience with such cameras, or maybe the circuit diagrams or service manuals? Or maybe someone has seen those HA11732 chips in a similar setting previously?

So far I've tried to dial in the rest of the calibration with the current state of things, but it looks very sad and far from what this camera can actually do:



(also you can clearly see on the spectroscope how the blue channel is clamped down)

Calling for help to the wizards of the tech past! Thanks a lot in advance for your help.

[akasaka]

As a bonus, here's the other camera and it's underexposed light-hungry single-Saticon picture sample:






And a riddle from the JVC engineers that I ran into while recapping the orange one. Took me a few tries to spot that one. Can you see it?

[m k]

Should you change all blue ones?

It's like that one has originally been bent outwards.

[akasaka]

So far I have learned the blue droplets are tantalum capacitors. I've found two nearly shorted ones in the pin 1 circuit, and another problem was a loose strand of desoldering braid ending up somewhere on the board when replacing them.
The rest seemed fine though, as per my DMM.

So now I cannot still adjust the black level, but at least I got some workable picture.

Also a very kind person behind this website scanned and uploaded a copy of the schematics and service manual for me: https://freeservicemanuals.info/en/servicemanuals/viewmanual/JVC/KY1900E/

So whenever I get around to fixing this camera again, this will be very useful. But for now I'll just use it as is I guess...

[akasaka]

OK, the schematic helped find the final fault... and it was in plain sight all along, a broken trimpot 

In total the following faults:
* C37: failed near-short, 2.7 (apparently those are tantalum caps and they tend to fail short rather than open)
* C40: same failure
* R76: one leg rotten and fell off, so spinning it further than the minimum value caused a disconnection and the black level went crazy... So I just spent the past 1.5 weeks hunting this, because originally I took the pot out and washed it and it seemed to measure fine, so I thought it's out of question.

Never be too sure I guess!

But now I can use the camera again, once I calibrate the color balance and all.