Homemade low power TV transmitter?

[andy]

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[domfjbrown]

Why bother with all that? Get a VHS machine or similar (with an RF modulator), daisy chain 3 or 4 aerial signal boosters in line off of it (those ones you use to boost output from TV aerials in poor signal areas - nothing fancy), and wire the output of that into a standard TV aerial.

My mates and I did this in college (using my mid-80s Panasonic PAL VHS machine, and 4 Labgear signal boosters) and were able to broadcast fairly well over a 40 metre range or so. Not sure if the metal-framed building we were in added to the effect or not, but friends in an adjacent dormitary block were able to pick it up well.

We "debuted" "THe LoSt BoYS" the night before BBC1 showed it - but ours was uncut. Also broadcast other interesting titbits including soft porn, but had to dismantle the rig after 3 weeks or so as some neighbours just off the college campus were complaining about interference.

The staff never did suss out it was us, but they had their suspicions

[fsjonsey]

I see Blonder-Tounge Tube RF amps on ebay quite often. Some of them have very high gain, 50-90db. They would probably work well for a LPTV station.

[kbmuri]

Originally Posted by ChrisW6ATV

A commercial "agile modulator" is probably the best device to use for in-house VHF signal creation. There is at least one on Ebay right now, item #120206024645. It goes down to 40 MHz, so you could even use it for Channel 1.

There were at least two. I bought it and another one and will try them out and let you all know how they work. Unfortunately, buying them on that ePay place, the shipping tends to cost more than the item, and buying 13 of them could get a little pricey. Maybe I'll check the local hamfests/surplus stores for some more.

[3Guncolor]

If anybody ever finds a tube type CATV modulator from the early 60's let me know. I want to restore one and have it on display and able to be operated at a real master headend. If the output was good I might even be able to put it out on the system for a while in the middle of the night. As far as real CATV modulators it's sad but we can't find anybody to buy em they have been going to scrap dealers for the metal.

[ohohyodafarted]

ChrisW6ATV,

I just did a deal with a seller on epay for 5 Holland modulators to build my own VHF in home cable system. I got a chanel 3, 6, 8, 10, and 12. I will feed them with 4 digital converters all tuned to a different chanel, and also a DVD player on the 5th chanel.

Specific question for Chris, I will need some sort of combiner, I was thinking of a 6 in 1 out passive combiner spliter, but I am not well versed on this sort of thing. Will this approach work? And how many sets would I be able to power on the outpout of the combiner?

I also will have eventually a large number of tv sets to feed, probably 20 to 30 sets in my collection. Typically, how would you feed this many sets?

Thanks,
Bob

[3Guncolor]

A combiner spliter will work just fine. Lets say you have an 8 port spliter you will lose about 11 to 12 dB going through it. Do you know what the output level of your modulators are? Most single channel CATV modulators will do +60dB.
It's all math you only need 0 to +3 at the back of the set. Each 2 port spliter you lose 3 to 4 dB.

Good Lock,
Steve

[ohohyodafarted]

Spec for the modulators claim 55db output. I also got a commercial Holland 12 input combiner thrown in the deal for a few extra bucks. So worse case now would be that I might need a distribution amp at some point in the system, after I have lots of set hooked up to the output of the combiner. That will be down the road a bit. I only have 5 sets restored at this point, and about 30 more waiting to be restored.

[Steve McVoy]

These modulators rarely work well at their maximum output. Count on running them at +50 dbMV. The combiner will have a loss of about 20 db, leaving +30 for your splitters. If you split for 16 output, the splitter loss is about 14 db, leaving +16, which is plenty for the TV sets. You could easily drive 32 sets.

[kbmuri]

Originally Posted by ohohyodafarted

I will need some sort of combiner...

I found this:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=150202318726

It does 12 inputs to one output, not amplified (passive). Looks brand new. I'd get it but I just snagged a used one here:
http://cgi.ebay.com/ws/eBayISAPI.dll...m=280189609459
along with two modulators.

From what I gather, some B-T rackmount units are set to one specific channel at the factory, others have DIP switches to select your desired channel. Presumably the latter were more expensive. The channel 9 one I can leave as-is, the chanel 17 one will be a science project. Hopefully just a crystal swap? The other ones I bought are a channel 7 and the switch-settable one mentioned earler. I paid way too much for the channel-7 one, in hindsight and after some research. I've seen a couple go for 5 dollars already. But I should have a good start sometime next week when it all arrives and it should be entertaining to say the least.

[3Guncolor]

Sad they were waiting for the junk man. I could not find anybody that wanted these they were the best, SA6350 modulators. Trust me these run best at 60dB. They were still in great shape after 20 years feeding 40,000 subs.

[ChrisW6ATV]

Originally Posted by ohohyodafarted

Specific question for Chris, I will need some sort of combiner, I was thinking of a 6 in 1 out passive combiner spliter, but I am not well versed on this sort of thing. Will this approach work? And how many sets would I be able to power on the outpout of the combiner?

Bob-

The Blonder-Tongue ZHC-12 mentioned by kbmuri, and I would guess the Holland combiner too, is made for just the purpose you describe (and that several of us seem to be planning here as well). What I do not know is if an ordinary splitter as sold for home use will work OK as a combiner with the relatively high signal levels of these commercial modulators. You certainly got a good combination of modulators (3,6,8,10,12), since it is best to skip channels when you can. As others noted, you may be starting with enough signal to split into 16 or more outputs without adding an amplifier; ultimately, just hook it all up and see what you get. Weak signals will be snowy of course; too strong/overloaded signals will usually have herringbone or diagonal lines and maybe ghost-looking/extra sync bars in the video.

[ChrisW6ATV]

kbmuri-

The CAMS-60a ones you got seem to be called "channelized agile", which sounds like an oxymoron to me, but it might mean "agile but only settable onto exact channel numbers", which would not be a problem anyway. The AM40-450 is settable to any step you want in its entire range, so you could set it to "channel 4 and a half" or similar. If the channelized ones are truly fixed on the channels on their labels, it is probably not worth messing with changing crystals when the actual agile ones are out there cheap.

[kbmuri]

This helps a lot.

http://www.blondertongue.com/media/p...rgComplete.pdf

[kbmuri]

Originally Posted by ChrisW6ATV

kbmuri-

The CAMS-60a ones you got seem to be called "channelized agile", which sounds like an oxymoron to me....

Sounded like an oxymoron to me too. The "Channelized Agile" versions arrived this weekend. FWIW, they're a lot more channelized than agile.

The channel 9 one can be modified, via some VERY inaccessible switches, to VHF channels 7 thru 13 and cable channels 23, 24 and 25 (174 mHz to 234 mHz). The channel 17 one can only be tuned to cable channels 14 thru 22 (120 mHz to 174 mhZ). Presumably there's a low-band one that tunes 54 to 120 mHz too (and probably some in the higher-cable bands too). My channel-9 one will be useful enough without having to switch it. The other one is for the most part useless. Both are significantly less versatile than the fully-agile versions so I probably won't want another oxymoron unit.

The straight-channel-7 one has a specific crystal installed, in an easy place to remove/replace. It came with a crystal-frequency chart for changing it to other channels, so it could be done. An advantage of this type of unit is that it uses less power (14 watts vs 20). After you rack up 8 or 12 of them, that could become significant. For our multi-vintage-TV purposes, they're completely adequate, and if they're cheaper than the fully-agile version, probably a better deal. Although I'm seeing eBay prices all over the place for all kinds, so probably a person could get an all-agile 12-channel setup, or an all-channelized 12-channel setup, for who knows what random final cost. My setup will be an interesting mix...

The 12-into-1 combiner arrived too. It's 100% passive inside (just a conglomeration of attenuators and matching transformers and cable ends). Pretty neat too.

Now to get some digital converter boxes so I can try these things out...