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Dimitris76

Problems with Pyle video amplifier

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For quite some time now I have been using a home made antenna tracker based on DakarOSD and Lynx tracker controller plans. http://www.aeromodelismovirtual.com/showthread.php?t=1849

Recently I thought it would be nice if I upgraded my ground station with DVR and an extra LCD monitor apart from my goggles so I put to use my trusty Pyle 4 channel video amplifier.

The tuning procedure is to adjust the R8 multiturn potentiometer (see attached schematic) until the LED lights almost solid, meaning that there is a steady flow of decoded data coming out of the Lynx antenna tracker controller on the ground. I do that and get a solid light over VTx/VRx link and everything works wonderfully until I plug in the RCA cable that goes to Pyle amplifier. As soon as I do that the amplifier the LED turns off and the tracker's operation is interrupted. I turned the pot in every imaginable direction without success.

The LED turns on and the tracker resumes it's function only when I disconnect the cable to Pyle amplifier.

Does the amplifier introduce interference? Is it some kind of impedance mismatch?

Thank you in advance,

Dimitris

PS I am even attaching a .pdf my version of Lynx board

post-1403-0-67181900-1407877981_thumb.gi

LynxBoard_v1.3.pdf

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Ok more info.

It seems that the problem is not Pyle-specific.

I connected the tracker with the same long video cable - about 15ft. long real coaxial - straight to DVR video input and had the same problem. The antenna tracker freezes and does not resume unless I unplug the cable.

Then I tried another composite video cable - longer and thinner - in case it's the cable's problem and the same thing happened. When I connect a Philips 9" monitor I have straight to the tracker I have both picture and a working antenna...

/Dimitris

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If you look at the schematic, there is no specific video output and input on the Lynx tracker. Input from VRx and output to monitor devices, DVRs, video amplifiers, etc is common and in parallel.

Could these long cable runs throw something off?

/Dimitris

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Long video cables can cause ringing (signal overshoot and undershoot) if the installation is not respectful of the required 75 ohm video impedance. That is to say, the video source, cables, and the connected monitor must observe a precise 75 ohm impedance. For example, the tracker circuit you posted doesn't appear to fulfill this requirement, so I suspect it is partly to blame for the problem.

But I think the Pyle amp can help out, despite your bad luck when you tried it. Post a drawing of how everything was connected when you used the Pyle amp. Be sure to label the cable lengths that were used. This will allow others to review your installation and offer advice.

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You were right about the long cables Thomas!

I just used a much thinner but also much shorter - only 1m long - cable between the tracker and Pyle amp and the problem went away. It's a viable solution but I would really like not to have the Pyle amplifier on the car roof next to the tracker.

The whole point is having only one composite video cable entering the car and then have the amplifier to distribute the signal to various devices.

So, how can I eliminate this "ringing" on long cable runs? This 75 ohm impedance - is it something I can measure with my multimeter and adjust with resistors between signal and ground?

/Dimitrios

PS I am attaching photos of my "Good" quality 6m long cable that I would like to use between tracker and Pyle and the ratnest on my desk just to give you an idea... hehehe

post-1403-0-62636300-1407920308_thumb.jp

post-1403-0-64994000-1407920323_thumb.jp

Edited by Dimitris76

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It's a viable solution but I would really like not to have the Pyle amplifier on the car roof next to the tracker.

Remotely mounting the video amp with a long cable is possible. Your long cable would work if everything was perfect (quality made video cables, good amp/buffer, properly designed vRx, etc.). The long cable's failure and the short cable's success suggests that one or more things are not correctly following the 75 ohm impedance requirements. Or perhaps the tracker controller's video input design might not be optimal and is contributing to the problem.

This 75 ohm impedance - is it something I can measure with my multimeter and adjust with resistors between signal and ground?

You can't measure impedance with an ohmmeter. However, if the Pyle amp follows the traditional video design practices then its main Video Input will be DC terminated with a 75 ohm resistor. So you should be able to use your ohmmeter to measure this. If the Video Input it is dramatically higher than 75 ohms then Pyle cheated and omitted the important DC termination. If this is the case then you can either use a short cable or replace the Pyle with a more expensive professional amp/buffer.

I am attaching photos of my "Good" quality 6m long cable that I would like to use between tracker and Pyle and the ratnest on my desk just to give you an idea...

Thanks for the photos. Unfortunately I can't tell what is wired to what in the photos. If you want me to check it over then post a clear drawing that shows the routing of all the video cables, with lengths noted on it.

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Thanks for the photos. Unfortunately I can't tell what is wired to what in the photos. If you want me to check it over then post a clear drawing that shows the routing of all the video cables, with lengths noted on it.

My Lynx controller's board is posted as .pdf file on post #1.

Here is a block diagram of what I want to accomplish. The problems occur when a long cable is being connected between the Lynx and Pyle amplifier.

If Pyle is not properly terminated, can't I just solder a pot across it's video input and adjust it accordingly?

/Dimitrios

post-1403-0-62808700-1408028429_thumb.jp

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If Pyle is not properly terminated, can't I just solder a pot across it's video input and adjust it accordingly?

It's a possible solution if the Pyle cooperates with it. Adding a video termination resistor to its input will reduce the signal level. For example, the video level will be reduced 50% with the ideal 75 ohm value. So start with 500 ohms and do Not go below 75 ohms.

The termination must be located directly on the Pyle's Video Input jack. As you vary the termination value you will need to use the pots on the Pyle to compensate the video gain to maintain the proper video levels. This is a situation where a o-scope is good to have, otherwise you will just have to guess at the pot settings.

But before you do that I have a test for you to do. This test is to see if the antenna controller's design is contributing to the problem.

1. Remove the antenna controller from the VRx.

2. Connect the Pyle to the VRx with a very short cable. Connect your monitor to the Pyle. All cables must be short.

3. Connect the antenna controller's video input to one of the unused Video Out ports on the Pyle using a short cable.

4. Install a 68 to 100 ohm resistor on the video output of the antenna controller.

5. Determine the correct Pot settings (on the Pyle and Antenna Controller) to achieve best quality video and antenna tracking. Confirm everything is working correctly.

6. After everything is working correctly, change the VRx cable to the 9M long cable.

7. Check the video quality and antenna tracker for proper operation. If necessary, slightly adjust the Pots. Can you get the 9M cable to work with this arrangement?

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I followed you instructions:

Step 1: Done

Step 2: Done - connected with 1 meter long cable

Step 3: Done - connected with 1 meter long cable

Step 4: Skipped

Step 5: Skipped - I didn't have to adjust anything - read next step.

Step 6: Everything is working like a charm. A solid LED on the controller and a perfectly functional antenna (tested indoors with a NMEA generator program).

Step 7: Vtx to Pyle input cable replaced with the 6m long one. No go... Tracker freezes. Is this because is skipped step 4?

I am trying to measure the resistance in Pyle's input and it's infinite according to my multimeter.When I switch up to 200k scale it shows 50 and then it drops to zero...I am lost here...

/Dimitris

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Ok I found a 98 Ohm resistor and did Step 4. Still no go. The tracker dies as soon as I replace the short cable from Vtx to Pyle's input with a long one. :(

/Dimitris

Edited by Dimitris76

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Here is what I have noticed when I tried once more the 6m long cable. When I slightly pulled the RCA connector out of it's socket the tracker came to life. I was excited and I believe now the problem is just a case of a sloppy rca socket or a cold solder around it.

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Ok I found a 98 Ohm resistor and did Step 4. Still no go. The tracker dies as soon as I replace the short cable from Vtx to Pyle's input with a long one. :(

Did you try fine tuning (re-adjusting) the Pyle's gain pot and the tracker's pot? If nothing helps then the tracker is probably not contributing to a video ringing problem.

I am trying to measure the resistance in Pyle's input and it's infinite according to my multimeter.When I switch up to 200k scale it shows 50 and then it drops to zero...I am lost here...

That means the Pyle has a AC coupled input and is not DC terminated with 75 ohms. This sort of thing is done to simplify a video amp's design and save costs, but it is not a professional solution. If you don't want to continue experimenting & testing then replace the Pyle with a commercial quality video amp/buffer that provides the desirable 75 ohm termination feature. Unfortunately there's no 100% guarantee this will solve the issue because there may be other problems in the video circuit.

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That means the Pyle has a AC coupled input and is not DC terminated with 75 ohms. This sort of thing is done to simplify a video amp's design and save costs, but it is not a professional solution. If you don't want to continue experimenting & testing then replace the Pyle with a commercial quality video amp/buffer that provides the desirable 75 ohm termination feature. Unfortunately there's no 100% guarantee this will solve the issue because there may be other problems in the video circuit.

This reminded me a conversation I had with Ian another hobbist that modified the original Lynx according to attached schematic.

Then I looked for his old email and I quote:

"I then hooked up a DakarOSD board and attempted to get data to the Lynx. However, I got nothing. Then I remember that I had a problem with Cristobals original design and so I modified the circuit a little. If you look at the Lynx schematic on my web page you will see 2 resistors and an extra capacitor at pin 3 of the LM393.

This modification fixes a problem when the video signal has a large DC offset......which some receivers can have. My modification AC couples the input so that any DC offset makes no difference."

I have this mod in my Lynx too. Is it possible that LM393 IC is providing 75 Ohm termination via pin 3? Could this modification on Lynx be the reason of my problems?

/Dimitrios

LinceIJV2.1Schematic.pdf

Edited by Dimitris76

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I have this mod in my Lynx too. Is it possible that LM393 IC is providing 75 Ohm termination via pin 3?

No, it's not providing the 75 ohm video termination.

Could this modification on Lynx be the reason of my problems?

Upgrading the original design's video DC coupling to AC coupling will help improve compatibility with video signals that have excessive DC offset. But I recommend using different values than what are shown in the LinceIJV2.1 Schematic, as follows:

- Change R10 to 4.7K ohms

- Change R11 to 2.2K ohms

- Add a 10uF cap in parallel with C9 (+ end at R10/R11 junction).

After this change your voltmeter should confirm that LM393N Pin 3 has approx 1.6VDC on it (unplug tracker from video before voltage measurement).

Here is what I have noticed when I tried once more the 6m long cable. When I slightly pulled the RCA connector out of it's socket the tracker came to life. I was excited and I believe now the problem is just a case of a sloppy rca socket or a cold solder around it.

This suspicious issue needs to be resolved before you do anything else. So spend more time troubleshooting this and fix the connection problem.

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Thomas,

I am starting to believe my problems were related to the 6m long cable...

Since you posted your instructions I am running the video signal first through Pyle amplifier then send it back to the tracker. The idea is to boost and condition whatever quality of video signal the diversity receiver is sending out, before I send it to the tracker's circuit board. I wanted to reduce the clutter so I figured that a single 3x RCA AV cable would be enough for all this - yellow from VRx to Pyle's input, red from Pyle's output back to Lynx tracker, white for the mono audio channel.

That's what I did with a 1 meter long AV cable and it was working very stabile but I didn't like the idea of placing all this stuff on the car's roof (next to the tracker), so I tried once more the 6m cable with the thick video coaxial. Same... the tracker was not working. As a last resort I tried a 10 meter cable I had lying around - the cheap kind, the one that has thin 1/8" cables on all three RCAs. And it worked!!!

I went to the store's website:

http://www.clasohlson.com/se/Anslutningskabel/Pr321228000

and read

Skärmad: Nej in english - shielded: No

So apparently despite the fact that the video cable is much thicker and stiffer that the audio channel cables - the braid is not soldered on the RCA coonector's ground and that makes all the difference in the world in my application.

/Dimitris

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I had a similar wild goose chase with a parallel cable when I made my CNC Mill. The steppers would jump at odd times so I did lots of mods to try and kill the interference only to find out it was a rubbish cable.

Terry

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The tracker is not 100% reliable yet but I am getting there. I have two pots to play with now - Lynx' and Pyle's... Hehehe

Thank you for your help!

/Dimitris

Edited by Dimitris76

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