Quote:
Originally Posted by Penthode
Have you played with the edge purity magnets? (The ones which are only adjustable after removing the screen frame. You are supposed to retract them before degaussing). I suppose the tube was well degaussed?
How good is the purity: do you reach the optimum purity pointwithf the yoke forward?
And also, what does the HV measure?
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I'm getting decent purity; edge magnets are retracted. Thanks for mentioning degaussing - I actually forgot to do it! With the glass tube the set didn't seem to need it - purity was fine before and after degaussing.
Haven't measured the HV - it measured on spec before. (The spec for this chassis is rather low - I forget what exactly.) I do recall, however that the luma on the cathodes was low (compared to the chroma demod outputs on the grids) and has been since I got the set. Changing out the video output did not seem to help. I think a reasonable design should be able to generate luma drive at least half the p-p voltage of the color difference p-p on the grids. The chassis is capable of color difference p-p much greater than 2x luma p-p. Time to get out the scope and verify what I recall.
One problem is not having any service info showing waveforms with full color bars. The luma waveform with gated rainbow input is meaningless, so I have no idea if I'm getting the expected amplitude - but it seems low to me compared to the range that is available from the color difference.
By the way, I have solved a mystery - I noticed that a full red field (from a test signal, not from shorting the grids) looks washed out. If I am reading the schematic right, the color difference outputs are only 75% DC coupled to the grids. This, combined with about 75% DC coupling in the luma, causes a full red field to be washed out when color level is correct on color bars. I can't figure out why the designers reduced the DC coupling in the color difference. There have never been white balance problems in broadcasts that this might reduce. If they were worried about DC drifts in the demods, a 25% reduction would not be enough to fix it. Later sets used the RCA three-triode matrix amplifiers with "X-Y" demods, and the matrix amp circuit had built in DC restoration by drawing grid current when the negative blanking pulse was applied to the cathodes, so demod drift was not a problem. Just another way the CTC-5 super chassis is weird and inferior. If I convince myself I am right, I will try cutting out three .01 uF caps that I believe cause the 75% DC coupling in the color difference signals.
Another by-the-way: I couldn't follow the RCA gray scale set-up procedure, since it assumes there is no R background control and you set G and B backgrounds to match. This later chassis has background controls in all three colors. So, I tried modifying the procedure to a sort of hybrid of the RCA and SAMS. It looks better, but probably is not optimum. It probably won't be optimum until I can get more luma drive.