Sencore VA62A Tech Tips

#178
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Means Success In Electronic Servicing
tech •.
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Testing Digital TV Tuners
The digital tuners found in today's TVs and
VCRs are more reliable than older mechani-
cally switched tuners, yet they still fail. When
they do, the VA62A Universal Video Analyzer
provides the signals and tests needed to isolate
and correct the problem. The VA62A's all-
channel operation, with the ability to offset any
channel's frequency, provides dynamic testing
of digital tuners.
This Tech Tip explains how to relate service
literature to tuner symptoms, and then how to
use the VA62A to isolate problems.
Understanding Band Switching
Most conventional (non-digital) tuners have only
three tuning bands. The low-band VHF tuner
covers channels 2-6, the high-band VHF tuner
covers channels 7-13, and the UHF tuner
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covers from channel 14 through the top UHF
channel. (Older UHF tuners covered through
channel 83, while newer ones stop at channel
69, since the FCC has allocated channels 70
through 83 to other uses.)
Digital tuners often add cable bands to the VHF
and UHF tuner sections. This requires several
more tuner sections. The channel numbers
where bandswitching occurs vary from one
receiver design to the next. Knowing where
bandswitching occurs helps in troubleshooting
suspected tuner problems. Channels in each
band should be tested to confirm that the tuning
system works correctly.
While it is possible for a tuner to improperly tune
channels in the center of a band, the most likely
problem channels are at the upper and lower
edge of each band. These channels require the
tuned stages to be set to their extreme limits,
making them the most difficult for the automatic
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Courtesy Philips
Consumer Electronics
Fig. 1: This tuning
chart from a TV
schematic lets you
determine the num-
ber of tuning bands
and the channels at
each band edge.
circuits to capture. You need to know where
bands switch, in order to know which channels
need extra attention. Service literature usually
shows which channels fall at band edges, al-
though some interpretation of the literature may
be needed.
Interpreting Service Literature
Fig.1 shows a typical tuning voltage chart found
on a tuner schematic. Notice that this chart has
four curves, indicating the tuner has four bands.
Drawing a line from ends of each curve to the left-
hand scale identifies the ends of each band.
The channels for this particular tuner break out
as follows:
Chart Channel
Band Channel Frequency Frequency
I Bottom 2 101 55.25
I Top 6 129 83.25
II Bottom A-2 153 109.25
II Top 13 257 211 .25
Ill Bottom
J
263 217.25
Ill Top RR 449 403.25
IV Bottom SS 455 409.25
IV Top 125 845 799.25
UHF Bottom* 14 517 471 .25
UHF Top* 69 847 801.25
*NOTE: Cable band IV and UHF use the same tuner
band.
Fig. 2: The top and bottom channel for each
of the tuning bands represented by the curves
in Fig. 1
Notice that the frequencies listed in the service
literature (3rd column) do not agree with standard
channel assignments (fourth column). The tun-
ing chart references the frequency of the tuner's
local oscillator, not the video carrier frequency.
To convert from the oscillator frequency to the
carrier, subtract 45. 75 MHz, which produces the
intermediate frequency (IF) after mixing.
#178
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