Lexicon RV-8 Service Manual page 127

Lexicon
S-video inputs (Video board schematic sheet 1)
Specific references are to input 1; other inputs are similar. S-video luminance inputs (pin 3 of the mini-din
jacks) are terminated and buffered the same as composite inputs. AC-coupling is applied after buffering;
C25 couples s-video #1 luminance. Chrominance input #1 (pin 4 of mini-din jack J15) is first ac-coupled
by C24, and then buffered by emitter follower Q19. The dc-level at the chroma input pin is direct-coupled
to subsequent sense circuitry through the voltage divider formed by R70 and R71.
Main (Monitor) Composite / S-video (Video board schematic sheet 2)
Composite and S-video luminance connect to multiplexers U20, U21, and S-video chrominance connects
to U22. The composite multiplexer is addressed by the MVID_SELn bits, and the S-video multiplexers are
addressed by the MSVID_SELn bits. When MCVID_EN/ is asserted low, U18 is enabled, and all
MSVID_SELn bits are forced to 0 by logic in CPLD U14 (sheet 9). Composite multiplexer U20 selects one
composite source. With MSVID_SELn set to 0, the s-video path is disabled because U21 is selecting a
disconnected input, and U22 is selecting a grounded input to feed the chrominance channel. When
MCVID_EN/ is high, U20 is disabled, disconnecting the composite inputs, and U14 passes addresses to
the MSVID_SELn bits, allowing U21 and U22 to select one of the S-video sources. The
composite/luminance (MY) signal from U20/U21 is amplified by non-inverting stage U28. R207 makes the
gain be slightly greater than the desired factor of two in order to make up for slight losses in other stages.
The signal from U28-1 is fed through R204 to the sync-stripper and dc-restorer (sheet 8). The dc-
correction signal BPCOR returns through R206 to close the dc-feedback loop and maintain the video
back-porch near 0Vdc. The signal OSD_Y_IN is distributed to output amplifiers U16, U27, and U38, and
also feeds the on-screen display (sheet 7).
Chroma selected by U22 (MC) is ac-coupled by C127 and amplified by U28, also with gain slightly greater
than two. With a composite source selected, U22 is forced to input 0, grounding the chroma channel. The
signal OSD_C_IN is distributed to output amplifiers U15, U27, U38, and U5, and also feeds the on-screen
display (sheet 7).
The dc-level on the chroma channel of the selected source is fed to two dc amplifiers through multiplexer
U23, filtered by C5. The overall gain of the dc path from the chroma input is unity, such that the dc
voltage on the chroma input is buffered and can be applied to the chroma output.
The amplifier formed by Q5, Q6, and Q4 applies dc to the chroma of the S-video 2 output, and Q8, Q9,
and Q7 applies dc to the chroma of the S-video 1 output. Each dc path can be selectively enabled by the
MORPHEN/ and MORPHEN1/ logic levels. A high logic level disables the amplifier and no dc voltage is
added to the corresponding chroma output. The purpose of the dc amplifiers is to transfer the dc voltage
at the input to the output, to accommodate different S-video aspect ratio switching schemes that rely on
different dc-voltages.
Main S-video at J4 is driven by gain-of-one amplifiers U16 (luma) and U15 (chroma). Internal multiplexers
in these amplifiers determine whether the s-video is taken from the OSD path (MSTHRU/=hi) or straight
through from the input amplifiers (MSTHRU/=low). MSTHRU/ follows MTHRU/ unless MSVID_YOFF is
asserted by logic in CPLD U14 (sheet 9). MSVID_YOFF allows main s-video luminance to be shut off by
U40 (sheet 7) when a composite source is in use. Amplifier outputs are fed through 75-ohm series
resistors (R145, R142), forming a matched transmission-line driver system. R144 and R141 compensate
for slight impedance errors due to the resistance of the on-board connecting traces. The chroma output is
ac-coupled by C80, with a dc-level introduced through R18, according to the action of the associated dc
amplifier.
Main S-video at J3 is driven by gain-of-one amplifiers U27. S-video at J3 is not subject to the OSD and is
not selectively disabled when composite video is in use.
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