Scanner dilemma : What's the best move ?

[Mark Wesley]

Thank you for your kind offer Stan, but I don't want to put you through that exercise unnecessarily (for now). The capacitors and MOSFET have been ordered and I get to sit back and wait.

If you notice at the bottom of the board, there is a connector labeled 'VID' which routes the camera composite video signal to U4, which is a sync separator. My guess is the output pulse goes to the 555 and then to U6 (NTE859 quad op amp) which drives the U7 MOSFET. Thankfully, all of these IC's are still available.

[Stan Jelavic]

NP Mark. Your explanation makes sense. The vertical sync separator provides the trigger to the LED. My guess is that the supply does not have enough current capability so a cap gets charged during the vertical frame inactive interval and then fires during cam exposure. One puzzling thing is that the LED does fire once. That means the cap in the out is ok. Potentially the LED fires during initial sequence because the cap is charged and the 555 sends the trigger due to the power up ramp but is missing the sync pulses from the separator. Possibly the sync separator does not work with the cam that you have.

[Mark Wesley]

So I received and replaced the filter capacitors and still no go. Just for chuckles, I hooked up the original Hitachi camera and it started the projector lamp to working again. It turns out that the Canon HV20 AV output jack is wired differently than the standard 3.5mm yellow/white/red video cable. There were lots of comments on Amazon about this very subject. I found an original Canon cable on eBay and voila, she works !!

Never assume anything ... 💀

[Stan Jelavic]

Yeah I thought it was not the caps because it fired once... but it is a good first step... good work.