A question for you long time experts

[jpdylon]

Does anyone know when RCA made the transition from the early color sets that had full DC restoration to ones that had partial dc restoration? My CTC 11 chassis has partial, but I know sets like the CT-100 and CTC2b have full dc restoration.

Thanks in advance.

[old_tv_nut]

CTC-5 is partial

[jpdylon]

Thanks. I'm wondering if it would be worth the trouble to modify the set for full DC restoration. The set has a fairly good picture as it is.

[old_tv_nut]

I think the CTC-5 picture would be improved by adding DC restoration - I am often fiddling with the brightness control depending on the subject matter. However, I haven't investigated how to do it in this chassis.

[old_tv_nut]

Just a note- the reduced DC restoration means that scenes with large bright areas don't draw as much beam current as they would with full restoration. Therefore, if you add full restoration, you will have to run a lower average drive (lower contrast setting, meaning lower picture brilliance) to prevent going out of HV regulation on those bright scenes.

[jpdylon]

It would be nice if you could safely run the HV at something like 30k - but given the technology of the day I guess that wouldn't really be feasible without heavy modification and perhaps a different FBT? Then there is the question if the 21CYP,BFP,JFP tubes can handle 30kv safely. . .

Perhaps I'll just live with it for now. I've always toyed with the idea of making a franken set that has full DC restoration with a modernized 30kv fully regulated HV supply. I'm betting the picture would look fantastic, but the engineering trouble would far outweigh the outcome.

[cbenham]

Originally Posted by jpdylon

I've always toyed with the idea of making a franken set that has full DC restoration with a modernized 30kv fully regulated HV supply. I'm betting the picture would look fantastic, but the engineering trouble would far outweigh the outcome.

What you want to find is an RCA TM-21 broadcast color monitor. Full DC Restoration and a well regulated HV supply with a 21AXP22 as well...
These sets make beautiful pictures.

[John Folsom]

I expect it would be very hard to find a TM21 with a 21AXP22 in it. Those monitors were used for many years, and were all likely upgraded to the all-glass CRT. My TM21D originally had a 21CYP22 in it, but had been upgraded to a 21FBP22. I have put a 21CYP22 back in the set, but have not gotten all the bugs out of it yet. Another project to get back to one of these days.... :-)

[Zenith26kc20]

Might be sticking my head out but I remember the 21 inch roundies are maximum rated at 25 KV. From what I remember, the 3A3 tends to overload and sag on bright scenes causing blooming.
Maybe a much stiffer high voltage supply (keep in mind pre war tv's and their ability to really ruin a persons day when ultor voltage goes to gound thru a human). It may be possible to wind a second filament winding on the flyback and run parallel 3A3's for more current. The only consideration here would be added dissipation in the primary of the high voltage stage and the ability of the regulator to control two 3A3's at one time.
I am looking for a suitable chassis to make a "Frankenset" right now. My girlfriend will probably have me committed if I do but..........
"What? No cabinet?!!!

[John Folsom]

I don't think the 3A3 regulator is the weak link in the chain that is the HV supply for the 21" color CRTs. Replacing the 3A3s with HV solid state diodes makes little difference in performance. Basically, the flyback supply is just not capable of supplying all the current needed to drive the CRT as hard as one would wish.

[andy]

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[ChrisW6ATV]

I don't understand why/how better DC restoration would increase HV beam current in bright scenes. Better DC restoration should only improve the darkness of dark scenes, not affect bright/high-contrast scenes. You would adjust the raster for maximum desired contrast or highlights within the ability of the HV power supply, the same as on any color CRT display, good DC restoration or not. Maybe, the improved black levels would increase the current through the HV regulator tube. Would that possibly exceed the ratings of a 6BK4 tube? If so, could that problem be solved with a pair of them, or is there a heavier-duty substitute available? Did the 21CT55 and/or CTC4 "eat up" 6BK4s, if those sets had good DC restoration?

[old_tv_nut]

Without no DC restoration, the total beam current tends to float to a constant 50% level, that is the average beam current does not follow the average brightness of the scene. With restoration, it goes from zero (all black scene) to twice the average (all white scene). Partial DC restoration means it will float over a smaller range, but it still floats, so there is more DC (average level) than there should be in dark scenes (washing them out) and less than there should be in bright scenes (clipping blacks, but drawing less than max beam current). Of course, on an individual still scene, you could adjust the brightness knob to restore the proper DC level. And then you could see if you have enough regulator current left on that all white scene.

[Zenith26kc20]

Does the RCA TM21 monitor use the same video output setup like the CT-100?
The CT-100 appears to be akin to modern solid state drive to the CRT with "Y" and color on the cathodes (hope I'm right but I'm at work away from my CT-100 schematic) (I know, blasphemy) from what I remember. The later color sets had "Y" on the cathodes and color on the control grids.
Both color and "Y" appear to be capacitor/resistor coupled (at least on my Magnavox T933 and Zenith N chassis.
I cant find any schematics on the TM21. Any chance of some postings?
Also, I might be wrong again, but I remember attending a Zenith service meeting in the 1970's (hope that's right) when they actually had a modified video module to remove DC restoration from their solid state sets because it would go black between scenes and the customers thought it failed until it came back on.

[andy]

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