RF Modulator Questions

[vintage.digi]

Okay, I've come pretty far with my long-time goal of setting up internal TV stations in my office. I've got coax run throughout my office and I've got my modulators (AMCM-860D) run into an output combiner (OC-12d) and an amplifier (RMDA-750-30) and it transmits just fine to everywhere with a coax output.

I'd like to try to broadcast some channels also for my TVs that aren't near a coax out. I have a different 4-channel modulator which only outputs UHF frequencies, a Terk MD-4, and I thought this would be a good unit to test for wireless output. If it works, I'd need another amplifier for it, but I could use the amp I have now to test and see if it works broadcasting the combined signal.

So, my first question is, what sort of antenna do I need to broadcast four UHF channels? I've seen the instructions for building a dipole channel for a single VHF channel, but I'm hoping to broadcast four UHF frequencies. Something with an F-type connector would be a big plus.

The other thing I'm wondering is if there is anything that I can tune on these modulators to improve the channel spacing. These Blonder Tongue modulators are actually pretty close compared to the Macom Pico ones I was just using. I can have channels side by side with almost no issues, but there is a slight ripple in the picture of some of the modulators when there is a modulator operating in an adjacent frequency. I'd like to use all of the VHF frequencies if possible.

Thank you for the help!

[Electronic M]

As I understand it UHF tends to be more fiddly with analog wireless transmission...If I was setting up a half wired half wireless system I'd make UHF the wired portion and VHF wireless.

I'd probably try a 300 ohm UHF bowtie and a balun for UHF transmission.

[ARC Tech-109]

You'd be suprised at how well the bowtie antenna would work esp on the lower end of the UHF channels, I don't know what the output is of the modulator in question but a source at +20 dbm is 100mw, and in a residential setting that is going to cover some real estate.

[vintage.digi]

I'd read before that UHF is better for transmission in cases where the signal might be blocked. I thought that might be the case in my metal building.

I'm going to do some experimenting soon and see if this works. If not then I'll likely just go to using some VHS channels for transmission and some for cable. I'm trying to see if I can find a good place to use this Terk MD-4. If I can't broadcast its combined input then I'll likely forego including UHF in my setup.

In regards to the channel spacing, I've been experimenting with that some today. I don't have a spectrum analyzer for calibrating my RF modulators and I don't know if I need one. I ended up turning AV all of the way down because I was still getting audio over modulation even with AV and audio all of the way down. Video I turned up until it went red and then backed off until the red was gone. Then with the RF level, I just set it to the minimum level that I could that wouldn't produce static.

Even with everything set the best that I can, I'm still getting some ghosting though from adjacent channels. It's not highly visible in programming but it is there. It's easier to see in test patterns or all black. I don't know if this is typical at this point. I'm also looking to try an active output combiner rather than the passive one I have currently to see if that helps the interference.

[ppppenguin]

Nobody has mentioned whether they are using modulators with vestigial sideband. Commonly available low-cost modulators will be double sideband. Hence you can't use adjacent channels. The signals will overlap and interfere.

Also beware of using channels that are being used to broadcast digital TV locally. You don't want to interfere with other people's reception, and don't let an unfiltered double sideband signal spill on to a locally used channel.

There a risk of them interfering with your system, even if you use fully wired distribution. At the Vintage Wireless Museum in Dulwich, South London, there were problems with digital TV from the nearby Crystal Palace transmitter getting into the wired system. The effect was subtle - noisy pictures. To an analogue TV a digital TV signal looks like white noise. That's true for the DVB-T system used in much of the world. I think it's also true for 8-VSB as used in the USA.

[vintage.digi]

I'm using Blonder Tongue AMCM-860D, which I believe has a vestigial sideband.

[ARC Tech-109]

That slight ripple you describe on the adjacent channels might just be old fashion overload on the receiver front end, I've seen very similar issues in the past with head end CATV and at broadcast tower sites with two xmitters in the same building running nearby channels in this case CH-4 and CH-5 at the Telefarms Complex in those good ol' analog days.

[vintage.digi]

The cross-channel interference that I'm seeing is very minimal. It's something that is very difficult to notice unless you have a black background. I know the technology isn't perfect and I'm wondering if this is just something typical to expect. I never analyzed the signal so closely back when I was actually watching TV broadcast over RF.

In any case, they work a lot better with adjacent channels that the Pico Macom ones that I had, which I'm assuming were dual sideband. They didn't look bad when isolated but they would absolutely destroy any adjacent channels.

My take on these modulators so far is that they're nice but they seem to be likely to have problems. Of the four brand new ones I had, one was DOA with an E2 error message immediately after opening. Of the six used ones I just bought, one had a problem with flickering power that would also affect other modulators on the same power supply. And a while back I bought a big stack of used ones for cheap, hoping that a few would be good. There's probably 15 or so in that stack and every one was dead.

On the positive side, it's a vestigial sideband modulator and the HE-12 form factor allows me to use up to 12 in 2u rack space. Also, a lot of them are stereo, and it's not easy to find a good stereo modulator at a good price. The new ones that I bought did not include the stereo board, so I opened up a few DOA units and took the stereo boards from there.

Also, these things run super hot so I ordered the Blonder Tongue fan unit that's suggested to be used when more than 6 units are deployed.

[Electronic M]

It may be worth while changing the lytics on your units, especially the dead ones.
The AM series and BAVMz units tend to develop bad lytics with age and some of my regular use units arrived with such bad PS hum that the overmodulation lights were on with the inputs unplugged and the pots at minimum, lytics in other stages can cause other problems in them too.
One thing they love to do especially with bad caps is introduce varying levels of hum (depending on how bad caps are) into the video.
Just a fact of life that lytics over 15 years old are not guaranteed to last beyond that... Especially in rack mount conditions where adequate cooling isn't always present.

[zeno]

Consider the TV you are using also. In the 60's Zenith tube sets were
the hottest RF / IF you could get. Consider this. WMTW TV8 was apx
130 miles. CH 7 was apx 30 miles off the back. CH 9 about 50 miles in the
SAME direction. You could pull out TV8 on rabbit ears. With a big antenna
the pix was a bit grainy but little to no adjacent QRM.
https://www.youtube.com/watch?v=-j_2W6GYMvc

For UHF look into NTSC ATV for hints. It can be touchy to work UHF.
You may find if you have a long run using a QUALITY foam twin lead better. There is a lot less loss with it over coax.

73 Zeno
LFOD !

[Electronic M]

Zeno reminds me of a good point. A lot of tube era sets (except Zenith), and especially cheaper ones lacked provisions for adjacent channel rejection and often didn't handle adjacent channels well. As a standard practice I do not use or recommend any 2 adjacent channels with vacuum tube sets (except 6-7 and 13-14 since there's large swathes of non-TV spectrum between those channels). Adjacent DTV channels can sometimes interfere too. DTV5 here tends to mess with CH6 so I haven't been using 6 despite wanting to.

If you happen to collect newer VCRs and like keeping them with the TVs, most VCRs have tuners designed to handle adjacent channels and could be used as a tuner.

But as a general rule if it isn't cable ready it probably won't like adjacent channels as much as a cable ready set will.

[Tube TV]

I've had good luck on my BT modulator with a piece of wire about a foot long hanging from the ceiling attached to the coaxial cable. On channel UHF 27-28 (I forget the actual cable channel number) it's crystal clear anywhere in the house even with the modulator turned down.

I still need to actually make up a nicer antenna, I just set this up when I got it and it's worked fine. The BT modulator's are really impressive at how clear they'll transmit.

[vintage.digi]

I would definitely like to replace the caps in the modulators. I'm new to soldering but I have a big stack of DOA units so I feel this would be a good place to start. I'm just hoping there aren't a lot of surface mount caps due to the compact size. I'm not ready for SMD yet.

I have noticed some waviness in all of the modulators I have, generally diagonal lines. That was mostly viewing on a '79 Zenith though, so some of it could have been the TV. I was going to check on my KV-25XBR, but I'm about to leave the office and already shut off my headend so I'll look tomorrow.

Something else I noticed is that a couple of my 80s era portable color sets, a GE (similar to some Performance models but it doesn't have a model name) and a Sharp Linytron both have problems with channel 2. Linytron doesn't tune channel 2 at all, and on the GE, the color goes out intermittently on channel 2 only.

I still haven't got to trying UHF yet. I'm waiting to finish tweaking my VHF channels first.

Is there any equipment that I should consider getting in order to test and set up my equipment? Right now I'm just eyeballing, essentially setting things to the lowest I can without seeing issues. I'm not sure how sufficient that should be in order to get the best quality, but in regards to the issues that I've been facing, like ghost images in adjacent channels or diagonal noise in the signal, those didn't really seem to be significantly affected by things like amp or modulation levels.

[ppppenguin]

Some kind of signal strength meter, as used by TV aerial installers, is very useful when setting up this kind of installation. I'd say it was essential to get good results with any kind of multichannel distribution system. Without one you're just guessing. Not too bad with a single channel, tolerable with a couple of widely spaced channels, impossible when you have three or more.

The better ones are actually simple spectrum analysers, with a CRT (or LCD) display showing you a load of channels at once. Or you can zoom in to a single channel to check that VSB is really working.

I can't give you any specific makes, certainly not ones in the USA (I'm in the UK).

If you're lucky enough to have a proper spectrum analyser that will do a very good job.

[zeno]

Dont do the SMD's just the larger ones in the power supply.
Recap dont mean every cap. It varies with age & product.
Always go for the main problem FIRST then stage by stage.
Many sets hear never ran again after a recap !!!

73 Zeno
LFOD !