Wiring an RF modulator from a VCR.

February 12, 2007 7:48 AM

I've pulled an RF modulator out of an old VCR, but I don't know what some of the pins are for on the unit. Help me wire it up to modulate a composite video signal.

Here are the pins, in order:

+5V
BS
AUDIO
CONV B+
VIDEO

I only need to modulate the video - audio connection is unnecessary. I've tried hooking up a 5V 1A power supply with the ground wire hooked to the modulator's chassis. I then connected the center pin from my composite cable to the VIDEO pin. I also tried a variety of other connections, but couldn't get anything to display.

What are the other pins for, and what should I do to get this thing working?

Reason for doing this: Old TV with no composite input, tried using a VCR as a pass-through, but Macrovision gets in the way. I'm aware that I can buy an RF modulator, but I'd like to try this out first.

posted by odinsdream to technology (12 comments total)

Are there any model numbers and/or manufacturers on the device? Maybe you can google them for a pinout/spec. sheet?
posted by Xoder at 8:23 AM on February 12, 2007

I hope you find (and post) a part/model number as you can usually google those and get PDF datasheets that list the pinouts and what they require.

From a completely naive standpoint (meaning, lacking a datasheet, you can take this approach:

++++

Connect the modulator to a telivision tuned to the RF modulator channel.

Connect to -ve or common terminal of your power to the shield of the modulator. Touch the positive lead to each pin until you get a blank channel instead of snow.

Do the same for audio and video signals until they go to their proper pins.

The then unused pins are grounds. The one adjacent to the video in is signal groun for video.

++++

I got that from Usenet (google groups) try searching around in there for "RF modulator pinout" and the like. My guess is that you'll have to supply a voltage on one of those other pins to get it right.
posted by fake at 8:40 AM on February 12, 2007

not sure what BS and CONV B+ stand for, but I'll hazard:

CONV B+: rf modulated signal
BS: shield for rf cable

Did the old VCR have any means for selecting the output channel?
posted by contraption at 8:41 AM on February 12, 2007

Some schematics here.

Would love to hear a success story on this.
posted by fake at 8:45 AM on February 12, 2007

The input channel selector is on the modulator unit, and selects from channel 3 or 4. The pinouts I've shown in the question were what I read from the silk screening on the PCB around where the modulator was mounted in the old VCR, I'm just not sure what some of them actually mean.

In addition to the bottom pins, where the device was soldered to the board, there is an incoming cable with an RCA male end on it that was plugged in to another component inside the VCR. I've also tried plugging this lead into my composite output, but get nothing.

I've not even been able to get a no-snow channel, so maybe it isn't powering up? Is 1A enough current?
posted by odinsdream at 8:45 AM on February 12, 2007

Is 1A enough current?

Yes, definitely. I don't think BS is a shield, that doesn't make sense. It is more likely "bias". Many such circuits require a bias voltage- check the links I sent you.

If you can't get a no-snow channel (have you tested both channels 3 and 4?) then you're not modulating a carrier. Doesn't mean it's not powering up, but it might mean you're not giving it all the components it needs to generate a carrier.

The RCA cable from elsewhere on the board is probably RF from an RF input or something similar.

Hope Paulsc finds this question, IIRC he's a ham and definitely knows his way around circuits.
posted by fake at 8:50 AM on February 12, 2007

I don't have specific advice but here are some thoughts:

* You should probably connect the shield from the composite cable to wherever you grounded the power supply

* You may need to connect power to some of the other pins. Conv B+ seems like a likely candidate... Some googling seems to indicate that modulators will often have a modulation stage and a boost stage, and that each might take +5V.

* The chassis may not be the ground connection... there may be some place in particular you are supposed to ground it. I don't know much about this kind of equipt. though. Like you, I would probably assume chassis first, especially on something that encloses an audio/video frequency device (grounding via the chassis gives you some RF noise-rejection)
posted by RustyBrooks at 8:50 AM on February 12, 2007

Here are two images of the modulator. Note that the pins and labels don't seem to line up correctly due to perspective. The very top and very bottom protrusions are just extensions of the case. The labels refer to the pins between these two areas.

Back
Front

I'll be able to test out some of your suggestions this evening, like hooking the composite shield up to the chassis. Thanks.
posted by odinsdream at 9:41 AM on February 12, 2007

Here's what's printed on the case:

VSCS 1156 MADE IN JAPAN
VSCS 1155 MADE IN JAPAN
VEQS0387 89 2
And, the number 3 in a circle.
posted by odinsdream at 9:44 AM on February 12, 2007

B+ is antique-speak for bias voltage in vacuum tube circuits, still used sometimes for secondary power source on some RF gear.

I'd bet 10 bux it is another power-in lead. "+" meaning positive, of course.

These things usually get powered by +9 or +12 V.

Tbe BS lead rings no bells, but I see no ground lead (for either the +5 or B+ supplies, so that would be where I would start.

The enclosure that the unit is in (the can) is usually grounded, so if the BS lead measures 0 ohms to it, you have correctly identified it as the ground lead.

Audio/video self-explanatory. The enclosure can be used for shield ground on your coax.

#3 in a circle may be channel 3 modulator. Common practice is either channel 3 or 4. My bet is 3, for obvious reasons.
posted by FauxScot at 11:20 AM on February 12, 2007

Thanks to the help you've given, I've been able to get the device to modulate the signal. Here's what it took:

Hook +5V up to both the +5V pin and the CONV B+ pin. Hook the composite video signal cable shielding up to the chassis, which is in turn hooked up to the ground side of the power supply.

The device transmits the signal fine now, but the picture is snowy. Any thoughts on correcting that? I've tried winding the power supply cord and laying it in different directions to avoid interference, placing the whole assembly into a metal-lined box, and hooking the chassis up to one of the grounding screws on the receiver/amp supplying the video signal. No change in the amount of snow was noticed.

Thanks again!
posted by odinsdream at 3:38 PM on February 13, 2007

Additional notes:

The BS pin provides just under +5V when the device is powered as described above. Still not sure what this is for.
posted by odinsdream at 3:47 PM on February 13, 2007