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Author Topic: CP-50 Volume Control Problem... Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 10-02-2006 03:31 PM                     The other day the volume in one of my auditoriums suddenly increased drastically for no apparent reason. The theatre is equipped with a Dolby CP-50 with Cat280 SR cards and Smart amplifiers. I have had no problems with the unit up until this point. When the volume control was turned down in order to solve the problem, there was no change in the volume in the auditorium. I thought that the volume control may be at fault so I replaced it with a new 100K linear pot. The problem was not solved and we are currently running the CP-50 in bypass mode. (the small bypass volume control is working) Any help would be appreciated. Thanks.  |  IP: Logged Brad Miller Administrator Posts: 17775 From: Plano, TX (36.2 miles NW of Rockwall) Registered: May 99 posted 10-02-2006 03:32 PM                     Do you have another CP50 (or a CP200) in the complex you could swap the cat 117 (or cat517) card with for a few minutes just to verify if that card is at fault?  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 10-02-2006 03:57 PM                     Yes I do. I will try that tonight and update my post later with results. Thanks.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-02-2006 05:19 PM                     There is a zener diode...I want to say something like 18V on the motherboard of the CP50 that sets the fader voltage...it too can be the culprit. However...First...disconnect the remote fader and see if the volume goes to zero...if not...the Cat 117/517 is your most likely culprit. If it does go to zero...I would check out this zener. Measureing referenced to the -12V rail to the banded end of the zener, it should measure 18V. Steve  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 10-06-2006 06:36 PM                     I swapped the Cat 117 card from a working CP-50 and the volume was still very loud and the volume control did not work. I assume that the diode Steve mentioned is the problem. Exactly where is this located on the CP-50?  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 10-06-2006 07:34 PM                     Did you first try removing the remote fader? If disconnecting it does not kill the volume, it isn't the diode. If it is...then you life will become difficult. The diode and its assoicated resistor are located on the mother board on the inside of the CP50. Remove the Center Cat64 EQ card and look just to the left of the card slot...the diode will be there about 2/3's of the way up (vertical orientation)...its resistor is there a little higher and to the left. Enjoy. Steve  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 11-15-2006 11:20 PM                     When the volume control is disconnected, there is no audio output from the CP50. Therefore, I decided to replace the zener diode and associated resistor to see if this would correct the problem. However, the problem was not solved. The volume control still does not work. What else could be causing this problem? Any help is appreciated. Thanks.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-16-2006 09:40 AM                     What did I say? If removing the fader does not kill the volume, it isn't the diode. It killed the volume thus it isn't the zener. Okay...you are now down to the fader Power supply and the Cat 117 because there just isn't anything else in the circuit. On the back of the CP50, measure the power rails and verify that the +/- 12V rails are there as well as the +24V rail. Do NOT be concerned that they don't read these voltages...depending on your power supply and the cards in your CP50, it could be several volts off on any of those rails. The +/-12V rails will often get down to damn near 10V...if they hit or go below 10V the CP50 WILL hum. I would also measure the fader reference voltage (terminal 6 where the fader connects) to see if it is between 6 and 8 volts. If that is good...you really are now down to the fader itself and the Cat 117 (unless it is just one channel, in which case the line amps in the Cat 22s could be a problem or even the Cat 64s since they have input/output trims). Measure the fader disconnected and verify that it is 100K (or simply replace it with a new 100K pot. Then you are down to the Cat 117. Steve  |  IP: Logged Dave Macaulay Film God Posts: 2321 From: Toronto, Canada Registered: Apr 2001 posted 11-16-2006 02:06 PM                     SG: "However...First...disconnect the remote fader and see if the volume goes to zero...if not...the Cat 117/517 is your most likely culprit. If it does go to zero...I would check out this zener. Measureing referenced to the -12V rail to the banded end of the zener, it should measure 18V." SG: "Did you first try removing the remote fader? If disconnecting it does not kill the volume, it isn't the diode." TW: "When the volume control is disconnected, there is no audio output from the CP50." SG: "What did I say? If removing the fader does not kill the volume, it isn't the diode. It killed the volume thus it isn't the zener." Huh? Which way do you really mean, you have told him both now: no fader + no sound = diode problem no fader + no sound = not a diode problem  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-17-2006 12:24 AM                     Wow...what a blunder on my part. Nice catch Dave and sorry Tom... Okay...lets try this again...you removed the fader and the volume dropped...that would normally indicate that it is not the Cat 117. That is, it is not seeing the fader voltage anymore so it lowered the volume as it should. This leaves the Zener and associated circuitry and the power rails. I would measure the power rails reference to ground. I would measure the fader reference referenced to ground as listed above...I would also measure the fader reference refrenced to -12V. The Fader reference should be 18V above -12 or about +6V referenced to ground (E). And changing the pot still wouldn't hurt any.  |  IP: Logged Tom Wienholt Master Film Handler Posts: 371 From: Towson, MD, USA Registered: Dec 2002 posted 11-17-2006 01:07 AM                     Ok...I measured all of the voltages that were suggested and all were correct. I replaced the linear pot the day that the problem started with no result. I also just changed the diode and resistor with no result. I swapped the 117 card with a working system with no result. Now I am stumped. I may have to just replace the system completely. Thanks for all of the help.  |  IP: Logged Steve Guttag We forgot the crackers Gromit!!! Posts: 12814 From: Annapolis, MD Registered: Dec 1999 posted 11-17-2006 08:25 AM                     Boy...there just isn't too much going on with that circuit to be having this hard a problem. If removing the pot turns the fader down to zero but turning the pot all of the way down (100K) doesn't that is just beyond strange. I would say the "Optical Fault" button is "IN" but that wouldn't care if the fader was removed or not. If just one channel has the issue then I would suspect the Cat 117 might be overdriven by say the previous parts of the processor. For yucks...try resetting the input/output levels on the Cat 64s (if they are Cat 64Bs there are no input/output controls). Once they are set you will know that the level feeding the Cat 117 is correct. Make sure you measure 150mV on the Left and Right and 210mV on Center (running Dolby Tone, removing the appropriate Cat 22 to measure the channels as specified in the CP50 manual). Try removing the Cat 64s one at a time to see if the volume restores to normal control. This really shouldn't be that hard...it is a simple circuit on the CP50. Steve  |  IP: Logged

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