Troubleshooting
Workbook
Purpose
You will know how to use a troubleshooting model for common drop-related problems.
This will decrease the amount of time spent on drop-related service calls and increase
customer satisfaction.
Objectives
At the end of this module, you will be able to:
Use a four-step troubleshooting model to identify and repair common
drop-related problems.
Demonstrate troubleshooting proficiency by using the bypass technique and
the “divide-and-conquer“ method.
Standard Troubleshooting Model
Troubleshooting is a four-step process designed to assist field technicians. By following
this simple model, you will be able to more quickly resolve problems with cable service.
Let’s take a look at this model in detail and then apply it to some common
cable-related problems.
Step 1: Analyze the problem
The first step in analyzing the problem is to get the customer’s description of it. The
technician should question the customer and listen closely to the responses.
It is very useful to ask questions like:
What is the problem?
How many TVs are affected?
Which channels are affected?
What time of day does the problem occur?
How often does it occur?
Step 2: Isolate the problem
Isolating the problem is a process of elimination, in which the technician determines
which parts of the cable are in good working order. There are two approaches.
The “divide-and-conquer” method systematically “cuts the problem in half” by
strategically checking at various points in the system.
For example, if there is a low signal problem, you might start by checking
the signal level at the wall plate.
If it is bad, you could then go to a point halfway between the wall plate and
the tap, and check the signal level there.
If the signal level at the ground block is good, you would check next at a
point between the ground block and the wall plate.
Another method is the bypass method.
With this method, you attach a length of drop cable known to be good,
bypassing devices and sections of cable that are suspected of being
defective.
If this eliminates the problem, the suspicions are confirmed.
Step 3: Fix the problem
Fixing the problem simply consists of taking the steps needed to repair the cause of the
problem.
Make sure that the fix is adequate to make a second service call unnecessary.
Step 4: Verify the fix
This step is critical to successful customer service. It is crucial that we check back with
the customer and visually check the TVs and channels that were being affected by the
problem.
Make sure there are not two separate problems that need to be fixed.
Check signal levels.
Check for signal leakage.
Common Problems and Isolation Techniques
The following pages present approaches to troubleshooting each of the most common
drop-related problems—ghosting and double images, bad or shorted cables, hum
modulation, and lines.
Ghosting and Double Images
Understand that ghosting and double imaging are not the same thing. A ghost refers to
two images coming from the same video signal, one image is fainter than the other. A
double image comes from two different video signals sharing the same frequency. The
troubleshooting procedure is the same in either case, however.
Step 1: Analyze the problem
Learn from the customer.
Check to see if the ghost is on more than one TV.
Look at the picture on all channels.
Step 2: Isolate the problem
The technician should use one or more of the following methods to help isolate
the problem:
1. Look for loose connectors, examine all jumpers, and check the integrity of the
drop cable. If that doesn’t isolate the problem, try one of the following:
Run a temporary drop from the tap to the input of the ground block to
determine if the ghosting is in the drop or the main line.
With one end of the drop hooked up to the tap, and the other to the SLM,
tune the SLM to a local channel. Any garbled audio on a local channel
indicates ingress on the drop.
Using the SLM, measure a local signal on the disconnected, terminated
drop. There should be no signal. If signal is present, it is from ingress.
Step 3: Fix the problem
Replace inferior VCR jumpers with quality jumpers.
Do not over-tighten the connectors. This will damage internal solder joints.
Step 4: Verify the fix
After making a repair, be sure to check all channels and TVs.
Check signal levels.
Check for signal leakage.
Check back with the customer.
Bad or Shorted Cables
If you suspect that an underground drop is defective, you will want to use the following
method for evaluating the condition of the drop cable. This method will determine
whether a shorted condition is causing the problem.
Step 1: Analyze the problem
Learn from the customer.
Look at the picture on all channels.
Check all TVs that are connected to cable.
Step 2: Isolate the problem
Use this method when there is reason to suspect that an underground drop is defective.
1. Measure the resistance of a terminator.
2. Record this measurement.
3. Terminate one end of the drop.
4. Estimate the length of the drop.
5. Go to the other end and measure the resistance from the shield to the center
conductor. Record this measurement.
6. Subtract the cable resistance from the terminator resistance, and record the
difference.
7. Divide this value by the resistance per foot—usually 0.0579 /ft. for RG-59,
0.0379 /ft. for RG-6.
8. Subtract the cable resistance from the terminator resistance, and record the
difference.
9. Divide this value by the resistance per foot—usually 0.0579 /ft. for RG-59,
0.0379 /ft. for RG-6.
If the value derived equals the estimated length of the drop, the drop cable
is in good condition.
If the value derived is significantly shorter than the estimated length of the
drop, the drop has a bad spot.
For example, suppose you have:
RG-6 drop, 100 ft. long
Terminator resistance of 75
Cable resistance of 77
Then, you would calculate:
77 minus 75 equals 2
2 divided by .0397 = 50.37 ft.
The problem, therefore, is at about half the distance of the length of the drop—50 ft.
Note: The distance this calculation yields is measured from the
non-terminated end of the drop.
Step 3: Fix the problem
Replace any bad cables and connectors found.
Step 4: Verify the fix
After making the repair, check all channels.
Look at the pictures on all the customer’s TVs.
Check the signal levels.
Use a leakage detector.
Check back with the customer.
Hands-On Activity: Calculating Length from Resistance
in the Drop Cable Loop
Calculate the length of the drop cable issued to you by following the steps below:
1. Measure the resistance of a terminator.
2. Record this measurement.
3. Terminate one end of the drop.
4. Estimate the length of the drop.
5. At the opposite end of the drop, measure the resistance from the shield to the
center conductor.
6. Record this measurement.
7. Subtract the cable resistance from the terminator resistance, and record this
value.
8. Divide this value by the resistance per foot—typically 0.0579 /ft. for RG-59,
0.0379 /ft. for RG-6.
a) If the value derived equals the estimated distance of the drop, the drop cable is in
good condition.
b) If the value derived is a distance significantly shorter than the estimated length of
the drop, there is a bad spot in the drop, at the length derived.
Hum Modulation
Hum bars are most generally caused by an improper ground. Water-damaged drop
splitters on tap spigots have been known to cause hum bars.
When you suspect a hum modulation problem, follow these procedures:
Step 1: Analyze the problem
Learn from the customer.
Look at the picture on all channels.
Look at the picture on all TVs connected to cable.
Step 2: Isolate the problem
Use one or more of these methods to help isolate the problem:
Check all cables and connectors.
Temporarily disconnect the ground from the ground block to see if this clears
up the problem.
Disconnect all other customers’ drops from the tap to eliminate the possibility
of backfeed.
Use the bypass method to determine whether the hum bar is caused by a
house amplifier, converter, or other customer equipment.
Step 3: Fix the problem
Replace any splitters, connectors, cable, or other faulty equipment causing
the hum bar.
Note: Do not replace any part of a defective ground or defective TV parts.
Step 4: Verify the fix
After making the repair, check all channels and TVs.
Check the signal strength.
Use a signal leakage detector.
Check back with the customer.
Lines
Step 1: Analyze the problem
Learn from the customer.
Look at the picture on all channels.
Look at all TVs connected to cable.
Step 2: Isolate the problem
Check the signal level at the wall plate.
A reading of 15 dBmV or above at the wall plate indicates that a house
amplifier is misadjusted or a drop is hooked up to a tap with high outputs.
Make sure that the input to the house amplifier, if there is one, is no higher
than the recommended input level, usually 0 dBmV.
If equipment is suspected of causing the problem, use the bypass method.
With a test set, check at various points in the drop system, using the
divide-and-conquer process.
Inspect all cables and connectors.
Step 3: Fix the problem
Replace any cables, connectors, or equipment found to be defective.
Step 4: Verify the fix
Check signal levels.
Use a leakage detector.
Look at the picture on all channels.
Check back with the customer.
9.