CABL Forum - Re: Magnavox 6TNA/395 Amplifiers

Hi Bruce,

This is Greg Tobin (Interactive Broadband Technical Services) the ex-Philips/Magnavox field engineer who you called earlier today. My battery on cellular died during our conversation.

I was thinking about our conversation and came to this conclusion.

First of all we talked about the inputs for that unit at least meeting or exceeding the noise figure (I believe it is 9 or 10dBmV minimum). This means that the 1st hybrid should have at least this minimum input AFTER the pad and equalizer to maintain a "flat" input noise figure. For each 1dB in addition to this input, your C/N will improve as such.

Second, your 6-TNA-395 units are most likely to have a 30dB input and output testpoint unless it has the WRONG cover on it or it has been modified. My suggestion would be to put a 20dB tap at output and compare for a sanity check.

As stated prior, the model number on the upper right of the aluminum cover should yield what you have exactly. For example: "6-TNA395/38-33/50A" The "6" denotes that the unit is a "power doubling" technology. The "TNA" is "Trunk Network Amplifier" which simply put is nothing more than a multiple output line extender with the trunk output on the upper right port. NOTE: There were earlier 50dB gain versions that had the trunk output leg on the lower right! 95% of TNA's will have the upper right as a trunk output leg. The typical TNA will have the trunk leg approximately 10dB lower than the lower distribution leg(s). The "395" designates as follows: "3" is 3 active forward outputs. "9" is roughly 1GHz housing platform and "5" is a 550MHz amplifier module. The "38" or whatever number following is the overall station gain with no ALSC, input eq. and factory installed interstage eq. The "33/50" is the diplexer cut-off: 5-33MHz return and 50-550MHz forward. The "A" is a designation that the amplifier module has a "resistive" testpoint and is built on a (if I remember correctly) a 10A module. The "B" and "D" versions were directional coupler testpoints (common to the HIGH side of the diplexer only) and were all 750MHz modules (297 or 397 modules) to the best of my knowledge. The "D" version had a beefed up DC power supply to handle usage as a launch amplifier in a node application with extra current capactiy. In your case, there is NO letter after the diplex split number....this is the same as an "A" with resistive testpoints.

Getting back to what I was remembering from the old field engineering days. I remember that your problem of "SNOWY" channels was common with customers who did not have knowledge of the design of the unit. If you ARE NOT using the ALSC circuit, there MUST BE A 9dB attenuation pad installed!!! If you do not, YOU WILL BE FORCED TO OVER-ATTENUATE THE INPUT AND ACTUALLY CAUSE A NOISE PROBLEM TO THE INPUT HYBRID! If you put a 9dB pad in the interstage, you will have significent input to drive the 1st gain block and decrease overall the station gain/output. You should see a significant improvement in your picture quality...espically the lower channels depending on how you are equalizing the unit overall. In addition, if someday you decide to install the ALSC circuit, you remove the 9dB pad and replace it with a 0dB pad. Since the ALSC has roughly a 9dB insertion loss, it will be apples and apples when exchanged causing no loss of station gain.

I hope that this has helped you. If you are still having problems with these units, feel free to call me and I will help further.

Good luck!

Greg Tobin
Interactive Broadband Technical Services
314-422-4423

http://www.interactive-broadband.com

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Re: Magnavox 6TNA/395 Amplifiers - Bruce...