Oops! Sometimes we are wrong.

I post my cell number and email address at the end of every column, and over the years, I, or others working for me, have trained at least a thousand technicians. Based upon our experience and you faithful readers, we’ve received hundreds of calls asking for help finding solutions to many unresolved case of trouble. Usually these cases have been looked at by a few technicians and other experts before we hear from folks, so when we get the call for help, it means it’s usually a tough case. 

While we often — even usually — are successful in helping uncover the cause of the trouble, we aren’t right all the time. Following are some cases where we got it wrong. I could slide these under the rug, so to speak, but we can all learn from these opportunities. I know that I certainly do!

Here are some examples of not hitting the nail on the head.

Example #1

Donald,
We have a bandwidth problem on a 200-pair filled cable leaving our central office. When testing any cable pair there is no physical trouble and all pairs show acceptable longitudinal balance. Power influence is acceptable on all pairs less than 80dBrnC. Circuit noise is also acceptable less than 20dBrnC. Here is where things get strange. The power company recently installed a new lead along our cable route. They also installed a capacitor bank on the route. When connecting a technician’s butt set across any pair that pair is quiet when either the on hook or the off hook is selected. But when the tone probe is used there is audible "motor boating" on any pair. What do you think is happening?
Robert

My response was:
I definitely do not know what the root cause is, but I think that the offending frequency is somewhere below 800Hz or above 2500Hz but in the voice grade range. When using a POTS circuit, we talk on the frequencies from 800Hz to 2500HZ. Unwanted interfering tones above 800Hz and below 2500Hz are filtered out with a C message filter. Your tone probe does not have a C message filter. I believe that is why the tone probe picks up the "motor boating" and the butt set does not pick it up. I think that the capacitor bank is your problem and not a bonding and grounding issue. I suggest that you contact the power company and have them take the capacitor bank off line and you can retest to see if the trouble goes away.
Donald

Robert called back and stated that he found the problem. He stated that he found a loose bullet bond in the CO vault that was arcing. When he tightened the bullet bond the arcing ceased and the noise went away. Go figure. 

Example #2

Hi, Don,
It has been a long time since I have bothered you. A technical issue has come up when we cross-connect a POTS line to a cable with no end point plugged into the jack, we get a hum on the pair. What do you think is happening?
Frank

My response was:
That vacant cable pair is open on one side on a lateral (bridged tap), open one side beyond your terminal, or crossed with a non-working conductor. Keep me in the loop.
Donald

Frank’s reply was:

Don,
Digging deeper, we found that the hum on the FAX was from the FAX machine itself. Or more correctly the electrical outlet it is plugged into had 40 volts on the neutral.