Film-Tech Forum ARCHIVE: Missing right channel on analog (sometimes)

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Posted by James Waite (Member # 4536) on 03-27-2010, 07:13 PM:

I had a problem last night and was wondering if anyone could give me some advice as to what to check? The sound for the film suddenly dropped down to what sounded like just the left channel. I went into the booth and checked the CP 65 and from the LEDs it looked like it was only getting the left channel from the projector. The indicators as to which channel were on where all dark except the left and one of the two green LED at the very left was dark which I believe means it was getting no signal from the right channel.

It seems to be sporadic, I noticed a couple times over the last couple months the sound seemed to drop out for a second during trailers now and then. Last night it wasn't working at the beginning of the first show but when I clicked off the DTS during the credits it seemed to be back on. Then it was off again during the trailers for the second show.

Luckily we have the DTS discs for both films, although one of the discs for the first film must be damaged as it kept kicking on and off for the first 45 minutes, after that the DTS worked fine for the late show and a rental this morning. I put on a separate questions about one odd thing I noticed.

I cleaned the red LED and the 'solar cell' or whatever it's called now in case that was the problem. is there anything else that's a common cause of losing one channel? As you can probably tell I

Posted by Manny Montes (Member # 5639) on 03-27-2010, 07:36 PM:

The first thing I would check is the actual film, does the soundtrack look damaged?

Did you try running the film with the processor in bypass?

Posted by James Waite (Member # 4536) on 03-27-2010, 10:17 PM:

I don't think it's the film as it was both most of one movie and at least one of the trailers (Iron Man 2 brand new) on the other film.
I tried running in bypass when everything was working just to see what changed, sounded a bit different was about all. So far the only times I've had problems I've had audiences in the theatre and been limited as to how much I could play with things.

BTW, I forgot to mention in my first post another thing I'd noticed. Over the last while the soundhead would sometimes make noises when it was touched while I was cleaning the projector or rethreading. This was with the exiter LED off! It sorted sounded like something was holding a charge and it was being released by contact- static maybe? By soundhead I mean the actual black arm that holds the 'solar cell'.

Posted by Chris Slycord (Member # 4239) on 03-27-2010, 10:30 PM:

quote: James Waite
I tried running in bypass when everything was working just to see what changed, sounded a bit different was about all.

What does sounding a bit different mean, exactly? Does it mean the problem went away or not?

Posted by Stephen Furley (Member # 1277) on 03-28-2010, 12:14 AM:

quote: James Waite
one of the two green LED at the very left was dark which I believe means it was getting no signal from the right channel.

Can you confirm that these LEDs are on the Cat. 240A optical pre-amp card? This card has two LEDs which indicate signal present on the Lt and Rt channels; the Lt one is just above, and the Rt one just below, the centre of the card. If you're getting nothing on Rt at this point then it rules out everything after the pre-amp card. In general, the signal present LEDs on Dolby cards indicate that the signal is there at the input to that card, so the fault is before that card, but in the case of the pre-amp the fault could be on that card itself.

The far left slot in a CP-65 should hold a spare pre-amp card, since the processor will not work even in by-pass mode if this card is faulty. This spare card will have no LEDs lit; the next slot contains the active pre-amp card. Of course, it's possible that the spare pre-amp card is missing. If it is there, or if you have a spare card in another processor, power down and swap the cards. If you still have no signal on Rt, which will probably be the case, then the fault is with the soundhead light source, the cell, or the connections between the cell and the processor.

If swapping to the spare pre-amp does happen to cure the problem then you'll need to check, and possibly adjust, the Dolby levels. If you use a pre-amp from another processor then you should have the A-chain alignment checked, but you may not have the equipment to do this yourself.

You don't say if you're running one projector, or a pair, and if a pair, whether both machines are affected.

Posted by Jarret Chessell (Member # 5077) on 03-28-2010, 12:20 AM:

It sounds like it could be a few things. Shoot me an email if you want me to check it out and scope your A-chain.

Posted by Louis Bornwasser (Member # 3063) on 03-28-2010, 07:04 AM:

It could be either the Dolby preamp card or a failure in one channel of the led reader assy. (It would help to know what brand you have. Older Kelmar units could short-to-ground through the wire insulation. Sharp wires on bottom of board.) Louis

Posted by Demetris Thoupis (Member # 833) on 03-28-2010, 07:50 AM:

This is a case where you learn to appreciate the BACP reverse scan reader due to the onboard indication of signal presence on each channel!! What reverse scan reader you have? Try swapping the left/right channel from the reverse scan reader to see if the problem swaps. If it swaps its the reader. If it does not swap its the cabling.
Demetris