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 Post subject: Injecting RCA audio directly to a Hitachi TV
PostPosted: Jun Wed 19, 2019 4:37 pm 
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Joined: May Thu 16, 2019 5:03 pm
Posts: 12
I picked up a Hitachi P-05 solid state B&W tv the other day and discovered that someone had retrofitted a Pickles and Trout TVM-04 direct video injection module to it. It's a little circuit board with a coax input which lets you display composite video signals *directly* rather than going through an RF modulator. The picture looks great, but I still need an RF modulator to get the audio into the TV.

What I'm wondering is, could I wire an RCA mono jack into the circuit somewhere so as to directly inject audio after all the RF decoding has been done? Or are the levels going to be all wrong? I've been unable to locate a circuit diagram for the thing. I suppose the crudest thing to do would be to jam a little audio amplifier circuit into the chassis and just use a switch to change the speaker's connection back and forth, but that's mighty crude.


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 Post subject: Re: Injecting RCA audio directly to a Hitachi TV
PostPosted: Jun Wed 19, 2019 5:47 pm 
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Joined: Nov Thu 11, 2010 6:03 pm
Posts: 749
Location: Pewaukee, WI
You may need a schematic to properly evaluate a good injection point.

Without a schematic the best place to try injection is the volume pot use a .1uF cap in series with the audio hot lead to prevent back feeding DC bias voltage from the set to your source.....Note if you have RF static coming through you may have to unhook the audio feed from the demodulator.

One thing to be careful of is that some sets are hot chassis designs where the chassis ground connects to one side of the power cord( sometimes through a rectifier bridge)...The video injection box you have may be designed to isolate the video input from chassis ground. If your set is hot chassis and you don't isolate properly (a 1:1 isolation transformer is the easiest way) you can blow your source device.


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 Post subject: Re: Injecting RCA audio directly to a Hitachi TV
PostPosted: Jun Wed 19, 2019 5:54 pm 
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Joined: May Thu 16, 2019 5:03 pm
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I've had zero luck finding a schematic online, so unless somebody turns one up here I'll probably have to wing it.

I'll try attaching the line-in at the volume pot and see what happens.

I think it's not a hot chassis; it's got a polarized plug and a transformer, but I'll take a look next time I'm in there and make sure neither power lead is wired to chassis.


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 Post subject: Re: Injecting RCA audio directly to a Hitachi TV
PostPosted: Jun Wed 19, 2019 8:55 pm 
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Joined: Feb Thu 28, 2019 7:48 pm
Posts: 253
Location: Lawrenceville, Illinois 62439
Sams 1577-1 might be it.
On the right of your screen, Steve's Antique Technology offers Sams Photofacts.
Dan


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 Post subject: Re: Injecting RCA audio directly to a Hitachi TV
PostPosted: Jun Wed 19, 2019 11:07 pm 
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Joined: May Thu 16, 2019 5:03 pm
Posts: 12
solderfumes wrote:
I've had zero luck finding a schematic online, so unless somebody turns one up here I'll probably have to wing it.

I'll try attaching the line-in at the volume pot and see what happens.

I think it's not a hot chassis; it's got a polarized plug and a transformer, but I'll take a look next time I'm in there and make sure neither power lead is wired to chassis.


This worked, so I wired up a couple RCA ports and a toggle switch on the side of the chassis. Not perfect, but this thing's already been hacked and it keeps the wires short.


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