RCA TT-5 restoration - Page 2 - Videokarma.org TV - Video

decovision · #16 09-18-2006, 03:04 PM

I have found that a signal amplifier from Radio Shack has made a great improvement to both the sound and the picture brightness on my TT-5. With the amplifier I can watch the set even with the lights on. Before I used the amplifier I had to watch the set in a darken room and the sound was low.

Eric

Steve McVoy · #17 09-18-2006, 03:19 PM

Good point, Eric. Prewar sets have no RF amplifiers. so the gain and noise figure of the tuner are horrible. Feed them only one channel, from a modulator, and hit them with as much signal as you can. If you have local low band stations that are close, they can leak into the front end and interfere with the signal from the modulator.

wa2ise · #18 09-18-2006, 07:39 PM

Didn't pre-war TV sets in the USA use AM for the sound and 441i line scanning, at 30Hz (60hz fields)? Modern analog NTSC TV uses FM and 525i at 30Hz frames (60Hz fields). So you'd have to change the horizontal oscillator to run a little faster. And change the sound detector and maybe the sound IF frequency, to receive modern analog TV.

If you wanted a crude simulation of what 441i B&W TV reception would have looked like, you could take a regular NTSC B&W set and set the vertical height 20% too tall (20% overscan) so you'd see only 80% of the scan lines of NTSC to fake the number of scan lines a 441i set would have shown. Maybe make the horizontal overscaned some to avoid excessively skinny actors on TV shows. Or you could defocus the CRT beam a little instead. As for the sound, if in fact it was AM, rig an AM detector in addition to the FM demodulator on the sound IF. You should hear some video buzzes from the AM detector. Mix, as in an audio adder (mixer) circuit a little of that to the output of the FM demodulator. basically you would be degrading the FM quality to simulate crappy AM....

Steve McVoy · #19 09-18-2006, 07:45 PM

You are correct about prewar standards, but every prewar set has enough range in the horizontal hold control to work at 15,750 Hz. Occasionally a set won't have sufficient width, but that is usually not a problem.

Prewar CRTs were relatively dim, and had large beam size, so it is impossible to see the difference between 441 and 525 lines on them.

As for sound, it is amazing how well slope detection works. Simply detuning the fine tuning control about 100kHz puts the FM carrier on the slope of the IF, and results in detection. Volume is somewhat lower than with a FM detector, but is quite acceptable.

FM sound and 525 lines were adopted in 1941, and some prewar sets were retrofitted with FM detectors.

andy · #20 09-18-2006, 07:51 PM

...

David Roper · #21 09-18-2006, 07:52 PM

Right Steve & Andy, and we've had this discussion with Way-Too before:

http://www.audiokarma.org/forums/sho...216#post512216

wa2ise · #22 09-18-2006, 09:08 PM

Quote:

Originally Posted by Steve McVoy

Prewar CRTs were relatively dim, and had large beam size, so it is impossible to see the difference between 441 and 525 lines on them.

Looks like all I'd have to do then to create a prewar TV simulation is to defocus a standard NTSC B&W set. And set the brightness and contrast low.

Quote:

Originally Posted by Steve McVoy

As for sound, it is amazing how well slope detection works. Simply detuning the fine tuning control about 100kHz puts the FM carrier on the slope of the IF, and results in detection. Volume is somewhat lower than with a FM detector, but is quite acceptable.

How worse was AM reception of Am modulated sound on a prewar TV channel anyway? I can imagine a fair amount of interference from the video transmission. Especially with modern intercarrier TV sets.

Quote:

Originally Posted by Steve McVoy

FM sound and 525 lines were adopted in 1941, and some prewar sets were retrofitted with FM detectors.

Heard that Sarnoff of RCA freaked when the FCC said "FM for TV sound".... Which meant that he would need to pay Armstrong patent royalties for every TV RCA would make, but he didn't....

tubesrule · #23 09-19-2006, 07:58 AM

I've created a flavor of my small standards converter with the RF modulator built in for 441/30i AM audio and found it to work better than using a modern System M signal.

I used to run my pre-war sets using a standard System M modulator and Radio Shack amplifier as Eric mentioned, and the audio sounded fine, but there were problems. First, the audio in today's System M is typically 16db down from the video carrier while it was originally spec at 6db down. This means the audio is 3 times lower than it was originally. The second problem is with slope detection. While this method works surprisingly well, and can be low distortion if the slope is linear, the entire slope is typically less than 100kHz wide. The LO in these sets are not all that stable, so I found you have to constantly touch up the fine tuning while watching the set to keep the audio centered "on the slope".

By running the original AM audio carrier and at -6db level, there is no critical adjustment of the set required, and the audio sounds every bit as good as a radio from the time, and sounds better than with slope detection which can result in non-linearities due to the shape of the slope

Also, as Steve has already mentioned, the focus is not good enough on these crt's to be able to resolve the difference between 441 and 525. Running them at 441 does allow you to fill the screen, but does not change the apparent resolution.

Darryl