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Old 07-15-2017, 01:50 PM   #23  
rabbit73
It's the Antenna!
 

Join Date: Apr 2010
Posts: 1,036
Default Voltage Measurements for CM7777HD Amplify Preamp at Different Coax Lengths

I made some voltage measurements with the CM7777HD preamp and different lengths of coax. I also monitored the signal reception with a 32" Sony TV and signal power with a Sadelco DisplayMax 800 Signal Level Meter at the different lengths.

Tests with OTA signal, channel 40

Equipment used:

GE 34792 Attic/Indoor Antenna, indoors at ground level
Channel Master 7777HD Amplify Preamp
100ft Solid Signal 3035 RG6 with copper clad center conductor
100ft Channel Master 3713 RG6 with copper clad center conductor
100ft Solid Signal Custom RG6 with solid copper center conductor
Jumper cables, F-81 adapters, 2-way splitter, and a type F coax T-adapter (f-f-f) for voltage measurements under load at preamp
Sadelco DisplayMax 800 Signal Level Meter (SLM)
Fluke 115 DMM
Sony 32BX320 TV

100ft Solid Signal 3035 RG6 with copper clad center conductor

Voltage at preamp: 4.553V, measured with "T" adapter
SNR 31 dB
Signal Power: -0.2 dBmV (-49.0 dBm)

I then added:

100ft Channel Master 3713 RG6 with copper clad center conductor

Voltage at preamp: 4.205V
SNR: 28 dB
Signal Power: -6.1 dBmV (-54.9 dBm)

And then added (for a total of 300ft plus jumpers):

100ft Solid Signal Custom RG6 with solid copper center conductor

Voltage at preamp: 4.051V
SNR: 27 dB
Signal Power: -9.1 dBmV (-57.9 dBm)











As you can see, the preamp LED is still on at 4.051V, with 300+ feet of coax.

CONCLUSIONS:

The preamp seems to have a circuit that compensates for low voltage from the coax.
Quote:
Originally Posted by machinery-man View Post
I hooked it up the bottom of the tower and the power inserted in the house but the amp did not light up as noted by instructions. I checked the power and found the 5.8 volts out of the inserter and 5.8 volts at the end of the rg6. Puzzled why it didn't light up...I then connected leads in the house and simply powered the unit at the antenna with a battery and inverted. It lite up and got the signals as needed......I really don't think there is any problem with the line as I have it in conduit and I do get good signals. Does anyone have any take on this. Any input would be appreciated. Best Chuck
You must measure the voltage for the preamp near the preamp with it connected. This is called voltage under load, which is less than open circuit voltage. A T-adapter will allow you to measure the voltage between the center conductor and the shield when the preamp is connected.



If your C7777HD preamp didn't work at the antenna and the Antennas Direct Juice preamp didn't work at the antenna, maybe there is something wrong with your coax.
Attached Images
File Type: jpg CM7777hdTest1.jpg (114.6 KB, 67 views)
File Type: jpg CM7777hdTest2.jpg (124.1 KB, 64 views)
File Type: jpg CM7777hdTest3.jpg (140.0 KB, 64 views)
File Type: jpg CM7777hdTest4.jpg (160.7 KB, 64 views)
File Type: jpg Loc2noRefl.jpg (100.9 KB, 64 views)
File Type: jpg T-Adapter.jpg (23.5 KB, 56 views)

Last edited by rabbit73; 07-22-2017 at 05:30 PM..
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