RCA TT-5 restoration

[Steve McVoy]

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.

[wa2ise]

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.

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.

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]

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