Apr 20 2010

Camera diagnostics, part 1

Always knew getting the service manual would be a good move.

Dug out the old JVC GR-S707’s. They’re a pair of SVHS-C semi-professional camcorders I picked up off ebay years back. One worked fine, only needing a new microphone muff. This came with me all the way to the USA to do some filming at a convention, and damn near broke my spine in the process.
The other was acquired a couple of years after that and turned out to be actually broken, as opposed to a little temperamental in high humidity. I set it aside as a parts camera, or something to try and fix later. It now may be “later”.

I still haven’t tested the capture rig, but will be in the next few days. I was actually looking for a lens this evening when I dug out the spares/repair cam. I had a couple of ideas though, and tried them out quickly.

The viewfinder displays a heavily distorted and rolling image when filming or on playback. The spares kit included all the proprietary cables, so I tried hooking it up to the capture card. Same result there, only in colour. Very heavy blue distortion.
Noticed however the viewfinder and AV output share the same connector type. May mean I can fit one or the other with a colour viewfinder from a different camera at some point. I think I have an even more elderly camcorder with a colour tube.
Found a tape to test in camera. Noticeably worse playback than on dedicated playback unit.
So here’s the interesting bit: PLAYBACK on the damaged camera works fine. However recording is badly affected.

A/V board eliminated. Viewfinder eliminated. Playback eliminated.

Now, from the diagrams it APPEARS that the viewfinder connects to the CPU board. Probably since on-screen info can be included on the video and the viewfinder separately, so that’s done there.

Pain the bum is a lot of these block diagrams had sections in green, and they were scanned in black and white for the PDF, so large portions are near-illegible grey smears.

Still, that both outputs from the CPU board are effected, including the portions that should be different for each output, says it’s either the CPU board itself, or the CPU board’s trying to fit the on-screen data to corrupted video. And since I can’t see how the CPU could corrupt the video feed as well, we move further down the line.

The overall wiring block diagram has the CPU connect to the THD (no explanation of abbreviations included for PCBs). Schematic diagrams show this to be motor and sensor control for the optics themselves as well as pass-through for video and character data, to the Video board.
The video board seems to have tonnes of lovely little trim-pots, any of which might have gotten out of whack. Two are named “Sync level” and “1H Delayed sig level”, both of which sound to me like they could be the cause of this odd issue. It does seem to be some sort of timing issue, afterall (and maybe more, if the blue tint isn’t an associated effect).

That said, the grip on the camera has always been.. sticky. As in coated with something. It could be something leaked down inside the camera through the zoom controls and onto the PCBs under it, altering component values.

In any case, opening it up and giving it a clean-out would seem in it’s best interests.

Ah, forgot I also bought an original non-PDF copy of the service manual. PCB diagrams are still in greyscale, but in much more detail. Schematics are detailed in colour. Indications of binder-holes though suggest either isn’t (wholly) original itself, or JVC actually sent out service manuals that were photocopied from an original.

Mind you, nice that it even goes into detailing the functions of the pins of each IC used, including part numbers. If I should ever need to make a new CPU board for one of these, it’s nice to know it’s only cost me $300 in processor chips alone. :P

[20/06/2010: Amalgamating old posts from “Dreamwidth Creative Blog” into sci-fi-fox.com to re-purpose DW blog account.]

Alibi3col theme by Themocracy