View Full Version : Thought about this (for those with lots of sets)


jpdylon
07-14-2006, 01:47 AM
I always hate having to connect each set up that I want to watch in and out of rotation. I saw this cool VHF transmitter kit (http://cgi.ebay.com/Ramsey-TV6C-VHF-TV-Transmitter-Kit_W0QQitemZ5701533417QQihZ008QQcategoryZ48839QQs sPageNameZWD1VQQrdZ1QQcmdZViewItem), and thought "why the hell not?"

Anybody else think of doing something like this?

roundscreen
07-14-2006, 07:47 AM
Jpdylon.
Yes I am and thanks for the tip. This will be a great thing to have when they turn off ntsc in 2009. I am about an hour away and have a good friend in rochester that knows the roads in victor. The other stuff thay have looks interesting too.

ED

kx250rider
07-14-2006, 12:02 PM
Thanks for mentioning this... We all will need some way of using the old sets, and I personally don't want some kind of high-tech digital decoder stuffed between the legs of my Zenith roundie with all kinds of LEDs shining. With a whole-house transmitter, maybe even 2 or 3 of them set up to repeat local channels, it would be like nothing ever changed :)

Charles

Adam
07-14-2006, 12:27 PM
"maybe even 2 or 3 of them set up to repeat local channels, it would be like nothing ever changed"

I like that idea, with that I could still use the remote on my remote sets for something other than just watching the dial turn.

tgunner
07-17-2006, 02:55 PM
Looks easy enought to build one yourself. Anyone try finding just the circuit diagram for a VHF transmitter?

Eric H
07-17-2006, 07:39 PM
Can you add an amp and broadcast to the entire nieghborhood? :D
Seriously though is this thing going to get you in trouble with the F.C.C. or is it below the level where it would be a problem.

jpdylon
07-17-2006, 08:02 PM
with a range of 300 feet, i would assume its only a couple hundred mw or so. Thought it would be fun to broadcast, but I really don't want to pay the rediculous liscense fees, nor put up with being arrested for hosting a priate TV station.

would definitely be fun though.

oldtvman
07-17-2006, 08:21 PM
with the new video compression technology, there is no reason why they can't run both technologies concurrently, they will probably go all uhf.

Whirled One
07-17-2006, 08:39 PM
I've got a Ramsey TVT-1, which I guess must be one of Ramsey's early versions of this kit. [And yes, transmitting TV shows to my old TVs was indeed the application I had in mind with it! :) ] It wasn't very expensive-- I think it was about $12 or $13 (in kit form, without the plastic cabinet which was an optional extra at the time) when I got it over ten years ago. It's a fun li'l device for twelve bucks, but quality-wise I'd rate it as rather mediocre, which is especially apparent when using it with a color TV. The kit shown in that eBay auction appears to have a significantly more complex circuit than mine, so hopefully this indicates that it's improved considerably over the old TVT-1.

It's also more expensive than the old TVT-1, as far as that goes-- looking at Ramsey's web site, the TVT-6C is the current version, and it's available for $39.95 direct from them (same price as the one in that eBay auction). At least now they throw in the the matching plastic cabinet in the basic price of the kit though.
http://www.ramseyelectronics.com/cgi-bin/commerce.exe?preadd=action&key=TV6C
Ramsey kits are available from various amateur radio and electronics dealers as well, so you might find it slightly cheaper (or more locally) elsewhere.

There are certainly other products out there besides the Ramsey kits, I might add; I found a few by Googe-ing for "VHF TV transmitter low-power", including products ranging from other no-license <= 100mW kits up to LPTV transmitters. ...Which makes me wonder if the FCC would even continue to issue LPTV licenses once analog TV goes off the air.

kbmuri
07-17-2006, 10:52 PM
Found this article on the Ramsey. Has schematic and PCB layout. -k

http://www.web-ee.com/Schematics/TV_XMIT/TVTransmitb.pdf

kbmuri
10-16-2006, 02:53 PM
I splurged last week and bought one of the Ramsey kits from eBay.

http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=5701533417

Arrived just now (photo). I'll build it tonight. Evaluation to follow...

kbmuri
10-16-2006, 06:33 PM
Parts is parts.

Went together pretty easy. It doesn't come with a power supply. Gotta scrounge for that...

jpdylon
10-16-2006, 08:08 PM
IMPORTANT: I think the center connector has to be positive. I accidently connected one with a negative center tap and roasted my power regulators.

A note for those building:
I built one of these, and though assembly was easy fine tuning was not. I was also getting limited signal range and a hard time stabilizing the picture. Once you dial it in its ok, but my old sets have a hard time syncing with the signal sometimes.

kbmuri
10-16-2006, 08:34 PM
Disturbing that nowhere in the manual does it give the polarity of the power supply. Looking at the schematic, it has both major electrolytics with their negative sides to ground -- that's the only clue. The large-area sections of foil on the PCB are ground, the P/S jack's housing is soldered to ground, so jpdylon's assertion that the jack's center is positive is correct.

I went to Radio Shack, another $20 added to the price tag of the transmitter for this: http://www.radioshack.com/product/index.jsp?productId=2049714 . I snipped the end off of it and soldered the bare wires directly to the PCB, bypassing the plug. No confusion for later.

As jpdylon alluded to, I'm having a helluva time getting the thing tuned. Four separate adjustments that all interact with each other, sort of like trying to crack a 4-digit combination safe. Hopefully there's a combination that works, and hopefully it can be found!

peverett
10-16-2006, 10:02 PM
I also have purchased this. I still have not gotten it tuned correctly either, but have not worked on it much yet. I purchased the power supply from Ramsey, which was only and adder of $12 or so and no worries about polarity.

kbmuri
10-20-2006, 11:38 PM
Well, I think I gave this thing my best shot. If anyone out there is considering buying a Ramsey TV-6C, I'd be inclined to say, "Don't waste your money".

The source of the major problems seems to center around Q5, the 2N3866 output transistor. The power-supply is a rock-solid regulated 9V with no ripple whatsoever, which is great. The audio and video amplifiers do their job perfectly, and the two are mixed appropriately, which is also great. The local oscillator, tuneable to channels 3-5, puts out a very nice sine wave at the proper frequency of choice (mine being 62-ish MHz, or channel 3), which is great too. 90% of the "squirreliness" appears to be at the output stage. The Q5 transistor is a brass button type, presumably for shielding and heat dissipation. It does neither well. The collecter is connected to the case, but in this circuit the emitter is grounded, so the case is free to collect stray radiation and rebroadcast it in something of a feedback situation. It also runs uncomfortably hot, and its characteristics change with temperature, which leads to drifting and inconsistent performance. On no video/audio input, it also doesn't reproduce the oscillator's sine wave all that well (ramps up faster than it ramps down, a "wiggle" halfway down). This leads to harmonics, such that when tuned to channel 3, channel 8 also has the signal, almost as strong. And there's a mess on channels 2 and 4 and even 5, so there's no hope of buying several units and broadcasting all your favorites on more than one frequency.

If you moisten a finger and place it on top of Q5 (and let the pain subside as you bring its temperature down), then tune the various trimmers for best picture, you get a very stable situation. Take your finger off and all hell breaks loose. Obviously a guy can't use it that way. Applying your finger does two things, it sinks all the heat, and it damps out all the transients floating around the transistor's case. There's an immediate catastrophe when you remove your finger, which worsens over the 60 seconds that the transistor takes to heat back up. Replacing your finger removes the worst of it, then after 10 or 20 seconds, as the temperature subsides, you get back to "tuned". Pretty unacceptable.

I tried cold-water-pipe grounding the case, no improvement. Couldn't think of much else to try there, outside of digging up a cadaver and permanently mounting its finger to Q5.

I removed the onboard antenna and soldered a 25-foot piece of 75-ohm coax in its place, then placed the (removed) antenna at the other end of the coax, across the basement, and tuned the trimmers to that configuration (after Q5 heatup). This actually improved things almost to the point of satisfaction. I guess operating Q5 an inch away from the antenna is the TV6C's biggest flaw, and perhaps the reason for an external-antenna jack on back.

However there's one other flaw, I can't trace down nor explain, in that there's a slight, sudden increase in amplitude at the antenna, then a slight decrease, at a specific period (60 Hz?) that results in a series of lighter vs. darker blocks of picture, about half the picture height each, which slowly "walk" up the picture as you're watching your program. It's noticeable-to-annoying, and no amount of tuning removes it completely. Like I said, the regulated 9V is rock solid, but I don't know, maybe the regulator is radiating something?

It's also disturbing that the video and sound are both xmitted at the tuned frequency (they should be separated by 4-5 Mhz). They seem to count on "bleed" to get both to the TV set. I'm guessing that's why it's so hard to get clean sound AND clean picture at the same time, although rather easy to get one or the other.

So that said, I'm s__tcanning the project. The guys at Ramsey didn't do their homework.

Save your money. Not worth the price of admission.

-k

wa2ise
10-21-2006, 12:02 AM
Ramsey kits were never much good. :thumbsdn: Your best bet is to salvage a TV modulator module from a dead VCR. You may need to build a small power amp if you want it to feed all the TVs in the house over the air. The Ramsey circuit you posted might do (but will need some mods). I'm a little hazy here with VHF linear "power" amps, but I think an emitter resistor of around 20 ohms, and a radio frequency choke from the collector to B+, and remove whatever was used to inject the video and audio into the collector.

old_tv_nut
10-21-2006, 01:19 AM
Q2 and Q1 form a sort of video op-amp. The video gain of this amplifier is the ratio of R8 (4.7k) divided by the video gain pot value (R3). You can see that if R3 is set to half way (500 ohms) the gain is about 10 times, which means you are already trying to get 10 volts of video out of Q1, which only has a 9 volt supply! So - the circuit is cheapened by not having another resistor in series with R3 to limit its minimum value. Starting point for R3 setting should be at max resistance, and only adjust to a lower value if you are not getting enough contrast.

R7 (BIAS) should NOT be adjusted for all black video conditions as the article states - its real purpose is to prevent getting zero-carrier clipping on whites. It has a similar problem to R3, in that it should be limited so you couldn't go to zero resistance. Always start with this pot at max resistance, and only adjust it as needed to prevent white clipping and sound buzz. Note that at zero resistance it shorts out the input to Q2 and the feedback from Q1 - very bad for operation.

Regarding the RF output transistor, I can't guess immediately from the diagram what the dissipation is. However, the output transistors should be driven class C by the RF oscillator, while the video modulates the collector voltage. This circuit is expected to generate harmonics, much of which are supposed to be filtered out by C12, L1, and C13. Scoping the collector directly will show harmonics to some degree even though c12 is right there. There should be less harmonics at the output on C13.

Regarding the sound, Q3 is a simple frequency-modulated oscillator, tuned to 4.5 Mhz nominal frequency by T1. The 4.5 MHz sound carrier is mixed with the video by R9 (10k ohm), so it is at about a 10% level compared to the video that comes in through R3. So, if you scope the emitter of Q1, you should see video of several volts (with sync positive, since Q2 is inverting), with 4.5 Mhz "fuzz" of about one-tenth the video amplitude riding on it. This combination is what modulates the collectors of the output transistor(s). (The capacitance of your scope probe may cause some distortion and amplitude reduction of the 4.5 mhz).

By the way, L3 (2.2 microhenry) is meant to prevent the RF from getting back into the video/audio circuit. How good it is in the real world, I don't know, so you might see some RF messing up your scope readings here too.

Another caution is on tuning the channel frequency. This circuit modulates double sideband, and you need to be careful that you have not accidentally tuned it to the next higher channel (channel 4 while trying to receive on channel 3, for example). You may get a recognizable picture but not a good one from receiving the lower sideband this way.

kbmuri
10-21-2006, 12:00 PM
I went back and looked at that web article, and obviously it's an earlier version of the transmitter. I was a little confused when you said "output transistors" (plural), and "class C operation". My kit has only one output transistor, the hot-runner I described earlier. Schematic attached. I wonder if the push-pull version worked better, and why the change (performance or cost?).

I probably confused some people talking about the 9V power regulator too. Something apparently they added later, as it's not in the web article either. They recommend a 12-15V supply (mine is 13.8V), the 7808 VR1 flattens it very nicely to 9V.

The oscillator and audio/video amps look unchanged. I guess I don't follow how the audio oscillator at 4.5 Mhz adds in, but I guess that's the signal that affects the shape of the sine wave at Q5's output, so I guess that's ok and the harmonics are expected. I don't think they're filtered well though, as like I said, tuning channel 3 also pretty clearly tunes channel 8 (the 3X harmonic).

Anyway, hope this schematic clears up what I was describing. I still can't see the people at Ramsey being proud of this product. Maybe the TV-7 or 8 or 9 will get it right someday... Not there yet, IMHO.

Eunomians
10-21-2006, 01:41 PM
I am glad to read this post as I had high hopes for the Ramsey Unit.

I've also thought about one of their FM transmitter kits. Maybe I'll stay away.

andy
10-21-2006, 03:26 PM
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Chad Hauris
10-21-2006, 04:40 PM
I use a Channel Master amplifier from a school to amplify the VCR CH. 3 output before it goes to the TV's through splitters...this can serve about as many TV's as you can hook up to it with no loss of signal quality. A Radio Shack VHF amplifier will supply 6 sets or so. Without using an amp the splitters seem to attenuate the signal too much if you use more than one.

peverett
10-21-2006, 11:08 PM
The problem that I have is that I would like to view non-broadcast channels (and prepare for 2009) to my workshop which is behind my house. It has no cable. Radio shack has some GHZ transmitters that work well room to room (no need to run cable), but my workshop is at the limit of their range. Depending on the weather, the Radio shack unit may have a decent picture or not, the sound is usable, but never very good.

The Ramsy unit that I have will broadcast a picture that far to a 1955 B&W Admiral that I just restored. However, the sound is non present and the problems mentioned by kburi upon warmup are all present.

As mentioned before, I have not yet had a change to work on adjusting it.

kbmuri
10-22-2006, 12:44 PM
This will work!
http://audiokarma.org/forums/showthread.php?p=857224#post857224
:D

Seriously, the reason I'm interested in this hobby is to preserve a "golden age" of TV (1950 +/-). Cable TV was sort of around back then (Zenith "phonevision" for example), but it wasn't really mainstream until around 1975. Then came VCRs, IR wireless remote controls, satellite dishes, MTV and Nick-at-Nite... Not really desireable to run them on a 10-inch roundie in mono Lo-Fi. These old TVs were made for The Honeymooners and Lucy. I put a 2005 el-cheapo VCR on top of my Magnavox briefly, and it looked so out-of-place that I took it back off in disgust.

The least obtrusive "modern" solution I think is the Radio-Shack 2.4 GHz transmitter/receiver pair, hiding the receiver inside the TV cabinet. But that still places the burden of "tuning" on the VCR or Satellite box driving the transmitter, and reduces the vintage TV to status of "monitor". Never using the tuner again. And the cost of putting a separate 2.4G receiver in each TV set.

There are no VHF stations where I live. The only answer for me is to broadcast VHF myself. Sad the Ramsey units suck so bad, but it should still be technically possible to set up somewhere between 1 and 12 "repeaters" of your favorite 1 to 12 TV channels and hide it all in a central closet or basement of the house, and then use ALL your vintage sets as intended without modifying them. My '50 Philco, '51 Zenith, and '64 Zenith sets have VHF antennas built into the cabinets -- no external wires whatsoever (except the power cord of course). As it should be. Even wiring cable to them sort of defeats the purpose of commemorating 1950.

I'm still working on a solution...

- k

peverett
10-22-2006, 05:48 PM
The Radio Shack units are what I use in my house. These will drive more than one set. I am using one receiver to drive three 1953 Hoffman sets without any problems (I am using a passive splitter). I would be driving a fourth set, an early Admiral, but the signal overloads the AGC on this set. The Hoffmans have switches that allow changes in TV gain for different signal strengths. This is why they do not have overload problems.

I have purchase an adjustable annenuator (also from Radio Shack) to put in series with the incoming signal on the Admiral.

old_tv_nut
10-22-2006, 05:49 PM
...I wonder if the push-pull version worked better, and why the change (performance or cost?)....
Q5 and Q6 are just in parallel - not push-pull.

kbmuri
10-22-2006, 08:34 PM
Q5 and Q6 are just in parallel - not push-pull.

Agreed. I didn't look at the older-version schematic very closely. Paralleling transistors seems strange to me. Two transistors paired together in the audio world usually means a push-pull circuit. At a glance, the two in the early Ramsey unit looked similar. Just sharing the power burden?

"Class C" is explained here if anybody else is curious.
http://en.wikipedia.org/wiki/Electronic_amplifier#Class_C

I'm not a EE. Never claimed to be. Just handy with a soldering gun sometimes.

wa2ise
10-23-2006, 02:48 PM
The oscillator and audio/video amps look unchanged. I guess I don't follow how the audio oscillator at 4.5 Mhz adds in, but I guess that's the signal that affects the shape of the sine wave at Q5's output, so I guess that's ok and the harmonics are expected. I don't think they're filtered well though, as like I said, tuning channel 3 also pretty clearly tunes channel 8 (the 3X harmonic).


Seeing the entire diagram I can make better sense of it now. Q3 looks to be an oscillator at 4.5Mhz, and its internal PN junctions' capacitence is being varied by the audio signal voltage. That creates an FM signal. General Electric used a scheme like this to do Automatic Frequency Control in their solid state AM/FM radios they made around 1969. Using voltage to shift the frequency. I'd adjust L1 after I succeed in getting a video signal seen on the TV set. Adjust L1 to get the sound carrier demodulated properly by the TV sound circuits. Be sure the audio input level isn't too high.

The RF power amp is being "plate modulated" to modulate the video and FM subcarrier onto the TV channel carrier. Some older AM radio station transmitters used this method. Video is amplitude modulated onto the TV channel. Real TV stations also partly filter out most of the lower sideband, but VCRs and other such devices don't bother as they are usually in closed circuit environments (not radiated into the "air").

You could connect a TV coax cable (the F connector type) to where the 75 ohm resistor R12 is, and remove that resistor. The 75 ohm termination at the far end would be a 75 to 300 ohm balun in turn connected to a TV set.


However there's one other flaw, I can't trace down nor explain, in that there's a slight, sudden increase in amplitude at the antenna, then a slight decrease, at a specific period (60 Hz?) that results in a series of lighter vs. darker blocks of picture, about half the picture height each, which slowly "walk" up the picture as you're watching your program. It's noticeable-to-annoying, and no amount of tuning removes it completely. Like I said, the regulated 9V is rock solid, but I don't know, maybe the regulator is radiating something?


What is likely happening is that some of the output RF signal is finding a path thru the power supply rectifier diodes. As the rectifier diodes turn on and off at the powerline frequency, this RF path also is switched at the powerline rate. One solution is to insert choke coils in both power supply wires from the power supply. A few milliHenrys should do. You can usually find a suitable coil used in the power feed of a dead computer monitor or PC power supply. Those usually have a pair of windings on the same core, one for each power supply wire (the + source and - return aka ground).


This kit's schematic looks to be adequate for the job. Not great, but it should work reasonably well. Things like the power supply chokes should have been added to make success more likely.