None of my TV small amplifiers is working. Why?

[neazoi]

Hi, trying to amplify the signal out of a uhf tv modulator I have made ( http://qrp.gr/tvmod )a 100mW 3-stage amplifier, a 100mW 4-stage amplifier and a very low ~1mw? one stage amplifier. None of these circuits work as I do not notice any receiving in my room TV by the transmitter.

The 3 and 4 stage amplifiers are from a magazine and the single stage amp is the first stage of this schematic http://www.radiosparks.com/images_d/Tvtx1.gif
They all heat up and draw significant current denoting probably class-A operation.
But I cannot notice any change in the receprion of a nearby TV. The received signal is exactly the same as without the amplifier at all.

Could the 40cm wire I use as an antenna be the problem?

 

[BigBoss]

Is the modulator working at tuned channel ?? You may check this by connecting its output directly to the TV.

 

[FvM]

On the receiver side, you can only expect an improved signal quality if the amplifier noise factor is better than that of the regular TV tuner. Probably it isn't.

 

[neazoi]

Is the modulator working at tuned channel ?? You may check this by connecting its output directly to the TV.

Yes it works ok and it is tuned to around 770MHz. I have verified this by connecting it directly to the tv aerial.

- - - Updated - - -

On the receiver side, you can only expect an improved signal quality if the amplifier noise factor is better than that of the regular TV tuner. Probably it isn't.

It is not signal quality I am trying to achieve. I am currently driving the modulator unmodulated to test the amplifiers. I do not sense any amplification, sonce the "amplified supposed" signal appears as strong as that with the modulator only.

Test is done with the tv at 3 meters away, with it's whip antenna.
The single bfy90 stage should show some signal strength on the tv shouldn't it?

 

[BigBoss]

Yes it works ok and it is tuned to around 770MHz. I have verified this by connecting it directly to the tv aerial.

OK.
The amplifier is a Power Amplifier and therefore "Output Matching" is very important.Even the amplifier consumes typical current, since the output matching ( of course internal matchings should be done properly) is not well done, the output level may sufficiently not drive the antenna.
Antenna is also important since the driving level is sufficiently large.
However TV Tuners have good sensitivity and even the matching is not properly done you should see something ( snowy and nosiy picture ) in a short range.If you don't see anything, there is another serious problem.( A short circuit,open circuit,failed component,connectors etc.)

 

[neazoi]

OK.
The amplifier is a Power Amplifier and therefore "Output Matching" is very important.Even the amplifier consumes typical current, since the output matching ( of course internal matchings should be done properly) is not well done, the output level may sufficiently not drive the antenna.
Antenna is also important since the driving level is sufficiently large.
However TV Tuners have good sensitivity and even the matching is not properly done you should see something ( snowy and nosiy picture ) in a short range.If you don't see anything, there is another serious problem.( A short circuit,open circuit,failed component,connectors etc.)

Thank you.
As I said I have tried 3 different amplifiers. Now I have made another one with a single bfr96s taken out of a magazine. Again the same problem.
I have tried this last amplifier with a commercial modulator too, and again the same problem. No improvement in signal range.
This is a hairpulling problem.
I would like to try another antenna, what do you suggest? a simple dipole with a matching transformer will do?
Even without a matching transformer I should see some signal out shouldn't I?

Yesterday, I have noticed in one of the two multi-stage amplifiers that even if I do not connect the modulator to it, I could pick up signals in the VHF (not uhf as expected) range a few 10s of meters away. What is going on, maybe some kind of internal oscillation?

 

[volker@muehlhaus]

What is the output power of the modulator (=input power to your amplifier), and what is the expected max. output power of your amplifiers?

 

[neazoi]

What is the output power of the modulator (=input power to your amplifier), and what is the expected max. output power of your amplifiers?

As you see from the link in post #1, the modulator output stage is just a diode that dumps the AM signal that has been previously inductively coupled to an inductor. So I suspect the output of the modulator is really small, smaller than the commercial modulators.

There may be a case that all of the amplifiers load this inductor too much and somehow the input signal is lost?
I have tried 100nf as the coupling capacitor, as well as 10pf and I see no difference. Shall I try an even smaller cap for less loading?