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I've noticed some video issues lately.

Made some tests and, after using other cameras and VTxs I realize that the issue is related to the receiver. Basically I'm getting 3.35V amplitude from Video Out of the Lawmate 1.2GHz Receiver, which is not right as it should be 1Vpp.

I thought of using a series resistor to try to solve that issue but have not done yet that mod as probably that can be solved by a trimmer on the rx if any available.

Any help will be appreciated.

Thank you

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How did you measure the receiver's video level? Did you use a a 75 ohm resistor on the video output to properly terminate it during the measurement?

The video level will vary wildly depending on the channel that is selected. Even the vTx antenna that is installed can impact the measurement. So be sure to do your calibration using the exact setup and freq you intend to fly with.

Keep in mind that the Lawmate vRx expects a Lawmate vTx. So if you are mixing vRx/vTx brands then the video levels will be incorrect. Unfortunately when brands are mixed it will be difficult to know if the level issue is caused by incorrect calibration at the vTx or incorrect calibration at the vRx. Choosing which end to adjust will be a coin toss and if the choice is wrong then the calibrated video level will have reduced performance. BTW, some brand mixing combinations are not compatible, even if the video levels are calibrated perfectly.

I recall you installed a SAW filter in your Lawmate vRx. Video calibration after doing that is definitely recommended. I recognize that very few do a recalibration after the SAW upgrade and most are satisfied with their video. But I just don't understand why someone would to go to the trouble of installing the SAW to achieve better RF performance, only to ignore the video performance. They both matter. :)

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Thanks for the quick reply Mr. RC Cam.

Yes, a 75Ohm resistor was used during this test.

I have always flown on 1280MHz (both Tx and Rx), so this variable should be not an issue (also the antennas was always the same - stock one)

That could be a possibility. In fact I'm mixing a lawmate rx with a fox 800. Thanks for the insight regarding this.

Well it was bought from RMRC some time ago and they did the saw filter mod, but really don't know if they have calibrated the video levels after the mod.

You're right, this is not a linear subject, the lack of knowledge from some vendors does not help also.

Edited by HF_ATL

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I've had to recalibrate nearly every Lawmate 1.2G setup I've tested. One of the issues is that the video levels vary greatly depending on the exact channel that is chosen. So the factory calibration that was performed has a 1 in 8 chance of being correct for the channel you decide to use. Plus, like many other brands, their random video level settings suggests that the calibration was done with their eyes closed. :)

But in your case the brand mixing makes the recalibration more challenging. Keep in mind that the emphasis circuitry varies with the different brands. Calibration helps even if the emphasis is not a good match, but is in reality just lipstick on a pig. That's one of the reasons I recommend not mixing brands. But it is widely done and most are happy with the results, so that is why it continues to be a widespread thing to do.

I may be able to offer some cal tips, but need to know a few things:

Does your vRx have a video level pot?

Does your vTx have a video level pot?

What kind of scope are you using? Is it a 2-channel scope?

Do you have a 1280MHz vRx that is a direct match (the same brand) to your vTx?

With the A/V cable removed, what is the input resistance on the vTx? Is it close to 75 ohms?

BTW, when we communicated in private you sent me a video to review. What I saw had some symptoms of reduced video levels instead of excessive levels. So your 3.3Vpp measurement is interesting. But the AGC used in some recorders (and monitors) can mask the exact issue, so anything is possible.

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First of all thanks again!

Regarding the VTx all I done was placing a potentiometer in the video in - gnd and adjust it so that I achieve a measurement of 75Ohm across.

Not sure about the videoRx pot because I've never opened it...

VideoTX has a video pot for adjusting.

Since I do not have a osc, I simply connected a multimeter to measure the voltage. It's not an ideal method I know but I have no access to other equipment unfortunately (and buying it overseas will be a bit expensive, like from dpcav).

I had other VideoRxs in the past but not anymore. By that time I simply did not inspected these parameters at all unfortunately.

I guess you mean "vRx" in your last question right?

I also tested with a simple video amp, and with that 3.3v input, the video amp gave 2.5v to their outputs... (no load)

Edited by HF_ATL

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Regarding the VTx all I done was placing a potentiometer in the video in - gnd and adjust it so that I achieve a measurement of 75Ohm across.

Ok

Not sure about the videoRx pot because I've never opened it...

You should check to see if it has one. If the POT is not available then it forces the decision on what you can adjust.

VideoTX has a video pot for adjusting.

Ok.

Since I do not have a osc, I simply connected a multimeter to measure the voltage.

The voltmeter method will not work. So stop and don't adjust anything until you get a scope.

It's not an ideal method I know but I have no access to other equipment unfortunately (and buying it overseas will be a bit expensive, like from dpcav).

I suggest you get friendly with a hobbyist that has a o-scope. In return for sharing their o-scope, offer to buy them a few beers or week's worth of fancy coffee.

I guess you mean "vRx" in your last question right?

Actually I mean vTx (video transmitter); What is it's input resistance to the video input? Use a ohmmeter.

FWIW, without a o-scope there's not much you can do.

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