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- File name: descramb.txt
-
- The following are the most frequently asked questions about the universal
- descrambler/synch regenerator. A 20 Mhz scope and frequency counter are a
- must for trouble shooting.
- 1. R7 There has been some confusion as to the correct connections around
- R7 and the polarity switch S3. If one closely examines the PC board, R7 can
- either return to +5V or to ground. R-E Fig3 and Fig4 (P40,41) seem in
- conflict. If R7 is returned to ground then +5V is supplied through S3. If R7
- is returned to +5V, then ground should be supplied through S3. Actually R7
- was intended to either return to ground or +5V to allow various interfacing
- methods for experiments in automatic polarity switching.
- 2. Pin 9 IC5 This pin will normally read +0.2 to +0.3 volts; not -0.2
- volts as in R-E P43 Column 1. Q2 produces a narrow(2msec) positive going
- pulse during burst intervals to IC5 pin 9 and therefore a VOM or DVM will read
- only a very small positive voltage.
- 3. Pins 5 & 10 IC1 are -5V and +5V respectively.
- 4. The correct frequency signal at pin 23 IC9 is 503.496 khz (503.5 khz
- nominal) not 1.08 Mhz as shown in R-E. The output from IC6 (divided by 455)
- should be 7867.13 hz. If this is not the case, check D1-D6.
- 5. One must have a 4V to 5V P-P square wave at P23 (503.5 khz) before the
- decoder will operate.
- 6. IC9 The voltage at pins 13, 11, and 5 may vary considerably or even
- read zero, depending on the meter used to measure them. Verify 503 khz clock
- is present on Pin 23 IC9.
- 7. IC8 divides by 64, not 128. Pin 12 shown is actually pin 2.
- 8. IC10 may run somewhat hot. Do not apply more than 15 vac to J3
- without providing a heatsink on IC10. This heatsink is not provided with the
- kit.
- 9. Ringing on picture - Check IC1 and verify that C2, C3, C4, and C5 are
- in the circuit and OK. Try a .01 bypass between IC1 pin 14 and ground
- (connect across R4)
- 10. -0.45 volts is supplied to IC3 Pin 2, not IC2 Pin 2 as in Column 1
- page 43.
- 11. Locking of picture in some cases can be improved by changing R20 and
- R21 (1K each) to 470 ohm. However, before doing this check C17, L1, Q1, R22,
- R23, and C18.
- 12. Incorrect hue can be shifted to improve color by adding capacitance
- (try 33 pf) across C32. If this makes things worse, then try changing C30
- from 56 pf to 33 pf without the 33 pf across C32.
- 13. Color stability is a function of accurate lockup. The burst key pulse
- (IC9 Pin 11) is somewhat narrow. Try resetting lock with S1. If no help,
- make sure that the video signal is clean. Some experimenters have reported
- that IC6 can produce "glitches" or momentary intermittent miscounting. A
- change of value of R36 (try 1K ohm) and also of C36 (try values from 820 pf to
- .0015 uf) has helped in these cases.
- 14. Smear may be caused by incorrect frequency response somewhere in the
- video system. This is generally an interface problem. Try the following:
- A Remove IC2 and IC3.
- B Jumper IC2 pin 2 to pin 15 and IC3 pin 3 to pin 15.
- C Now using a good off the air NTSC signal (not scrambled),
- connect video into J1 and examine video out of J2. If OK, the problem is not
- in the descrambler video circuit.
- D restore normal connections and check IC2 and IC3. If OK, the
- problem is in the video source. A typical case with a non-scrambled signal,
- the picture is OK, but with a scrambled signal, there is smear, blotchy color,
- shadowing,or poor contrast.
-
- Remember that your VCR, TV tuner or video source may not operate properly with
- a scrambled video signal. The horizontal and/or vertical pulse are missing or
- distorted. Your equipment may contain circuitry that depends on these pulses
- for some phase of operation (AGC, DC restoration, etc.) and therefore may
- further distort a scrambled signal. The reconstituted signal may therefore
- show these distortions. Therein were no such problems in the prototype
- circuits, so there shouldn't be any problems inherent in the descrambler. The
- following points should be considered:
- A The video source should ideally be right off the video
- detector of the TV tuner or receiver with no extra amplifiers or other
- circuitry. Sag, excess tilt, or other waveform distortions can occur due to
- low frequency response or phase shift in the coupling capacitors.
- B The fact that "every else works OK without the descrambler in
- the system" is meaningless due to the aforementioned considerations.
-