The contents of this page are based on work done by Barry VE6ATV and are presented here in brief.
His website and full article on which this page has been based can be found here.
The Comtech module and evaluation board as supplied by G1MFG and others have
provided a simple and low cost way onto ATV.
These modules have fast become the standard choice for ATV enthusiasts, mainly
due to their low cost however, they do have some inherent problems.
The most noticeable problem with these modules is the poor low frequency
response, which can cause the pictures received from these transmitters to roll.
This can be corrected through simple modifications, but please be aware, the
modifications require the exchange of some surface mount components, fitted into
a cramped space. If you are not used to working with surface mount
components do NOT attempt these modifications as there is little room for error
and you can easily end up with a damaged module!
Step1
The main PCB has a 7812 regulator mounted to the right of the yellow preset. The module requires a supply voltage of 15v or more (because of this regulator) to function correctly. If you are going to use this module from a 12v regulated supply, remove this regulator and fit a shorting link on the main PCB between pins 1 and 3 of the 7812.
 |
 |
The remaining modifications are in the can. Carefully remove the lid and perform the modifications as below.
 |
 |
 |
|||
|
To allow access into the PLL compartment, this tab must be temporarily bent up and out of the way.
|
Replace 36 pfd capacitor at this location with a .01 ufd capacitor. The 36 pfd cap is so tiny that it is easiest to replace it. Photo shows a larger .01ufd cap. used for mod. |
Stack & solder a 0.5 ufd capacitor on top of the 1.0 ufd capacitor at this location or simply replace the 1.0 ufd capacitor with a 1.5 ufd one. It is usually easiest to simply stack & solder components if they are of similar physical size. |
|||
 |
 |
 |
|||
|
Replacing this 1.0 ufd capacitor with a 2.2 ufd 6.3V capacitor will make a further slight improvement to the transmitter low frequency response, but the improvement is too small to make this mod worth doing. |
Video
Peaking Components C1 & R1. |
VR1 Video Deviation Control |
|||
| CCIR
PRE-EMPHASIS AND DE-EMPHASIS
The circuits here were designed by Dave G8AJN. Please visit his website at www.g8ajn.tv |
|||||
The CCIR standard pre-emphasis is intended to raise the hf portion of the video so that it can be reduced at the receiving end by the same amount and in doing so also drop the noise level. The Comtech modules do not have CCIR pre and de-emphasis and should be fitted for best results. Below are the basic circuits. |
|||||
|
||||||||||||
