Hello!

Does anyone know if the I.F. output from a Drake or Blonder Tongue video modulator will inject directly into a vintage chassis such as a Zenith 20X1C36?

I suspect it will, but before investing in one of those (kind of expensive!) I was hoping to confirm.... I have been fighting noise in the picture on an old Zenith. I have been using either CH 3 or 4 from an RF modulator and the picture is very strong but I can't get rid of what I think is a ground loop or RF noise. There are diagonal lines that come and go and move back and forth. I've isolated everything, and experimented with every method I can think of to get rid of this, but still there...

So before I tear into an unnecessary alignment proceedure, I was wondering if I can just inject I.F. directly into the chassis and bypass the tuner. It seems like that would produce a superior picture anyway.

Anybody have any thoughts on this?

Thanks!

Dave

First thought is that diagonal lines that come and go probably have nothing to do with alignment, so don't go down that rabbit hole until you have exhausted other remedies.

Yes, you can inject IF - just try it and see if it helps. You should be able to go in at whatever point the service info says to use when aligning the IF.

If the noise is really coming in through the RF, the Blonder-Tongue should produce enough signal to overcome it, so you won't even need to inject IF, and you will be able to see the noise level go down as the RF level goes up. But if the noise is coming in some other way, you will still have it.

Thanks for your thoughts on that alignment...

The set really does work very well and the last thing I want to do is make it worse by my own hand. Many of the coils in this set are disintegrating physically, especially the ones with a translucent core. Those are held together by a careful application of epoxy. I'm sure just attempting to move some of the cores... would be a bad idea!

As for the noise, I would say it "rides on top" of whatever the signal strength is.
I've tried isolation in the coax feed, twin lead cable, plugging everything into the same outlet. The usual things... it's not something that can be fine tuned out really, it is on top of wherever the tuning is. It's odd that some times it's worse than other times sometimes it's gone completely.

Regarding the I.F. injection, I have a Sencore VA48, so I can inject I.F. test signals from that but was thinking of a permanent solution - and improvement by bypassing the tuner and going directly into the chassis with the modulated IF from a DVD or other source. The Drake modulator, as an example is made to output IF at 45.75Mhz which seems right to go directly in to a Zenith chassis from 1967. Is that true?

Dave

It could be line-conducted interference as well as RF.

To find it, you need to turn things off and see if they are the source. Anything with a switch-mode power supply - your computer or whatever - is suspect. Include LED and fluorescent light bulbs.

If you do find the culprit, plugging it into a different outlet (line-conducted) or just moving the set away from it (RF) could help.

45.75 MHz is the standard IF for all analog sets since the late 40s when 20 MHz IF designs were abandoned. All brands of NTSC color TV use the same IF frequency. This is because the FCC analog channel assignments were predicated on avoiding image frequency interference assuming sets had that precise IF frequency. The spacing of 6 channels between UHF stations in a given city depended on it.

You say the interference comes and goes and that your source is a DVD player...
Does your interference look like this? https://www.antiqueradio.org/art/MacrovisionInterference.jpg
If so that is Macrivision from the DVD player. Scroll down to" interference from copy protection" in this page for an explanation. https://www.antiqueradio.org/HomeTVTransmitter.htm

Hi Everyone!

Thank you for your thoughts on the noise issues that I have described in my 1966 Zenith. I wanted to let people know what I have discovered in the troubleshooting process and what I think is the solution.

I'll start by letting you know what I forgot to mention... I live in the metro Atlanta area and there is a low power VHF station that broadcasts on channel four. I live close enough to the antenna that I believe this was a big part of the picture noise I mentioned. As a test, if I disconnect the balun, I can see a snowy color picture, with sound, on channel four. Even channel three has some residual content that leaks through. I had been using an inexpensive RF modulator (Phillips brand) and it works perfectly on a 1985 Magnavox which has one of the early all digital tuners. However on the 1966 Zenith, the separation of channels isn't quite as distinct.

So- I decided to buy a Blonder - Tongue modulator and try to use other VHF channels. I found one of the agile series modulators on eBay for a fair price.
I bought a model AM-60-550-option 4. This version has a digital display on the front for changing the frequencies instead of using DIP switches which makes this very user friendly. It is in "like new" condition.

So what I discovered is that given where I live and the TV I am driving a signal into, it has worked out very well if I use the upper VHF frequency range (7-13),
specifically, channels 8-11 seem to bring in the best picture. Using this broadcast quality of RF modulator, I found the best channels to use and all of the noise has gone away. It really is an amazing improvement! So I settled on channel 11 but could have chosen others.

I also experimented with feeding the Zenith directly into the chassis with IF instead of RF through the tuner. That does work very well, but I don't know if it really makes the picture better. In reality, using the fine tuning control in the TV can help in dialing out the buzz and softening the picture in a pleasing way.
So I have stayed with using the RF out of the Blonder - Tongue on VHF channel 11 and it is great! Investing in this professional modulator has worked exactly how I had hoped and it is very flexible for RF or IF output.

All the best-

Dave

VHF low band ( ch's 2 - 6) are most susceptible to outside interference.
especially with rabbit ears. Sometimes it picks up garbage from the
horz / HV stages & gives lines. VHF high band ( ch's 7-13 ) are the best
all around. UHF ( ch,s 14-83) are clean but shorter range & tend to
suffer from fading etc.
Just keep an eye on things. It may be some corona from the HV. If
it is worse on humid days that can be it. If so you may hear hissing
& smell ozone.
BTW its normal on a set with a click tuner to pick up a station off channel.
Example is if the channel is 5 but you can fine tune it in on 4 or 6 also.
Just keep in mind that is within the above bands. Example ch 7 can
be tuned to ch 8 but NOT ch 6, its a different band.

73 Zeno:smoke:
LFOD !

Thanks so much Zeno!

I'm happy to know I discovered what you know to be the best. In my opinion,
we are all here because of our passion for these older TV's and it only makes sense to help it perform as well as possible. The $125 bucks (or so) to spend on a professional RF modulator versus a $15 RF modulator "from the day" does not make sense. I'd recommend spending the extra to make your set work best.

With this unit, I also have to option to transmit. Right now I'm connected directly via coax - 75 ohm cable. I see the Drake units also work as well...

Thanks!

Dave

One last thing -

I do keep and eye and "nose" on the HV corona... I had a vertical output transformer fail and it was true that the picture and sound telegraphed the failure of the transformer. I could smell it too!

Absolutely - never leave and old set running when you are not in the room!!

Dave