Disabling Dolby Cat 702 LED when film not running

[David Ferguson]

In an effort to save LED life, I want to have all my sound exciter lamps only on when film is actually running in the projector. My projector (Kinoton FP 38 EC) has a run relay, which should make it all possible. The DTS reader was easy enough - just wire the LED power pin (pin 5 on the d-sub 9 pin cable) through the run relay, and now it only lights whenever film is running.

I now want to do the same for my Cat 702 reader. If this was the 699/700/701 it would be straight forward, because they all have separate power supplies for the exciter LED, but the 702 has the LED powered off the video cable. I see there are the following wires in the Dolby Digital video cable:

1. Video 0
2. GND
3. +15V
4. Video 1
5. -15V
6. GND
7. TTC
8. FTC
9. TC GND
10. TSC
11. SC GND
12. FSC

The +15V/-15V sound promising... but I don't want to do anything that could damage the sensitive CCD in the reader. I've heard that unplugging/replugging the reader cable from the DA20 whilst it is powered up is the kiss of death - but that's sort of what I want to achieve here!

Anyone done anything like this, or can help with this?

[Steve Guttag]

I never did it on a CAT702 but did on a BACP reader. I just put a switch on the wire feeding the LED itself, at the reader. I would suspect that is doable on the CAT702 as well.

[Jim Cassedy]

The original CAT-700 DolbyDigital™ readers used a halogen reflector bulb, ( Type EPT )
similar to what was used in many old slide projectors, and there was actually a switch on
the reader head to turn them off when not in use. I've always wished they had put switches
on all the subsequent penthouse readers to kill the LED's when not needed. There' still a
couple of these 700's in use at one of the film festival venues I work at, so I always keep
a spare bulb in my 'bag of tricks' . Dolby used to charge over $50 for them, but now you
can find them online for appx $10 (US)

Although I still get to run alotta film, most prints are older titles that pre-date DD and are
often not even SR. Just either straight mono or pre-1986 "Dolby Stereo" prints. So to save
my 702 LED's, I physically disconnect them from the CP-650 (yes, with the power off)
whenever I'm running anything other than D-D. ( Mono, Dolby-A, 70 or 35 magnetic sound
or 70mm DTS) My 35mm analog readers are hooked up through a motor relay and so the
LED's only turn on when one of the projectors is running, so during extended 70mm runs,
such as "Licorice Pizza" or "Oppenheimer", etc, I open the back of the console & unplug
the AC cord to the Kelmar boxes that power the LED's & optical pre-amps. I figure why
burn the LED's if I don't need to.

[David Ferguson]

[QUOTE=Jim Cassedy;n32835]The original CAT-700 DolbyDigital™ readers used a halogen reflector bulb/QUOTE]I actually had a cat. 700 originally - but it got stolen in the post! And I couldn't get a replacement, so had to go to a cat. 702 instead. A shame, as I much preferred the easily available and replaceable bulb!

So to save
my 702 LED's, I physically disconnect them from the CP-650 (yes, with the power off)
whenever I'm running anything other than D-D.

I have my 702 hooked up to a separate DA20, which then goes into the CP500 (yes, this does mean more converting between digital <-> analog, but that's not an issue right now). Then, I only power the DA20 on when I'm going to be running Dolby Digital film. Works well, and means it's just one extra switch to flick if I'm running Dolby Digital, no fiddling round with connectors. But I'm now looking to further optimise it by only lighting the LED when the projector is running.

Thanks Steve for the mention of actually putting a switch/relay inside the 702 - I'll have a look inside it and see how feasible that is.

[Jim Cassedy]

I actually had a cat. 700 originally - but it got stolen in the post! And I couldn't get a
replacement, so had to go to a cat. 702 instead. A shame, as I much preferred
the easily available and replaceable bulb!

Yes it is ironic that the light source in the 700's is actually almost easier to find and replace
than an LED these days! The 700's did get pretty hot- - too hot to hold your hand on, thanks
to the halogen bulb, but in actual use, you wouldn't be holding your hand on it anyway!

[Sam Chavez]

It is ironic that it's easier and cheaper to change an EPT than an LED. Not all technology improvement is painless or cheap. We do still have stock of EPT's and DPN 32075.