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On the ISU 512, you can look under TEST and view the NEBE/FEBE (near end block errors/far end block errors). This tells you if there are, or have been errors on the BRI lines. If the counters have never been cleared, you should do that and then look again after a few hours because there are liable to be errors on boot-up and while the BRI lines synchronize.
If there are no errors, you can place a call and verify the channels Bond. The front panel of the ISU should show BONDING X (where X is the data rate - 64k for 1 channel, 128k for 2, 256k for 4, 384k for 6, etc.). Once the channels are Bonded, verify the TD and RD LEDs are both lit, or blinking - these are Transmit Data and Receive Data. These indicate if the ISU is sending and receiving data on the DTE connection (either V.35 or RS-530). If TD/RD look good but video/audio is garbled, verify the NEBE/FEBE is staying at zero. You can also go under TEST and do a DTE LOOPBACK to loop back the DTE interface toward your equipment. When this is done, you should receive your own audio/video back. If that is garbled, then that indicates the problem is local, and could be a bad cable, the originating equipment or the ISU. If the loopback is good, then the problem is "upstream". There's not a good way to test this unless the bonded channels can be looped back (the ISU 512 can do this with a "loopback protocol" - this loops the bonded BRI channels back to the ISDN network).
On the ATLAS 550 you can look under the MODULES to see if the BRI modules toward the ISU 512 show any NEBE/FEBE errors, you can also look at the interface(s) toward the carrier to see if errors are detected there.
Hope this helps,