View Full Version : Direct AV injection


damien191
10-24-2017, 12:36 AM
i have this TV, sadly the last of my tube collection - i want to do direct injection for standard rca video (the plan is to "implant" a hdmi to rca downscaler) but i cant find the schematic so i wouldn't even be able to guess the connection points - i don't entirely care if the rf tuner stays functional since there's little to no use for it any more - but it would be nice for if i could ever find an elusive vhf transmitter

the sort of end game i suppose would be to hide a chrome cast inside so i can have one wire and not loose the aesthetic of the TV since right now the gangly cables annoy me

i don't have the model number right now - i can get it sometime this week when i go to the storage space - i had it but it's lost to the move at the moment

TL;DR need av injection points

VintagePC
10-24-2017, 07:33 AM
The more common ones are tough to find but I see a lot of Drake VMM860s on Ebay for cheap... Not a full size transmitter but into a pair of rabbit ears definitely enough oomph for reception nearby.

They do both UHF and VHF up to Ch. 125

damien191
10-27-2017, 01:55 AM
nice, ill keep that in mind

olkrwa
10-27-2017, 02:53 PM
Hi Damien.

My 16-inch Zenith porthole and 1948 teleking is running with direct input. When it works it works great. To make this work properly you need a video source of good quality. It is easy to get disturbance on the video if your media device has a cheap power supply and so on. My 16 Zenith using a media device with SD card and USB that just running in loop with old movies and commercials. To get a good result I had to add some ferrite core filters on media power supply and a galvanic isolator on the video AV connection. I leave you a link to a link to Antiqueradios for directions how to connect to the video amplifier tube and the audio connection. See also the link to my sets on youtube if you want to see my sets in use. Good luck

Best regards Kristian

https://antiqueradio.org/A-V_AdapterForVintageTVs.htm
https://www.youtube.com/channel/UCZmwVaCXzyZtbykPkhUzCdg

kf4rca
11-03-2017, 05:01 PM
A CATV modulator generally puts out about 60 db. Thats because some loss (12 db or so) occurs in the combining process with other channels that are also used. Generally the line level leaving the headend should be about 42 db so that the output from the first tap (say a 32 tap) is about 10 db.
A TV normally likes to see 0 to 10 db in to the tuner. Yes you can transmit from a modulator for a short distance. 78 db is 1 watt. 75 db is 1/2 watt.
Ideally you need a high gain antenna to put a decent signal around the neighborhood. But a yagi is directional. For a non directional antenna a super turnstile would be best.

Findm-Keepm
11-04-2017, 11:54 AM
78 db is 1 watt. 75 db is 1/2 watt.


dB (Decibels) express a ratio, not a value. Wattage is usually expressed in dBm or dBw, or in a 75 ohm system, dBmV.

30 dBm= 1 Watt
27 dBm= 1/2 Watt

The above are for a 50-ohm system.

For a 75 Ohm system, dBmV = dBm + 48.75 dB

Best calculator out there:

http://earmark.net/gesr/opamp/db_calc.htm

I graduated Channel Master's MATV/CATV course in the early 80s. They beat it into your head that 75ohm (TV) and 50 Ohm (everything else, at that time) had different calculations and that 78dBmV = 1 Watt = 30dBm in a 75 ohm system.

I went from that world into the military, where everything is pretty much based on 50 ohms, and you never forget that 0 dBm = 224mv(RMS) = 1mW and 30dBm was still one watt...:scratch2:

The pulsed power bench I worked in my first 6 years made it even harder, as duty cycle entered the picture, and peak power would be in the MegaWatts region, but RMS power was down in the Watts region.....:sigh:

My Jerrold VHF-UHF Field Strength Meter is still the only instrument I own with a meter that will measure dBmV.

kf4rca
11-04-2017, 12:41 PM
How useful is an old TV field strength meter to use on the new digital TV signals? Haven't tried it. But I see several of them on ebay at reasonable prices.

Findm-Keepm
11-04-2017, 01:33 PM
How useful is an old TV field strength meter to use on the new digital TV signals? Haven't tried it. But I see several of them on ebay at reasonable prices.

Very useful, especially in the VHF region. I aimed my 102" long V/U/FM Radio Shack Yagi with it, using Channel 13 VHF (WVEC, our only VHF ATSC station) as the guiding point. Of 38 channels, all but two are in the same direction, so I aimed using the weakest signal, Channel 13. Of the other 2 channels, both come in clearly despite being 4-11 degrees away from the bulk.
When I get a 4-Bay UHF unit, I'll aim for the PBS stations in NC - 44 degrees away from my main aim point, and 37 miles away from me.

damien191
11-06-2017, 07:01 PM
Hi Damien.

My 16-inch Zenith porthole and 1948 teleking is running with direct input. When it works it works great. To make this work properly you need a video source of good quality. It is easy to get disturbance on the video if your media device has a cheap power supply and so on. My 16 Zenith using a media device with SD card and USB that just running in loop with old movies and commercials. To get a good result I had to add some ferrite core filters on media power supply and a galvanic isolator on the video AV connection. I leave you a link to a link to Antiqueradios for directions how to connect to the video amplifier tube and the audio connection. See also the link to my sets on youtube if you want to see my sets in use. Good luck

Best regards Kristian

https://antiqueradio.org/A-V_AdapterForVintageTVs.htm
https://www.youtube.com/channel/UCZmwVaCXzyZtbykPkhUzCdg

thanks for the info - and definately going to take a look at your you-tube page