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Telrad digital 128, COs come in one a T-1 line, but run through a channel bank so its copper to the KSU. Lately a terrible hiss has developed whenever you access a CO. Doesn't matter which CO or which station. I checked the COs with a butt set at the block and they seem to be quiet there. Loop current? Impedance termination? ANy ideas?

TIA

Hard to say, can you make a clean call at the breakout with your butt set? You may also want to power down and reseat the cards.
If you have a spare COL card see if that clears it up. Your going to have to do some trouble shooting.

Assuming "Lately" means it wasn't hissing before. Is the hiss on all circuits coming from the channel banks? (1-24 or how ever many your using) Are there any POTs lines hooked through the system?
I'd also report it to the service provider, 9 out of 10 times it's their problem if it was clear then went bad...on all circuits and you or no one else had made any changes in programming, adding a station, moving a station etc.

I just had this with another customer. Found a bad MPD card. Have you changed any cards out lately?

No I haven't changed any cards. Unfortunately I don't have any spare COL cards (this is the only Telrad I service, customer provided it, I installed it.) The idea of a bad MPD card is interesting. Changing the card cleared the problem?

If it's the MPD that is causing it then yes it would fix the problem. If you don't have spare cards then I would try the reseating of all the cards in the system, If that doesn't help then they are going to either have to get some spares of bring in a tech that can trouble shoot the system.
You do realize that the MPD is the main processor and you may have to reprogram if you can't reuse the memory module

The MPD is where all the highways are in the system. But before I changed out the MPD I would do some trouble shooting. If this is the only Telrad you have you might not have all the experance you need to find the trouble on this system. WHat software level is it?

Most definitely, always trouble shoot before changing out or buying cards. What is your level of experience with the system?

(btw - this is a great board.. .)
Thanks for all your input, let me try to answer your ?s. The software version is sbu 5.10. I have been looking after this system for about a year and a half. One crash which a reload fixed. A few AMC. Lots of programming changes, Yes I have backups. There are not normally any POTS lines on the system, but I borrowed a POTS line from another piece of gear in the phone room and cross connected it to one of the CO ports. No noise! I measured the DC voltage on the POTS line, 48v on the nose, I also measured the DC voltage on the lines derived from the T-1 at 50.6. If my Ohms law is correct, voltage goes up, current goes down. 2.6 volts isn't a lot, but if the line voltage is too high could this be a low line current issue ?

Thanks

And the answer is . . . it was in MCI's channel bank. I coaxed the tech to replace cards one by one, when none of those made a difference I said, 'how 'bout the power supply'?. He swapped the ring generator and the noise went away.

Thanks for all your responses and constructive suggestions.