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  #1  
Old 08-06-2016, 12:49 PM
madlabs madlabs is offline
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HDMI converter recommendations

Hi All,

So, as work progresses on my Predicta, I am starting to look into converter boxes to be able to feed it a signal, when and if I get it working. I am planning to bypass the tuner and direct inject the signal into the video and audio feeds like Phil outlines on his site. My source will be a PC and perhaps a DVD player.

I'm guessing that not all converter boxes are created equally and that screen resolution and so on are all in the mix to get the image to display correctly. So, any recommendations on a good converter?

Thanks!

Jonathan
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Old 08-06-2016, 10:30 PM
Electronic M's Avatar
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Unless it is one of the models with a power transformer I recommend sticking with RF. Video injection would entail connecting chassis ground (at 120 VAC) to your box's video ground (at 0 VAC)....The most likely result of that is releasing the magic smoke of death from the shiny new box.

The only time to go with video injection is when the RF/IF is beyond your ability to repair it.
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Old 08-07-2016, 01:30 PM
madlabs madlabs is offline
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Yikes! Hadn't thought about that. What about if running the set on an isolation transformer? I have one that allows me to also set the line voltage to 117 so I was thinking of using it in any case to lower the line voltage.

My understanding is that direct injection allows for the best picture possible. Tuning the IF and getting as good a picture as possible seems like a real challenge and if nothing else it would be good to have something to compare with when attempting alignment.

Regardless, I need the same sort of converter, no?

Thanks!

Jonathan
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  #4  
Old 08-07-2016, 11:22 PM
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An isolation transformer may work, but if the primary and secondary windings start to (or currently) leak current between each other, or you ever forget and run the set without it things still could go wrong.

90% of the time the alignment is good enough as found after paper caps and weak tubes have been changed (clean the tuner contacts before you dismiss the system) that you will be hard pressed to notice a difference between RF and direct injection. Also if the CRT does not focus really well it will look mediocre to bad no matter how good the IF is or how you feed it signal. Ignore direct injection till you get it working.
50's-60's sets are by and large not designed to be as sharp as newer CRT sets so the old gamer/home theater saws about composite being better than RF often don't apply.

IF alignment in TV is much more involved than radio. You have to have an Oscilloscope and a properly calibrated TV Sweep/Marker alignment generator to do an IF alignment (the turn and observe method will only screw it up).

There are no HDMI to RF adapters on the market AFAIK so if you plan to use an HDMI based signal source you will be buying an HDMI to composite video adapter and probably an RF modulator like it or not. Personally I don't know what you would connect that would not use composite or some analog connection. I've got a cable box, DTV box, DVD, VCR, (several older video formats), and a windows 7 PC (for web videos and digital vids in formats my DVD player can't handle) with s-video out on it's video card as analog signal sources...NONE of them have only HDMI (all have analog outputs) and they cover everything programming wise I could imagine watching on a TV and more... So I've never needed an HDMI converter.

If you go RF I recommend getting a Blonder Tongue agile modulator (AM40, AM60, or BAVMz series), they are worlds better signal quality wise than a Radio Shack (or other off the shelf) RF modulator (and can be had for under, sometimes well under, $40). A BT will actually PROPERLY adhere to broadcast specifications (no crude approximations like the cheap crap), and most with an optimal antenna set (ask me for details) and at near max power can send RF wirelessly for 70' to a suburban block with ease...Which can be super handy if you want to move sets around or have them in multiple rooms (I speak from experience) or outdoors. I use all three families of BT mods, for wireless broadcasting and am quite pleased with their performance.

One other thing that is useful to have is a 90's or newer portable or small screen CRT set (can be had for less than $5) to test your signal sources, and if transmitting RF, your reception range.
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Last edited by Electronic M; 08-07-2016 at 11:37 PM.
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  #5  
Old 08-08-2016, 10:58 AM
madlabs madlabs is offline
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Tom,

Firstly, thanks very much for the concise and detailed reply.

OK, RF it is, at least at first. I do have a 'scope but no sweep/marker. So let's hope that replacing the caps and checking the resistors will get it done for the IF. If not, I wonder if I can find someone I could just send the IF to.

As to video source, all I have is a laptop with VGA and HDMI. I live off the grid and so I just use a laptop as my PC. Although I do have enough power to run whatever I want now, the laptop is a legacy from when I first moved out onto the property and had little power storage.

So, looks like I need something like this:

https://www.amazon.com/ViewHD-Compos.../dp/B00LK95VNQ

and run that into something like this:

http://www.ebay.com/itm/Blonder-Tong...-/272304780489

I'll start cruising the thrift stores when I'm in town and pick up a small CRT TV set.

Thanks again for the help!

JOnathan

Last edited by madlabs; 08-08-2016 at 12:50 PM. Reason: broken link
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  #6  
Old 08-08-2016, 05:18 PM
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looks good to me. Every so often the BTs will have bad lytics...Little cheap ones. If it has video hum (dark horizontal bar rolling slowly vertically, often disturbing V sync when it goes off screen) or other issues look at the caps.

If you go wired run it at or near minimum RF power (output level), and use a 75ohm to 300ohm balun/matching transformer to connect to the set. If you plan to go wireless crank the output level to max and look for a youtube video on antennas by shango66. Start with video and audio levels at min turn up till distortion appears then back down a bit. Audio carrier I typically set low to mid and make sure it is not creating buzz.

BTW: if you or anyone in a mile radius is using a DVT box for OTA reception transmit on an un-used channel so you don't jam other recievers....Come to think of it make sure your on an unused channel if you go wireless no matter what...I've had DTV interference with my TX experimenting on and near local channels.
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Last edited by Electronic M; 08-08-2016 at 05:22 PM.
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  #7  
Old 08-09-2016, 09:21 AM
madlabs madlabs is offline
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Thanks for the advice, Tom.

Is there any picture quality difference between wired and wireless operation of the RF modulator?

No worries about interfering with others as I live plenty far from the nearest neighbor. I'll stick with low VHF channels anyway to prevent interference with my own ham radio operations.

Jonathan
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Old 08-09-2016, 11:37 AM
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Often there is no difference. With wireless if you are too far from the transmitter or something that absorbs signal like a metal wall is in between antennas you may get snow in the picture (thus why I run enough power to reach across my next door neighbor's lot with decent clarity). Also if you have metal or concrete walls or someone moving within 5' of the antennas ghosting/multipath can occur...Strategic antenna placement usually will prevent or fix it.
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Old 08-09-2016, 05:40 PM
madlabs madlabs is offline
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Thanks Tom!
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