CTC5 Video Response
 

I created this new thread to discuss the video quality on a CTC5 I am in the process of restoring.

I have checked the RF/IF alignment and the response looks pretty good. But I am not satisfied with the picture quality. The picture looks soft. The scan focus is good and the CRT is bright without blooming.

The problem looks as if it may be the luma amplifier. There is no trace of any subcarrier getting into the luma channel (no characteristic dots)which suggests the frequency response must be rolling off pretty early. I adjusted the 4.5 MHz trap.

Perhaps it is the delay line? (No evidence of ringing). Or perhaps the peaking coils?

The service notes show the luma response rolling off after 2.5 MHz.

I am curious if others see the same on the CTC4, CTC5 or any color sets of that vintage? (I am referring to the post CT-100, 21CT55 sets).

Note: I have seen pretty sharp pictures on the CTC10 and CTC11 vintage sets. but not sets from the '50's.

Cheers, Terry

I will hopefully be able to sweep the video amps in the Philco TV-123 chassis (1955 design) shortly; am surprised to see the 2.5 mc figure and wonder if the TV-123 matches the CTC5. For reference, the CT-100 is eyeball flat to 3 mc where it drops quickly to zero at the chroma carrier point.

Pete

Thanks, Pete. It appears the Philco has a similar video response. I'd be curious to know how sharp the picture looks on your set.

I have attached the Bandpass Alignment page from the CTC5 Supplemental Notes. Actually the swept video response check on the notes looks flat to about 2.5MHz and the drops to -6dB at 3MHz. The response then appears to dwindle off to a minimum at 3.58 MHz.

I suppose the concern is the picture looks too soft to me. I plan to sweep the video response of my CTC5 to see if it matches the notes. I suppose it had crossed my mind that perhaps others had noticed the soft picture and that it may be attributed to a bad delay line or perhaps bad peaking coils.

I posted previously the diagram for the CTC5 but not the CTC5N "Deluxe". Please find attached the drawing I planned to refer to in the above posting.

2.5 MHz would not be a very sharp picture on a 21-inch screen. Even 3 MHz is only 240 lines of horizontal resolution, but that was normal until comb filters became practical in the late 1970s.

This is ironic because in Europe, early colour sets actually resolved only about this number of lines. One of the, reported, reasons the BBC did not want to go forward with 405 line NTSC colour was that side by side 625 and 405 line colour on the sets of the day would have barely appeared different and would have made a change to 625 line harder to sell to the government and public.

The BBC moving forward with 625 lines was to fall in line with most of the rest of Europe. 405 line had a Kell factor of 1 whereas 625 had a Kell factor of about 0.7, if I recall correctly. (525 line Kell facor is 0.6 which is why 525 line pictures often appear to have less resolution).

Interestingly, France switched from 819 lines to 625 lines but transmitted positive modulation with AM sound. And then, to top it off, SECAM color modulation. But at least standards conversion for Euro video exchange would be a lot simpler.

Back to the CTC5: I am going to perform a video amplifier sweep to get to the bottom of the picture quality issue. I am suspicious of the encapsulated peaking coils and also of the delay line and/or its termination.

I would like to know if other CTC5 owners have ever seen a "crisp" picture on these sets?

Have fun trying to improve the CTC5 - from working on mine (Super, not Deluxe), I would say it's impossible to improve without completely replacing the detector and video sections. I was surprised there is no explicit 3.58 Mhz trap in this chassis, but the response at subcarrier frequency is nil without one. I tried playing with components around the delay line, and only resulted in producing annoying reflections in the video. So, I now consider the set as suitable for viewing VHS quality pics.
The Deluxe chassis has more peaking coils, starting with the detector circuit, and hopefully better video response, though I have never had the chance to measure one.

The CTC5 super chassis can fairly be described as a collection of failed experiments in cost reduction, IMO. Video, sync, chroma, and audio all have minimal performance.

I have never measured the video response of my CTC-5 deluxe chassis, but have never been very happy with the apperance. The picture is soft, and the colors seem never to be quite right.

John, what do you think is the function of Cap on 1st Vid stage of RCAs that couples from cathode to its screen?

Sorry to muddy the waters with the 405 line discussion ... yes the UK moved to 625 line (part of a very logical Europe wide agreement) but one of the reasons the BBC did not want to broadcast 405 colour was they feared it would reduce the take-up of 625 line receivers when 625 line came online in the early 60s esp since side by side 405 line NTSC and 625 line NTSC or PAL were too close in quality.

Newvista, not sure, I don't see a cap from screen grit to cathode on my CTC-5 schematic...?

For my Whitby Deluxe it is C6, 2 mfd, whereas on another thread the schematic showed it as C402

Was just curious for an engineer's take on this circuit design/function

Check out this paper by Jeffrey Borinsky on "Ultimate 405"

Certainly some truth in Jeff's tongue-in-cheek article. Certainly Jeff was amongst the earliest of video oversampling proponents.

I can understand your disappointment with the Super chassis. My set currently has relatively poor vertical sync and poor luma resolution despite changing all the paper caps and IF/RF realignment.

I was was somewhat taken back that there is no 4.5MHz trap in the luma path, unlike the CTC7 onwards. Certainly the CTC4 and CTC5 were evolutionary designs in cost cutting. I have yet to sweep the video amplifier and shall be looking for drifted resistors and bad peaking coils.

I have noticed however that the Deluxe CTC5 has two Snd. IF traps instead of the one on later chassis'. A 4.5MHz trap is in the chroma path only. Perhaps it was felt that the two Snd. IF traps were sufficient with a narrow video amplifier response?

I have compared CTC5 with the CTC7 thru CTC10 video amplifiers and they are suprisingly similar. I recall the CTC10 I once had delivered a sharp picture. But that was within the context of thirty years ago and my memory may be faulty as I am exposed to HD displays today. But I can say that a direct comparison of the CTC5 picture on my 1949 Black and White RCA with a 16AP4, the old 1949 set delivers a nice crisp picture and the CTC5 can't compare.

I am suspicious of the encapsulated coating on the peaking coils and of drifted resistors. I have noticed that the 180uH peaking coil feeding the grid if the video output in the CTC5 has an 8.2 kohm resistor shunt whereas the CTC10 has the same 180uH peaking coil at the video output grid with a shunt of 39 kohm.

I shall make some minor modifications to the video amplifier and shall perform some sweep measurements to see if I can improve the high frequency response a bit.

Perhaps these notes ref the CTC12 will also apply... (From a Jack Darr book)

Of course it could be accomplished with 1/3 the parts like they did at Admiral :smoke:

But did you notice that one version of that chassis drives a four-speaker sound system with a 6L6? Must be the huge Patrician model, the one that resembles a stereo combo. Sorry for the hijack, but are any of those known to exist?

Their audio is nice design also: simple bass+treble controls + neat feedback Presence control :cool:

This is very interesting. The RCA Service folk recognized the same problem? And then suggested tilting the IF to boost the higher baseband video frequencies!

Hmmm...I am going to see if the problem in the video amplifier can be minimized...