Quote:
Originally Posted by maxhifi
Read videokarma long enough, and one recurrent theme is the VDR.
My favourite subject, the CTC38 uses about four of them (don't have the manual with me), but the two I am most interested in are in the vertical and horizontal sweep circuits.
I have noticed two things, which I am starting to suspect are related.
1. Vertical size must be turned all the way down to get the correct size, with linearity adjusted properly.
2. Line voltage fluctuations/transients (electric space heater coming on, furnace starting, etc) seem to affect the vertical size.
My Theory: the VDR is supposed to keep the vertical size constant with line voltage fluctuations. Since this isn't happening as well as could be expected, is it also possible that a bad VDR could be causing the vertical size control to be adjusted at the bottom of its range, rather than in the middle?
Now another and related question, is how to test a VDR out of circuit?
Digikey and Mouser have tons of MOVs available, but it will take some doing to find a substitution, because the way they are specified appears different. I think some more homework on my behalf is needed, I am posting my thoughts so far though for a reality check to see if I am staying on the rails or not.
I also ordered two interesting documents which may have some answers for me
1. Field Service Guide RCA Color TV Receivers 1969 1970 CTC-22 Through CTC-40
2. 1969 RCA PRODUCT TECHNICAL MANUAL TV RADIO TELEVISION PHONOGRAPH TAPE ORIGINAL
I am hoping I can get answers from either or both of these, to some of the mysteries of the CTC38 chassis which Sams has not revealed to me.
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Zenith made that funky transformer related voltage regulator as a way to
control supply voltages in the tv as line voltages fluctuated. I believe RCA
decided they did not need to put that much money into a power supply
immune to voltage changes, so they decided to use the VDR's to control
on screen artifacts that would cause the customer to complain - if they did,
when the picture would jump around when the frig turned on or off.....
After all, it was only the picture that was effected, not sound.... So
why fix the entire power supply..... Just stop the picture from changing
size...
I think they needed something they could use as a non linear device that kinda
tweeked the gain of those amps and react rather quickly - thus do it in a
gain circuit.... Their solution was for sure cheaper than Zenith's....
If you don't find new ones with the exact specs, I bet you could parallel them
with a small resistor to make them more stable.... Kinda divert some
of the current..... Down side lessen the effect it had on the circuit...
I think it would be neat if you could put manual items in those circuits and
test each to see what they do to the picture visually, and document the
effects..... And are they in fact linear, or non linear to provide a quicker
response..... Sony's picture use to jump around with extra bounce when
the voltage levels were disturbed..... and they were not damped in their
effects....
Or the dam things are an RCA designers fudge device.....
.