Purpose of noise inverter????

[reichsrundfu]

Hi All,

I'm tweaking around attempting to make the vertical sync on my Predicta Barberpole a bit more stable. I got to wondering, what is the actual functional purpose of the "noise inverter" just ahead of the sync separator? I'm wondering if this could be affecting the touchy vertical sync on this set?

Thanks,
-George-

[old_tv_nut]

The noise inverter could affect sync, but I would expect it to affect both vertical and horizontal.

The purpose of the noise inverter is to invert RF noise pulses that go beyond the sync tips, quickly clipping them and making them go towards white instead of super-black. This shortens pulses like ignition interference greatly, so that they do not overwhelm the sync separator. What would be a wide super black pulse gets turned into a narrow super black pulse followed by a wide white-going pulse (I mean that it goes in the direction of white, not that it necessarily goes all the way to white or even all the way to black level). The following RC filtering before the sync separator then tends to reduce the negative part even more (because it is now narrow), so the sync separator is much less likely to react to it.

Edit: The noise inverter could be degrading both H and V, but if one of these is more sensitive to the degradation, it could fail separately. So still worth checking the noise inverter even if only vertical is a problem.

[reichsrundfu]

OK. This makes some sense. I have found that the vertical hold becomes unstable when a scene changes - especially if a scene in the picture becomes a nighttime scene with an abundance of black. I also see it occur when for example, when a tv program goes to commercial: the commercials tend sometimes to be less stable than the program pictures, of if watching local news, the studio shots are more stable, but when they go to a reporter in very "contrasty" setting again the hold decreases. Some times in fact, you can go to no vertical lock at all, going from upward to downward roll by the slightest adjustment of the vertical hold control. Change scene, and it'll lock right in.

I also notice it once in awhile if I'm playing a DVD however the video out on the DVD runs thru an image stabilizer to remove any kind of copy guard interference which could affect things, so overall watching DVD's are the most stable. If I run my cable thru the image stabilizer it has no effect.

When I first restored the set I was able to make the vertical hold more stable by swatting the sync separator tube but I didn't touch the noise inverter / vertical output tube. Perhaps I'll take a look at that next.

-George-

[bandersen]

Definitely try a different 10DE7. Also did you replace the vertical feedback and vertical integrator couplates ?

[reichsrundfu]

Originally Posted by bandersen

Definitely try a different 10DE7. Also did you replace the vertical feedback and vertical integrator couplates ?

Did not replace the couplates since they were behaving within minimal parameters. I'll do those after checking out other potential causes. My understanding is the horizontal couplets are the worst however my horizontal syn. Is rock solid. Of course that doesn't mean much if a cap elsewhere gets funky!!!
George

[bandersen]

I've always hear the two vertical couplates - especially the feedback are troublesome. You'll see them mentioned often in other Predicta restoration threads and articles. I replaced them in the Pedestal set I restored recently and it has solid vertical hold.

[reichsrundfu]

Well, guys, I'm dragging out the asskicking machine and as soon as I'm done posting this will be strapping myself into it for a bit: problem solved, and YES, it was the noise inverter section. Specifically, the 6AW8 tube was bad. The first section of this tube is the video output and the second is the noise inverter. When I swapped out the tube the vertical locked right in and is absolutely rock solid. I noted that the picture is also with much better contrast and definition.

So I pulled my Hickock tube tester and checked the one I pulled out. Section A was marginally good going into questionable. Section B (the noise inverter) was so far into the bad / replace range it was almost flatline. The gauge needle hardly moved.

But for me the learning experience was the function of the noise inverter, so at least I learned something useful!

-George-

[DavGoodlin]

Damn tubes hardly ever go bad these days.

Good work!

[reichsrundfu]

Originally Posted by DavGoodlin

Damn tubes hardly ever go bad these days.

Good work!

Yeah I know, but since this is a transformer less chassis and therefore carries more current and runs hotter I really should've known better! Well if you learn something every day it's a good day to be alive. Even dumb stuff you should already know!

[bandersen]

Why do you say it carries more current and runs hotter ? The tubes run on 600mA same as if it had a filament transformer.

[reichsrundfu]

There is no power transformer to absorb some of the load which therefore is absorbed by the rest of the chassis which sadly includes the PC board. My experience had been that transformer less chassis always distributed more best throughout the entire chassis.

[miniman82]

If you don't run it on a variac or isoformer that has a voltage switch, it probably does run hotter. Remember that our line voltages can sometimes be much greater than the 117VAC specified by old sets, that extra voltage translates into more heat being dissipated by heaters and deflection circuits which is why I'm very adamant on people running at 117 input volts and making damn sure the HO tube cathode current is as low as possible.