Color TV Chroma Bandpass Alignment
 

I have completed the RF and IF alignment of my CTC5 Deluxe. Tackling the chroma alignment has been a bit of a challenge in as much as the CTC5 service notes requires a 0-5MHz video sweep with markers.

The CTC5 Deluxe I believe is unique in the realm of early RCA color TV design because it uses an asymmetrical chroma bandpass amplifier (2.5MHz to 4.1MHz) for both chroma demodulation axes. The CTC2 had different bandpasses for each axis and the CTC5 Super and CTC7 upward appeared to have equal double sideband (3.0MHz to 4.1MHz) for both axes.

The original RCA Service Data required a 0 - 5MHz sweep generator feeding a RCA Multimarker WG-295A and an RF Modulator fed by the multimarker with a channel 4 video carrier from an RF signal generator. The original RCA multimarker and modulator are all but non-existent and I am planning to use my HP8601a sweep generator with HP8600a marker generator to replace the RCA suggested equipment.

Note that the uniqueness of the RCA CTC5 Deluxe design is curious as it provides extra chroma resolution at the expense of introducing quadrature crosstalk in demodulation. I recall many years ago a discussion of this in the RCA Review although I cannot recall the context of the discussion. Early NTSC color receivers suffered to a degree by the low color resolution which meant that finer detail color elements which should otherwise be resolvable could not be reproduced. Later PAL development allowed wider chroma bandwidth because the quadrature crosstalk would be effectively cancelled by the alternating R-Y axis.

That set up has evolved into the VSM (Video Sweep Modulation) method of alignment. The BK 415 is already set up to do a VSM alignment. As is probably any later sweep/marker generator with the internal crystal markers pre set. Now, that applies if its a 45 mhz IF. If its a 21 mhz, then we need something else.

This alignment set up means that you will be seeing the effects of both IF bandpass shape and chroma bandpass shape on the chroma. To get the chroma alignment right, you therefore need first that the IF alignment and fine tuning adjustment are right. You could have a debate about which is better, specifying the IF and chroma bandpass separately, or in cascade like this. But since the service info is for the cascaded case, that's what you have to use.

That is the quandary. The simplest method to me would have been for the manufacturer to create separate RF, IF and Chroma bandpass alignment procedures with response curves published for each separately. Although to me, best overall engineering practise would be to evaluate the complete start to end response. It is more complicated and requires proper or alignment of RF and IF stages before checking the chroma response.
I did do this with the equipment I have and it worked out well.