Separating Video and Audio in Split Carrier Sets
 

I've got a set on the bench with an odd audio problem. After recapping the audio had a lot of buzzing along with the audio track. I suspected the problem was video feeding into the audio circuit. I've plugged enough RCA video cables into audio ports doing installations blind to know what it sounds like. My suspicions were confirmed when I adjusted my modulator's video level output to minimum and the audio came through loud and clear. So my question is how is video and audio separated in a split carrier set? How can video be "infecting" my audio?

By they way I should mention that the video IF alignment is COMPLETELY out of wack. Would that have anything to do with the audio problem?

Well, using the philosophy of fixing the most obvious things first, I would do the alignment and see what happens. Can you improve the buzz problem with fine tuning?

Also, are you talking about a true split carrier set or quasi split (with a separate sound IF to generate 4.5 MHz subcarrier)?
https://books.google.com/books?id=U9...v%20if&f=false

The buzz intensity does change with fine tuning, but it comes and goes with the audio.

If the misalignment on the video could cause the audio problem then I'll go in that direction, otherwise I want to fix the audio problem before moving on.

Reached the limit of my knowledge. Hope somebody recognizes the problem.

We need more info to be able to help you. Give make, model and chassis number please. Ok gents if you want help give as much info as you can. Pictures will also help. With what little info given it sounds like an alinement that's way off. All the best,Tom

That link is European, using entirely different nomenclature than U.S., and shows 5.5 mhz sound vs. the 4.5 mhz U.S. standard.

In U.S. vernacular, what needs to be established is whether the set in question uses the 'intercarrier' system wherein the 4.5 mhz sound carrier is taken off after the video detector, or

if the set uses the older, true split-carrier having dedicated video and sound IF strips, split at the tuner output.

True split carrier with completely separate video and audio if sections. The set is a 621ts. Here is a link to the schematic http://www.earlytelevision.org/pdf/R...-Rider-TV1.pdf. I can share some photographs, but I don't know what to show? I can post a video to YouTube (hopefully tonight) that shows the issue.

Since it's a true split carrier, you want to address the audio problem first. First thing I would do is reduce the signal level to be sure overload is not the problem. You can do that by disconnecting one side of the antenna lead or just by 'proximity coupling' without direct connection.

Yea I thought that too, the problem does sound like it could be the tuner getting pummeled with signal. When I cut down the input signal the buzz gets quieter, but the audio goes down with it at the same rate. My other thought was that my modulator was at fault, but another set connected to the same modulator sounds fine. The only thing I can do to make it go away is kill video going to the set by removing the video input to the modulator, or adjusting the video output level to zero. The audio signal is very loud and very clear with no video going to the set.

One thing if the picture(contrast) control is set to high can cause this, back off the control and see if that helps. On this set the picture control is a R.F. and I.F. gain control, doesn't control video amp gain. All the best, Tom

One question- When the buzz is present, does it vary with the volume control? That is, will the buzz go down to zero with the vol control all the way down?

If so, this eliminates the possibility that video might be leaking into the audio chain after the volume control.

You know I'm not sure I adjusted the picture control or not...that would be the simple answer I often miss :scratch2:.

The volume control does attenuate the buzzing. I also injected audio just before the volume control to see if the buzz was still present and it is not. Removing the tuner input to the 1st audio IF stage also kills the buzzing (and of course the audio).

Another thought that came to me in my sleep last night is changing the channel that the modulator is operating on. I'll give that one a shot if the picture control has no effect.

Well, adjusting the picture control doesn't effect the buzz in the audio. I also changed the modulator's output channel a couple times and that also had no effect. I didn't have much time today to experiment.