Zenith 25mc30 DC restoration.

[iong]

My roundy seems to lack DC restoration. I don't know which circuit I should check or how to improve it. Please help.

[Alex KL-1]

https://www.antiqueradios.com/forums...f5077f2f175f19
Basically, they discussed also about the DC restoration here.

[old_tv_nut]

There have been tons of discussion here about the lack of DC restoration on all tube color sets after the CT100. You are seeing it the way it was designed.
Consumer Reports complained about this yearly until solid state chassis with 100% DC restoration came along. People nowadays don't remember or never knew how these old sets performed compared to today's, and yes, it's striking.
To improve it requires modification from the original design. I don't know if you can find (here or elsewhere) posts of modifications to this chassis to improve/correct it.
You have to consider whether you want it as a true example of how it was or improved.

[Alex KL-1]

Intriging. Being a fanatic for DC restoration, making it even for B&W portables, I first think that the DC restoration is a rule for color TV (years ago). But then, reading some schematics and restoration reports, I concluded that several models not have it then (since I not read all schematics of all models, of course ;-). Then all tube-based color ones lacked DC restoration? Interesting to know.
Even considering the low light output, the colors will not strongly mistrack with this?
I know the set without DC restoration have a average light output, so this can account for that, along with tracking, I guess... (and so more I bet)

Here in Brazil color comes late, in 1972, but the schematics that I seen, of hybrid sets, have DC restoration (at least for the ones I got schematic), but I bet the track are poor, since not have stabilized PSU, video out tube will drift etc, and color sums "high level" (like most out there, also), using the grids of CRT as input for the color signals. Since I born in 1983, I only seen one of these hybrid sets working. All others are transistor ones with fully DC restoration... so I have almost no experience in watching one with this image properties.

[old_tv_nut]

These sets did not totally lack DC restoration. Tyical value would be 85% (sometimes less), but that is low enough to make dark scenes foggy and bright scenes clip the shadows.

[Alex KL-1]

Thanks for the info.

Wondering about it, perhaps is asking too much for having a solid DC restoration in sets not having stabilized +B (not counting the HV reg. here) and/or DC stabilized gain stages, so perhaps is a compromise between price and performance at all.

[old_tv_nut]

Originally Posted by Alex KL-1

Thanks for the info.

Wondering about it, perhaps is asking too much for having a solid DC restoration in sets not having stabilized +B (not counting the HV reg. here) and/or DC stabilized gain stages, so perhaps is a compromise between price and performance at all.

Lower DC restoration makes the set less susceptible to mistakes in black level in the signal, but I always thought that was a poor excuse because the resulting errors in the receiver due to DC changes with picture content were just as bad.

Nevertheless, you have to note that NTSC had terrible problems maintaining proper black level at the broadcaster or analog cable head end, due to the unfortunate inclusion of a 7.5 IRE unit black level in the standard instead of setting black at blanking level. There is no reference in the signal for 7.5 IRE, while zero IRE (blanking back porch) is always there. This was originally in the black and white standard to provide some room to prevent clipping blacks in the cruder video processing circuits, but even better tube gear that didn't clip blacks would drift. In hindsight, the NTSC standard for black level (7.5 IRE +/- 2.5 IRE) should have been changed to zero IRE when color came along. I seem to recall that Japan did this in their broadcasts.

[kf4rca]

I've got a Tek TSG170 that is a Japan generator. One of the color bar outputs has zero setup. But all other test signals are good.
If you think black levels were bad in NTSC, in the early days of HD, syndicated programs came in as a data file. They often had screwed up color phase, black levels, etc. And since everything was digital there was no way to correct it at the station. EVEN audio phasing (L-R) was often out of phase.

[Alex KL-1]

Originally Posted by kf4rca

I've got a Tek TSG170 that is a Japan generator. One of the color bar outputs has zero setup. But all other test signals are good.
If you think black levels were bad in NTSC, in the early days of HD, syndicated programs came in as a data file. They often had screwed up color phase, black levels, etc. And since everything was digital there was no way to correct it at the station. EVEN audio phasing (L-R) was often out of phase.

Even today, within same streaming platform, is possible to see some series having elevated black level for entire episodes/season, or during some scenes (some can be intended for clarity, but some seems having too much error at all).

[luRaichu]

Is there an advantage to using NTSC-J on American NTSC sets?

[old_tv_nut]

Originally Posted by luRaichu

Is there an advantage to using NTSC-J on American NTSC sets?

Black level stability, but only if everyone would use it and legacy stuff would be conscientiously corrected.

[Alex KL-1]

Just remembered something: I have a notebook with S-Video output, and it have almos all systems in vigence at it's time, so I believe that have NTSC-J (well, it have the PAL-M...). So, I need to check it. If have it, I will compare it with NTSC or PAL-M using my old B&W TV's tha't I've added DC clamping (using only Y signal from S-Video as usual for B&W).

I noticed for making a usable passive (not switched) DC clamping (eg. diode) I need a very large RC time constant, ie, for example, if I clamp at grid of video out tube, need to have some 1M and 560nF cap, or DC drifts fast with some scenes, making the restoration partial.

For some small B&W I clamped at base of video out, and needed at least 100µF for not tilting the DC level under certain conditions.

Then I assume that active (switched) clamping will be more stable with it, since it can be able to clamp at correct waveform position/time.

Oddly as it seems, some CRT monitors from last era in fact have simple DC restoration at the cathodes, and uses large time constant anyway. But it works good.

[old_tv_nut]

It will be interesting to hear the comparison of NTSC and NTSC-J. Ideally, the only difference will be to reset the black level.

[Alex KL-1]

Fortunately I had some free noise time to play with it this weekend.
My notebook indeed have the NTSC-J.
So, I proceed with the tests. With interesting results...

I'm able to test with 2 B&W TV's, one small 7" with transistor video amplifier. I employed DC restoration is similar to one used in the System A Modulator in the The Valve Pages (Tr4, fig1: emitter clamps the base signal of the video amp), and collector is DC coupled the video out to the cathode CRT.
These are already pretty good even with PAL-M and NTSC. High quality clamping. With NTSC-J, it basically moved the DC level as predicted, and bright needed to be readjusted. No statiscally important difference in black level stability. Only a little impression that the NTSC-J can be more impervious to extreme scene changes, but then I'm relying on my eyes only...

Then I tested with a Admiral that I applied DC restoring at the g1 of the video out tube, using a Ge diode, and DC coupled the anode to the CRT. Again, no statiscally difference for the black level performance per se (only the adjustment is necessary), but in these, I swear that the PAL-M have better less black crushing, ie low level is slight easier to see. I not noticed it with the small TV one, and seems to be just the opposite... Again, could be the difficult to adjust, or my eyes, or the notebook video amp indeed having a slight difference in gamma, and certainly the 2 TV's also have some difference in CRT curve response, basically.

[old_tv_nut]

Interesting - thank you.