Film-Tech Forum ARCHIVE: Hum in CP-45

This is topic Hum in CP-45 in forum Film Handlers' Forum at Film-Tech Forum ARCHIVE.

To visit this topic, use this URL:
https://ft-forum.com/ft/cgi-bin/ubb/ultimatebb.cgi?ubb=get_topic;f=1;t=003582

Posted by Manny Knowles (Member # 1171) on 06-14-2002, 05:40 PM:

My brother (the manager) called me today to say that one of his theatres has a hum.

Interesting...because when I ran Dolby Tone in that theatre yesterday I made a note that Left and Right trimpots were INTERACTING!

Raising Left also raised Right and vice-versa.

I've got an hour to spare while waiting for the current show to end. What should I check first?

I'm willing to bet that the solar cell isn't wired in to the processor correctly.

This is a Dolby CP-45 and a Kelmar LED Reverse-Scan (Analog only) in a Westar (Century R-3).

Posted by Paul G. Thompson (Member # 655) on 06-14-2002, 07:11 PM:

Check your solar cell wiring closely. Also make sure the common wire on the solar cell is not touching ground. That could be causing the hum.

As far as interacting is concerned, I would have a tendency to think the cell common wire might be broken or improperly connected somewhere.

An ohm meter will be very helpful to have and use.

Posted by Gordon McLeod (Member # 33) on 06-14-2002, 07:40 PM:

On the CP45 you must be in stereo mode to set the level not mono like there other analogue processors or you will have interaction
Since it is the pots that are interacting and that is in the output of the preamp it sort of leaves one in the mode that the cell is okay and it is electronic issue

Posted by Manny Knowles (Member # 1171) on 06-14-2002, 08:04 PM:

Okay...

I went over and had a look-see during intermission and here's what I observed...

Solar cell wiring (Kelmar PC 7929 AT-1200 LED Kit) is as follows...

1 L+
2 Earth (jumpered to #5)
3 L-
4 R+
5 Earth (jumpered from #2)
6 R-

On the D9 connection to the CP-45 Projector 1 Input the connections for L+ L- R+ R- are in the right pins. There is no shield wire attached to the body of the D9 connector, and the Earth connections don't terminate anywhere on the processor input.

FWIW this D9 connector isn't like the others we are using. The others I've seen all look like a breakout card and they use L+ E L- R+ E R- so this one is different. Still, it seems to be wired up correctly (with the exception of the shield wire not being soldered to the body of the connector).

Also...what do you make of jumpering the Earth connections on the solar cell? From what I can tell, it's not actually connecting to ground anyway...that connection is only happening on the solar cell side.

When running Dolby Tone, the little LED's on the SR/A card start dancing around and freaking out...they don't give any steadiness. I suppose I could try swapping out boards to eliminate the circuitry.

Also, the hum is more of a buzz appearing only in the left channel. I've heard DTS timecode before and it was a lower pitch than this buzz that I'm hearing. DTS was more of a motorboating sound and it tended to remain constant (i.e. you'd hear it throughout the movie). This noise is more of a high-freq razzing sound that comes and goes.

Want to hear this sound? Smile with your lips parted. Teeth together, move the tongue away from them and make the sound of the letter "Z" in short spurts and you'll probably hear what I'm hearing.

Note: Playing the movie in MONO seems to get rid of the problem as a temporary work-around.

Posted by Steve Guttag (Member # 268) on 06-14-2002, 10:23 PM:

First off...loose the jumper on the reader board. Now...where does the shield wire actually connect to anything? If it is just at the reader end, then that isn't good enough. The shield wire MUST terminate either on the body of the DB9 connector or on pins 3, 6, or 9 on the DB9. Ideally, the shield wire should float on the cell end (to provide the best electrostatic shield). There should be an additional wire run between the reader "E" terminal and the processor "E" terminal to provide not only a ground to it but the actual cinema sound system ground.

In those silly countries that require that the shield be tied at both ends for RFI reasons..then the shield should only be connected directly at the processor end, at the reader end, connect a capacitor between the shield and the reader "E" terminal.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Manny Knowles (Member # 1171) on 06-14-2002, 10:28 PM:

Steve:

Presently the shield is going to Earth (#5 on the cell) and that's what is jumpered at the moment to #2.

The shield is floating on the processor side

Posted by Steve Guttag (Member # 268) on 06-14-2002, 10:30 PM:

Well that is just plain wrong....try again!

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Manny Knowles (Member # 1171) on 06-14-2002, 10:34 PM:

Steve:

Please double-check this for me and let me know if this is correct/incorrect...

LED CELL >>> DB9 ON PROCESSOR

#1 Left + >>> Pin #1
#2 Left E >>> Pin #???
#3 Left - >>> Pin #2
#4 Right + >>> Pin #4
#5 Leave Open (Earth, same as LED terminal #2)
#6 Right - >>> Pin #5

Shield Wire floats on LED end >>> Pin #6 (Earth) on DB9

Ground the processor/rack with a brass lug...can I make one if I can't buy one here buy using a brass nut & bolt?

~Manny

"Don't forget to drink your...Ovaltine!?"

--Ralphie, "A Christmas Story"

Posted by Steve Guttag (Member # 268) on 06-15-2002, 07:52 AM:

You got it right....except use a copper lug, not brass. Any electrical supply house will have them. The Lug is really a tie point...you will need to properly ground the lug and then use that point to ground the rest of your equipment.

Steve

------------------
"Old projectionists never die, they just changeover!"

Posted by Manny Knowles (Member # 1171) on 06-15-2002, 11:01 AM:

Okay...It is solved.

The hum was triggered by the platter motors. Yikes!

Rewiring the solar cell to the P1 DB9 (and grounding) per Steve's directions took that hum away.

Posted by Paul G. Thompson (Member # 655) on 06-15-2002, 12:20 PM:

Steve....sometime I should invite you to my radio station and fix those hums that come and go just like the rain.

I got most of the ground loops out of the audio, but once in a while they come back and plague me when someone plugs in a little space heater to warm their feet in the sales office. For a while, I thought our call letters were KHUM instead of KAPS!

One of my biggest nightmares has always been keeping the KAPS rf out of KBRC's audio. But when the weather changes, the RF says, "I'm back!"

Nothing like listening to Paul Harvey's "The rest of the story" super-imposed on top of Elvis Presley.

Our building is old. So old that in some areas, it was wired with Romex without a ground. How in the hell can I deal with that without spending a king's ransom to cure? We even had an electrical contractor come in once, and when he walked away, he shook is head in dis-belief.

Posted by Manny Knowles (Member # 1171) on 06-17-2002, 07:24 PM:

Guess what? I did a random check and the very first system I inspected at another location was also wired like this one that had the hum. I couldn't hear any hum in this second system, but I rewired it anyway.