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Wavess

Strange problem with lawmate tx 1.2ghz

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Hi all, it´s been a while since I don´t post here.

I´ve been flying with lawmate 1.2ghz 500mW tx for a while on my quads and okto. On my okto, I used to use an extension pcb that mikrokopter used to sell a while ago, and now that I´m looking for it to link it here, seems that they don´t sell it anymore. Anyway, the voltage regulator that they used was a Traco TSR1-24120 SIP3

I used that extension pcb apart for feeding my leds, for taking down volts from my main battery (4S), to 12V. With those 12V, I powered the camera, microphone, and the original cable regulator from lawmate that requires 12V. I quit on using that regulator because if you have lower that 13.5V in, the regulator quits working.

Instead, I installed the very popular LM-2940-12V, for feeding 12V to the lawmate regulator, camera and microphone. The system has been working very well for months, either on my okto and on my quad, until couple weeks ago.

I went to fly with my okto, and right after taking off I got a very poor video signal, lots of snow, and unfliable. Had to land right the way without googles. Went home, tried that tx on my quad and saw that the video signal was too bad, tx didn´t get hot as usual eithter.

Changed the tx on my okto for a new one, tested at home once, all was good. Went to fly yesterday, made an entire flight without any problems. When I was prepared for a second flight, the same bad video signal right after take off. Went home again, and checked that I had from the lawmate regulator nice and stable 5V out. I changed the lawmate regulator for a brand new one, changed the tx again for a new home, and made a 1 hour continuosly trial at home, video was good again. After 30 min, did a second trial at home, and then it happened again, tx fried.

I then inspected with the oscilloscope on the 5V line out the lawmate regulator, did few trials, powering up and down, and didn´t appreciate any voltage peaks, clean 5V were read.

I kind of lost on this problem, have tried with different antennas too, and that doesn´t seem to be the problem either.

Sorry for this long post but I tried to explain well for better understanding.

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1. If the video problem is intermittent, then I suspect you have incorrect stability caps on the LM2940. Despite an endless number of users that say there's no need to install them on 3-terminal linear Vregs, they are VERY important and MUST NOT be omitted. Here is the LM2940's data sheet's instructions:

post-2-0-45072400-1344295672_thumb.jpg

If the stability caps you used are correct choices and properly installed with short leads, then intermittent operation may be due to the Vreg's temperature protection. Your application will create 1.5 watts of heat on the LM2940, which is a big thermal burden. A large heatsink and a lot of air flow will be needed. Use in hot environments will need extra care.

2. If the Tx's are being permanently destroyed then the ground wire connection to the LM2940 is suspect. That is to say, if the Gnd connection is intermittent then the LM2940 will put out full battery voltage and possibly harm the video Tx.

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Thanks for helping. I always use the correct caps on the datasheet on all my regulators. I haven´t seen an intermittent operation of the video tx, even on my second tx installed, this was working for about 45 min continuosly as a test at home, without any air flow, I mean static operation.

I suspect that there´s something related with power on and off, because this has happened when I turn off the system (all went good flying) and when I turn on for a second flight, tx has burnt. Seems like the tx lost its power stage, because it doesn´t even get hot

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Well, the end of this story.

I received new lawmate 500mW rx, and I could try some other things. I have installed a switch for the fpv gear, so when I don´t fly by fpv my okto I turn it off.

I tried, to connect/disconect the main battery with this switch off. Voila, problems gone, no more tx damaged so far.

I think this is something related with okto´s caps loading on startup, something that my oscilloscope didn´t see, but might be there and that damages the tx.

Do you think is there a way to repair my 3 damaged tx?

Edited by Wavess

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If the Tx's were harmed by supply voltage transients then it is hard to predict how much damage they suffered. But if the PLL IC and microprocessor survived then the repair may be practical. The only way to know for sure is to investigate the Tx's with your o-scope to see what you find. Keep in mind that if you find a bad RF PA transistor then the repair would require a Spectrum Analyzer to recalibrate the RF output (via trimmer cap near PA transistor).

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