I was working on this issue some more this weekend with a BAVM-z. I plugged the composite video and audio from a DTV box into the inputs and the low level test output directly to the antenna terminals via a balun. The BAVM-z has separate controls for video and audio levels which is very handy.
Using this setup, I was able to confirm what I observed earlier. If I decreased the video to almost zero, the sound levels improved significantly. Unfortunately, the picture was unwatchable. I'm pretty sure this effect is due to the AGC which is derived from the video alone. This set has separate video and sound IF, but they do share an RF amp up front which is hooked to the AGC. So, weak video will crank up the gain on the RF amp so the sound is louder.
I fired up my EICO 369 and aligned the audio IF stages. Coils A2-A5 behaved as described and I was able to get a decent overall ratio detector response curve. However, I had to crank the sweep gen. output to almost maximum to get a decent curve. Adjusting A1, the 21.25 MC audio trap, had almost no effect. That can't be good! I did swap out the 120 pF mica cap across this coil but that didn't help
Also, I got a tip about the 30pFs cap inside the 1st audio IF stage being suspect. I popped the cap off and indeed they are not encapsulated. So, I order up some new ones for this and the 2nd stage and that 120pF across A1. What a pain!
07-05-2010, 09:54 PM
Hybrid Mode
