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Author Topic: How can I power this board with DC instead of AC?  (Read 960 times)
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snoop911
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« on: December 17, 2017, 10:54:52 10:54 »

I bought this $6 AV to RF converter, but it's meant for a Chinese 220V market.   I'd like to see if I can modify it to either use 120V or preferably, just feed it DC somewhere.

Power comes directly to the resistor (see red/black lines) and it seems like it would internally run on DC, but I don't see any active ICs, so not sure what voltage and where I could tap to. 

I found what looks to a similar circuit, but not sure since it's all Chinese to me.
https://wenku.baidu.com/view/1567a6f7f61fb7360b4c65a6.html

Anybody have any suggestions to try?

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Signal
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« Reply #1 on: December 17, 2017, 12:03:28 12:03 »

Power comes directly to the resistor (see red/black lines)
I see red wire connected to R2->D1->47uF->black wire. It is half-period rectifier. Followed by inverter.
Unfortunately I can not clearly distinguish the value of R2. R2 is the hot "eater" of redundant volts.
If circuit is meant to work from 220VAC then for 120 VAC you should pick a resistor with resistance about 1.5-2 times less. Or just check if it works from 120AC.
« Last Edit: December 17, 2017, 12:16:59 12:16 by Signal » Logged

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Wizpic
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« Reply #2 on: December 17, 2017, 12:09:33 12:09 »

What's the dc voltage across the 2 capacitors (330uF & 47uF) ?
what's the DC voltage across that resistor ?
What's the T092 next to the transformer  and the 47uF cap is it a regulator or transistor ?
The 47uF is only rated at 50V so I would say no much more than that may be 9,12 or 18V
I would measure around the board with AC mode first then if your getting no readings then move to high voltage DC o the meter (unless auto ranging) and see what the average voltage is it my be 9V if this was the case I then would just connect a 9V supply to the point where you measured it and see if it works or what the average supply would be, Also what's the DC voltage measured across ZD1
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enzine
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« Reply #3 on: December 17, 2017, 12:13:41 12:13 »

Almost certainly the right side of the first picture and a self-oscillating ac / dc converter with a transistor and a transformer to obtain isolation from the mains.
The components D2, C5, ZD1 complete the power supply to the secondary of the transformer. You should read the zener ZD1 value and replace everything with a continuous voltage sent from outside to the Zener heads.
If the value can not be read, measure the voltage across the terminals of the zener diode.
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Signal
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« Reply #4 on: December 17, 2017, 12:29:18 12:29 »

What's the dc voltage across the 2 capacitors (330uF & 47uF) ?
what's the DC voltage across that resistor ?
To measure snoop911 needs 220AC. But has only 120VAC.

What's the T092 next to the transformer  and the 47uF cap is it a regulator or transistor ?
Transistor. The active part of inverter that works as enzine describes.
 
Almost certainly the right side of the first picture and a self-oscillating ac / dc converter with a transistor and a transformer to obtain isolation from the mains.
Precisely. No doubts.
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enzine
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« Reply #5 on: December 17, 2017, 12:35:41 12:35 »

The electric schematic should be similar to this:
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metal
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« Reply #6 on: December 17, 2017, 12:37:34 12:37 »

Look at this:

https://www.circuitsdiy.com/mobile-charger-circuit-diagram/
http://www.electroschematics.com/12799/diy-rcc-smps-circuits/
http://danyk.cz/iz_odv_en.html
https://www.vintage-radio.net/forum/showthread.php?t=77635

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Signal
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« Reply #7 on: December 17, 2017, 12:38:46 12:38 »

<...> and replace everything with a continuous voltage sent from outside to the Zener heads.
Disagree. Correct place to connect external DC is 330uF capacitor.
(Not only because it could leave Zener alive)
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« Reply #8 on: December 17, 2017, 01:42:50 13:42 »

I think Wizpic made the right call, what is the working voltage on the 330uF, I would remove the transformer and all the components from the mains input to the transformer. then apply a voltage below the working of the 330uF

the mains side is very poorly designed, also I would not put any mains voltage any where near that pcb.
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Signal
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« Reply #9 on: December 18, 2017, 08:52:39 08:52 »

I would remove the transformer and all the components from the mains input to the transformer
No need to physically remove components. D2 is enough isolation from transformer.
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snoop911
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« Reply #10 on: December 22, 2017, 12:05:20 12:05 »

To measure snoop911 needs 220AC. But has only 120VAC.
Transistor. The active part of inverter that works as enzine describes.
 Precisely. No doubts.



Yep, I should have mentioned I don't have 220V AC to test it with..  

I did remove D2 and wired a female barrel plug to the power (C5)..   As a quick test with no input video, I used a variable 5-18v wall wart, and tested with 9v, but I don't see any current draw, but perhaps it's because there's no video input.

The bigger problem is that I just noticed the RF output connector is of PAL type, so I'm thinking that it's not likely to work with an NTSC TV anyways.   Although there's nothing in the product listing that says PAL or NTSC.

Anyways, rather than cutting up a F-type cable and replacing the PAL cable, I ordered a PAL Female to NTSC connector.  When I get it, if it looks like the output is PAL, then I guess I'm hosed.       There's some cheap PAL to NTSC converters on ebay, but require demodulated rca input video.

« Last Edit: December 22, 2017, 12:10:22 12:10 by snoop911 » Logged
h0nk
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« Reply #11 on: December 22, 2017, 02:05:22 14:05 »


Hello snoop911,

An "AV to RF converter" is also known as Modulator. Better ones used integrated circuits,
like the "CXA1733M" from Sony. Sorry but the "CXA1733M" this also a P.A.L.-type Modulator.
But the datasheet may contain some hints to use and modify their circuitry.

You should find lots of similar devices in old VCR's, Sat-Receivers and Home Computers.
I expect they would match Your local T.V.-System also.


Best Regards
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Signal
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« Reply #12 on: December 24, 2017, 10:00:03 10:00 »

I did remove D2 and wired a female barrel plug to the power (C5).
It doesn't look for me as you connect black wire to the right point. I'd check ones more.

And don't forget to check voltage rating of 300uF (C5?) capacitor as pickit2 suggested. Working voltage should be 1.5-2 times less if the capacitor was chosen correctly by designer/manufacturer.

Also it looks that all circuit is powered from ZD1 as enzine noted. So increase supplying voltage carefully while checking voltage across ZD1 at the same time. ZD1 and series resistor (between 300uF and ZD1) have limited power dissipation ability.

I don't see any current draw, but perhaps it's because there's no video input.
There will no considerable difference (if any) of current depending on signal presence.

The bigger problem is that I just noticed the RF output connector is of PAL type, so I'm thinking that it's not likely to work with an NTSC TV anyways.   Although there's nothing in the product listing that says PAL or NTSC.
According to what I remember about color coding in TV signal the video part from AV input of this modulator should be already coded by your video source to PAL or NTSC (or whatever). The modulator only moves that signal (together with audio) to RF band.
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enzine
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« Reply #13 on: December 24, 2017, 10:54:29 10:54 »

This gadget is a very "poor man" AV RF modulator, only two transistors to do everything!
First of all you check if the circuit works without any AV input.
You have to do a channel search on your TV in the VHF band until you see a white screen and then add the AV signals.
Obviously, the search should be done on analogue channels and not on DTV.

Ciao.
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