23c/m COMTECH RX 6MHz VISION TRAP CURE
After adding the RX de-emph circuit as in CQ-TV 195 I was shocked to see all the mush coming out of the amp at first I thought it was unstable but on investigation found that it I removed the video from the TX I detected a carrier this was the 6MHz one (NOTE I had removed the 6.5MHz carrier on the TX and increased it's level to -20dbm as is standard then it matches the video when weak). I next did a plot of the removed filter parts L1 C2 L2 C3 and this helps to remove the sound carrier by -15db. I was seeing about 450m/V of carrier and was upsetting the syncs. Just adding a Cap does drop it but upsets the Colour burst and the higher vision frequencies as well. So I sat and thought could we use a 6MHz filter but put it in shunt mode on the input of the NE592 Amp and yes it worked. I had tried to place it in the feedback of the amp but it took off and was very unstable. I did try this in all modes and the best was as shown below. I finished up with a clean signal and was left with just the correct gain level to sort out and the LINE 50% LF DROOP PROBLEM and this is in the RX not the TX. I noticed an error in the Figure 1 of CQ-195. The biasing to the NE592 on pin 1 they have potted down half the voltage from pin 8 ? The input will not work with a DC differential 2V Voltage Below is how it should be done (Also see my new way of biasing this amp to stop LF distortion.)