odd TV UHF tuner problem

[Wayne Fulton]

This is an idle question of curiosity, theoretical, not about any actual
hardware repair.

I have a Panasonic TV, at least 15 years old, tuning conventional
channel numbers on an attic broadcast antenna. Works fine except that
all UHF channels went dead recently (blank dark snowy screen). VHF
still works fine and normal. Repair is not an issue in this situation,
a VCR can tune and output UHF to channel 3 if seldomly wanted.

But there is a curiosity. The TV tuner goes to channel 99, and I
discover that the missing UHF channels are now available on much higher
channel numbers, specifically always 51 channels higher than expected.

Channel 21 comes in fine at channel number 21+51 = 73
Channel 27 comes in fine at channel number 27+51 = 78
Channel 33 comes in fine at channel number 33+51 = 84
etc

The tuner only goes to 99, so those above channel 48 are unavailable,
but all below work fine in this way. The on-screen Channel Recall
number shows the higher "wrong" number.

I'm curious about the UHF tuner design, in a general way, the digital
part I assume. I understand the idea of superhet mixers and RF and 10.7
MHz IF, and UHF is probably double conversion. But I'm curious what
could be the possible source of this 51 channel UHF offset? How could
the tuner be constructed so that this specific fault is possible? If
some oscillator died, wouldnt there be no output? I expected more of an
"all or nothing" operation.

 

[Andrew Rossmann]

This is an idle question of curiosity, theoretical, not about any actual
hardware repair.

I have a Panasonic TV, at least 15 years old, tuning conventional
channel numbers on an attic broadcast antenna. Works fine except that
all UHF channels went dead recently (blank dark snowy screen). VHF
still works fine and normal. Repair is not an issue in this situation,
a VCR can tune and output UHF to channel 3 if seldomly wanted.

But there is a curiosity. The TV tuner goes to channel 99, and I
discover that the missing UHF channels are now available on much higher
channel numbers, specifically always 51 channels higher than expected.

Channel 21 comes in fine at channel number 21+51 = 73
Channel 27 comes in fine at channel number 27+51 = 78
Channel 33 comes in fine at channel number 33+51 = 84
etc

The tuner only goes to 99, so those above channel 48 are unavailable,
but all below work fine in this way. The on-screen Channel Recall
number shows the higher "wrong" number.

Your TV is in cable mode. You need to go through the menus and change
it back to 'AIR' or 'Broadcast' or whatever. Cable channels 14+ are on
different frequencies than broadcast channels.

Some details here:
http://www.arrl.org/tis/info/catv-ch.html

 

[Wayne Fulton]

Your TV is in cable mode. You need to go through the menus and change
it back to 'AIR' or 'Broadcast' or whatever. Cable channels 14+ are on
different frequencies than broadcast channels.

Yes, I can see now that you are exactly right. There is a little switch
that was in one of the cable positions, instead of TV position.
The mystery now is how that happened. <g>

Thanks Andrew, I appreciate the help.