Weak antenna input = strong sound and vice versa

[bandersen]

I've never been happy with the sound output level on a couple of my Admirals. They are split carrier sets with the audio picked off around 21.25 MHz and fed to a couple sound IF stages. Here's the schematic: http://www.earlytelevision.org/image...-rider-tv4.pdf

While tinkering with one last night, I tuned in to local analog OTA channel 6 and got the usual decent picture and low sound output. I disconnected the antenna and the picture faded away but the sound came booming out really loud! I did some more experimenting and concluded that the stronger the RF input at the antenna the better the picture, but the audio got weaker and weaker.

Has anyone experience anything like this ?

[bandersen]

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!

[jeyurkon]

Was it a flat sheet of mica? I had to replace one of those. It wasn't silver migration but a crack in the mica causing an intermittent open. Took me a while to figure that one out.

Most put the new caps on the outside, but I had room so I put it inside.

John

[bandersen]

Yes, they are the two caps stamped "30" inside the IF can. I'm pretty sure the new caps will fit inside. They should arrive Thursday.

I put a sweep generator on the audio trap coil and it certainly does have a peak resonance frequency, but adjusting the coil only shifts that a small amount. The capacitor has a much larger affect. The replacement I used was just a cheap ceramic. Hopefully, a new mica one will fix that up.

[bandersen]

I think I've solved my audio issues

First up , I removed the old micas and installed new ones. That improved the sound quite a bit.




Then, I made a properly terminated (90 ohm non-inductive) probe for my Hickok 615 generator and realigned the audio IF. That improved the sound even more.


Finally, for a signal source I used a DTV receiver feeding into an BT modulator. The low-level test output of that went to the TVs antenna terminals via a 75-300 balun. If I use the RF output of the DTV box directly, I get almost no sound at all!

Luckily, we do still have one LP VHF analog broadcast in my area. I can just use a two foot scrap of wire for an antenna and get excellent sound levels now! So I figure the DTV box is at fault here even though it sounds OK when plugged into a more modern TV.


Next up, I'm going to move the chassis into another room where I can plug it into cable TV.

[bandersen]

I hooked my set up to basic analog cable via a 75-300 ohm balun. Mostly channels 2-13 came through with a great picture and sound. Others had an OK picture but bad sound and one had both bad picture and sound. I suspect those could use a little tweak of the tuner slugs.