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Author
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Topic: No Left Surround CP55
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Steve Guttag
We forgot the crackers Gromit!!!
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Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 07-07-2008 07:58 PM
Since he has the signal at the Cat 441 test point...the signal is getting to the card.
Since it is the left surround, a quick and dirty test (on optical) is to PULL the card...if that restores the signal, then it is the Cat 441...it is a little known secret that on a CP65 or CP55...the Cat 241/441 are not needed to pass the surround signal! The cards will merely apply an EQ for the surround and, of couse, bass enhancement. The signal path was designed to allow the signal to flow without the card (only for the Left Surround though...as it was a design introduced for the CP-55.
Okay...so moving on...does the Cat 242 light its "S" LED? If so, it made it there but is failing in its output stage. Note, there is a High/Low jumper (white wire) for each channel, verify that it is in the "High" position (installed). It will be labeled "W8" and it may have a blue jacket instead of white.
Next on the hit parade are the dreaded, often cursed "Bypass Relays" on the back. If you have WHITE colored relays with the largish BLACK lettering of "STC"...RUN!!! Or at least change them to good relays. Those are a known problem and are called out in a Dolby Field Bulletin (I don't know which one off hand).
The relays are "DIP" style with 24VDC coils in a 2-C configuration. Suitable replacements include Omron G5V-2-DC24 and I'm sure there are others.
If you have another CP55 in the complex and can experiment while they are not on screen. Try swapping the Cat 242 (don't be worried about level... you just want to know where the sound is failing). If the problem moves with the card, you've found it.
As for the 100K versus 105K resistor...that only affects the PN level for the subwoofer. With the Cat 241, the subwoofer was derived from Lt and Rt...with the Cat 441, the subwoofer is fed, post decoding from L, C and R. The 105K resistor gets the level correct for the new configuration. The person that did this has you off by 5%. While 105 resistors are available they are not often sitting around at the Radio Shack down the street. However, both 100K and 5.1K are pretty standard...there would have been no harm is wiring the resistors in series. This would have you in less than 1/10% off. 4.7Ks are also pretty available (and more so) and would have gotten you closer....lazy tech.
Steve
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